Liquid moisture and water vapour, relative humidity (RH) and indoor air quality (IAQ), capillarity, hygroscopicity, vapour permeability, moisture management in construction, breathability, example constructions (new build and retrofit)

It is clearly not possible to make anyone an expert in moisture management in a single chapter of a book. This is partly because water/moisture is affected by so many different variables: climate, location, construction type and method, material, occupant behaviour, etc. But it is important to gain an overview of some of the main issues, especially on an ultra-low-energy build. Moisture levels both within the building fabric itself and transported by the internal air need to be carefully considered.

The Passivhaus approach is well considered when it comes to combining low energy performance with moisture management, and this is one example of how all the principles of Passivhaus together provide an integrated solution. Adoption of the standard needs to be as a whole – the temptation to partly adopt strategies, particularly in retrofit scenarios, carries significant risks. Highly insulating some areas while leaving others exposed will concentrate moisture towards localised areas, so a structure that could previously manage moisture levels may become overloaded. Highly insulating without ensuring low air-leakage levels increases the risk of moisture travelling into your construction and condensing (interstitial condensation – see page 147). Both these scenarios could lead to structural damage, a reduction in thermal performance and fungal growth (which can affect indoor air quality, or IAQ). However, you can also prolong the life of your construction materials by ensuring good moisture management – this is part of the quality approach built into the Passivhaus standard.

Moisture has been referred to as the worst of all potential pollutants in a building, and it is certainly identified as having the largest detrimental impact on the fabric of buildings, in terms of both structural and material degradation. The moisture (relative humidity, or RH) level in the air largely determines whether a variety of microbiological pollutants will be able to thrive, which has direct consequences for our health and well-being. The RH level also influences how comfortable or uncomfortable we feel (as it affects our ability to sweat and control our own body temperature) and will alter the way we experience a given temperature. We are more tolerant of higher temperatures, for example, if the RH level is low.

Unfortunately, the potential for these detrimental effects increases as we increase the level of insulation in our buildings. Traditional buildings were not heated to any great degree, and neither were walls, roofs or floors insulated. Moisture presence was managed by building with materials that could naturally dry out, and allowing air to pass through and around elements to aid this process. As we now expect warmer indoor temperatures, are increasing airtightness levels and using greater amounts of insulation, we are also creating different indoor environmental conditions. When designing ultra-low-energy housing, considering how moisture will be managed is therefore extremely important.

This chapter is intended to ensure that you approach the design of your building fully aware of the detrimental consequences of poor moisture management. In a new build, the risks are controllable – you can design them out. In a retrofit project, moisture management becomes more complex and your management strategy even more critical. The first part of this chapter describes the processes involved in moisture transport in buildings. The second part deals more directly with construction and discusses some examples of low-energy constructions.

Liquid moisture and water vapour

When we consider moisture, we usually think of it in its liquid form; the kind that falls as rain and hangs in the air as mist and fog. Liquid moisture has the potential to do the most damage to your building. But we also need to be aware of water in its gaseous form, water vapour. While invisible to the eye, water vapour is always around us and makes up approximately one to four per cent of the Earth’s atmosphere. Moisture is transported by the air in the form of water vapour, and will readily return to its liquid state given the right conditions.

Water molecules are polar molecules (they behave a bit like mini magnets) as a result of the asymmetrical distribution of their constituent parts – hydrogen (positively charged) and oxygen (negatively charged). This means they tend to group together (in pools of water) and also adhere easily to some materials, such as smooth glass, and be repelled by others, such as wax. Some materials will soak up (and give off) moisture from the air much more readily than others, and in turn this will affect RH levels – the quantity of water vapour in a given volume of air – especially at low air change rates. With high air leakage rates, typical in most housing, the influence of materials on humidity is much less significant. The way a particular material interacts with moisture influences how moisture might or might not get transported through your building’s thermal envelope. Poor moisture management within your construction will affect the durability of the materials, can encourage moulds to develop, and will often be detrimental to energy performance.

The difference between liquid water and water vapour is in their molecular density: water vapour is less dense than liquid water, while the molecular bonding present in liquid water also reduces significantly in its vapour form. Water vapour, like any other gas, will diffuse through a material if there is a higher concentration of the vapour on one side of the material than on the other. This is why we can have watertight sheet materials, often referred to as ‘breathable’ or ‘breather’ membranes, that are both airtight and liquid-moisture-tight but water-vapour-open, i.e. they act as a barrier to liquid water and to the body of air as a whole, but allow water vapour to pass through them (see Chapter 9, page 128).

Understanding this concept is key to appreciating how airtight constructions can also be ‘breathable’ constructions. When we refer to ‘breathable’ materials, we are normally considering how they interact with moisture. So with an airtight, low-energy construction, depending on the materials you use, water vapour may still enter and exit your construction.

Monolithic films

Conventional airtightness membranes are normally microporous, transporting vapour ‘passively’ through their open pore structure. There are also membranes on the market that include an additional monolithic film (TEEE film). These are non-porous but allow water vapour to diffuse ‘actively’ through their molecular structure, by molecular exchange. Unlike microporous membranes, monolithic films have selective permeability – they allow only some, not all, gases to pass through. These films are usually hydrophobic, i.e. repelling liquid moisture. They might be used where a watertight membrane is needed towards the outside of an assembly and there is a risk of condensation forming on the inside of the membrane (say, in a timber roof construction) – this will help to prevent mould growth on the surface.1

Interstitial condensation

In winter, temperatures will generally drop across the construction depth (from inside temperature to outside temperature). It is therefore possible, with the right conditions, for any water vapour to return to its liquid state, i.e. to condense, on a surface within the construction. This is known as interstitial condensation. In this way, liquid moisture can accumulate within the fabric of a building over time. If this water is trapped, the detrimental consequences can be extreme. However, if the water can dry out (normally to the outside), any damage can be limited or can have no effect at all, depending on the materials involved.

If the moisture accumulation is the result of air leakage, you will tend to get localised areas of concentrated moisture. Moisture transported through vapour-open ‘breathable’ materials will distribute the moisture more evenly across the whole construction. Moisture penetration by air leakage is never planned and may cause localised failures. However, should moisture transported through vapour-open materials become trapped within the construction, failure could well occur across the entire construction element. One problem with moisture condensing within constructions is that the effects can go unobserved for long periods, perhaps coming to light only when the consequences have become severe.

Moisture in the air

If you do not achieve appropriate airtightness in a super-insulated building, then the internal air will transport moisture produced by occupants into your building assemblies, wherever there are gaps or cracks to allow it to leak in. Designing an airtight contiguous (continuous) layer is therefore critical in avoiding moisture transfer into your wall, roof or floor. Airtightness, then, is not only a strategy for lower energy use but is also an important part of the way in which a low-energy building manages moisture in a healthy way. Once your structure has been highly insulated, it makes no sense to allow warm, moisture-laden air to penetrate into it.

In terms of the external air, your wind-tight layer (see Chapter 9, page 128) will act in a similar way to protect your assembly from ingress of moisture-laden air. In cool-temperate climates, such as in the UK, the vapour pressure differential, or gradient, for most of the year will be from inside to outside, so the internal airtight layer is more critical. The vapour permeability of materials should increase towards the outside of the assembly to further encourage moisture transport to the outside (see ‘Breathable assemblies’, page 156). In the summer, when the outside air is warmer and moister, the reverse gradient is much weaker because of the smaller average temperature difference between inside and out, as well as the more rapid drying effect from wind and sun.

Absolute humidity and relative humidity (RH)

It is best if we define the term ‘relative humidity’ before proceeding further, since RH levels are generally what is referred to when considering ideal indoor air conditions. ‘Absolute humidity’ is an easier term to comprehend: it is simply the mass of water vapour divided by the mass of dry air in a given volume of air. Humidity is measured at different temperatures because temperature affects the ability of air to contain or carry water vapour – hot air can transport more water vapour than cold air (see Figure 10.1 overleaf). This is why cold air can sometimes be described as ‘dry’. ‘Maximum absolute humidity’ means that the air is transporting its maximum potential of water vapour at that temperature – the air is effectively saturated. RH is a percentage calculation between the absolute humidity and the maximum absolute humidity; 100-per-cent RH would therefore mean you had reached the saturation point of the air at a given temperature, and condensation will occur.

Figure 10.1 Psychrometric chart showing how RH levels and saturation points vary with temperature. Image: Randall McMullan, Environmental Science in Building, 1983, Palgrave Macmillan. Reproduced with permission of Palgrave Macmillan

When we devise the ventilation strategy for our buildings, we are aiming to achieve good RH levels, normally 30-60 per cent. Within this range, the beneficial aspects of water vapour in the air are optimised and the negative aspects minimised.

The amount of moisture in the indoor air is obviously influenced by the external climate. If your ventilation rates are high (lots of air changes), then the absolute moisture levels are likely to more closely follow the external air conditions. In order to control RH levels, it is important to keep ventilation rates low, as is the strategy in ultra-low-energy buildings. When we say ‘low’, this is of course only in the context of the general unplanned over-ventilation of existing houses. In ultra-low-energy houses, ventilation rates can be reduced to appropriate levels by using planned ventilation strategies and by minimising air leakage.

Relative humidity and perceived comfort levels

Regarding comfort levels, ASHRAE (the American Society of Heating, Refrigerating and Air-conditioning Engineers) has undertaken studies that indicate a general preference for RH levels of 20-50 per cent. ASHRAE is an international technical society that develops standards and guidelines on ventilation matters; these are normally adopted by, or inform, other more localised standards. ASHRAE considers an RH level of 35-40 per cent to be optimum during the heating season. Drier air will feel colder to us and we may also suffer from dry eyes or nose irritation, making us feel less comfortable. If the RH level is closer to our ideal, we will be happy with slightly lower air temperatures.

Relative humidity and health

The ASHRAE health standard for RH recommends 30-60 per cent for any habitable space, to minimise the growth of allergenic or pathogenic organisms. The ‘Sterling Study’, an ASHRAE technical paper,2 looks at the effects of RH levels on a variety of unwelcome outcomes, such as dust mites, fungi, viruses, bacteria, respiratory infections, allergic rhinitis, etc. The results are shown in the Sterling bar graph (see Figure 10.2 below), which visually captures the increasing negative effects of either too-high or too-low RH levels.

There is increasing evidence to demonstrate the link between RH and these health-related outcomes; certainly enough for these effects to be widely acknowledged. With growing numbers of asthma sufferers since the 1970s (one in three Irish children has asthma), the need to ensure good ventilation and RH levels is key to the health quality of our new low-energy housing. Currently, indoor RH levels during winter are commonly below 40 per cent. If we can increase RH levels above this, we would reduce respiratory infections and asthmatic attacks in particular. If higher RH levels occur in summer, then there is an increase in the level of dust mites (almost eliminated in winter at lower RH levels, say, below 50 per cent). The number of people allergic to dust mites is also increasing, and therefore aiming to keep RH levels to lower levels, i.e. below 50 per cent even in summer, is very desirable. From current research it would seem that a target RH in the mid-to-upper 40s is the optimum range.

Figure 10.2 Sterling bar graph illustrating the link between relative humidity and health.
Source: Sterling et al. ‘Criteria for human exposure to humidity in occupied buildings’3

Relative humidity and the Passivhaus standard

The target RH level for the Passivhaus standard is 35-55 per cent. As we have seen from the above, aiming to keep the RH levels towards the middle of this range would seem to be ideal. Once you have achieved low ventilation rates, RH levels will be affected by the building construction itself and by how the building materials interact with the moisture in the air. Using materials to improve RH levels is therefore a strategy worth considering in an ultra-low-energy build.

The reasons for the Passivhaus adoption of a whole-house mechanical ventilation strategy are discussed in detail in Chapter 12, but it is worth noting that this ensures that moisture is being continuously removed directly to the outside, from the wet and moist areas of the house (kitchens/bathrooms). The overall effect on indoor air quality of RH levels (too dry or too humid indoor air) and the way this can be addressed is also discussed in Chapter 12.

Moisture in materials

Once we have ensured our building is airtight, and have therefore eliminated moisture transport into the fabric via the internal air, we then need to consider how moisture might enter our construction directly, through the materials we choose. The way in which moisture will affect and interact with a material is dependent on three different properties: capillarity, hygroscopicity and vapour permeability.

Capillarity relates to the uptake, release and accumulation of liquid water. Hygroscopicity relates to the uptake, release and accumulation of water vapour. Vapour permeability relates to the rate at which water vapour will be transported through a material, or its resistance to water vapour passing through it. These three properties of the materials you choose will affect whether moisture (both vapour and liquid) enters your walls, roofs and floors, how much will enter, whether it is then transported through the structure (and in which direction and at what rate!) or whether it is likely to accumulate at a certain location.


When water hits a building as rain, some bounces off, some will run down the face of the building and some may seep into the fabric. Liquid moisture can be relatively quickly transported some distance into the fabric, especially if the wall is constructed from a dry, porous material such as brick. This ‘wicking’ action is referred to as capillary action. Some materials have higher capillarity than others: gypsum plaster is a good example, as it is highly porous with myriad small pores and therefore has a very large internal surface area. Materials such steel or glass are not porous and have an effective zero capillarity. Driving rain is the main potential source of liquid water within any building and is therefore a very important aspect of water management to consider, especially when adopting less familiar building assemblies and if building on exposed sites.


Moisture can also be taken up into materials hygroscopically, as vapour from the air. All materials have some level of hygroscopicity. Some will absorb vapour far more readily than others, and in the building industry the term ‘hygroscopic’ is generally used to denote a material that has either a high equilibrium moisture content (EMC), i.e. it can hold a lot of water vapour, or a fast rate of vapour absorption. (ECM refers to the moisture content of a material for a given temperature and humidity. When the ECM is reached, the material will no longer gain or lose moisture unless the ambient conditions change.) Hygroscopicity is generally considerably higher in purely organic materials than in purely inorganic building materials. The capacity of a porous material to carry liquid water through capillary action will normally far exceed any moisture taken up hygroscopically. This is certainly true of bricks or timber, for example.

A highly hygroscopic substance that can thereby be used to preserve a state of dryness in its local vicinity is called a desiccant. Rice, for example, is a fairly well-known desiccant and can be added to salt shakers to prevent the salt from becoming damp. Modern window spacers (which separate the individual panes of glass) will sometimes contain a desiccant to inhibit condensation between the panes.

Hygroscopic materials will naturally exchange moisture with the indoor air, if exposed to it, effectively flattening out ups and downs in RH levels – this can be a good reason to use certain organic materials (e.g. clay plasters). In an ultra-low-energy setting, anything that helps to avoid too-high or too-low RH levels is worth consideration. The key to such an effect is the rate at which the material will absorb and de-absorb (release) moisture. The capacity of a material for absorption of moisture will clearly also be related to its mass and to the exposed surface area available for moisture take-up. By intelligently selecting your construction assemblies, then, there is the potential to modulate RH and increase comfort levels.

It is worth noting that the cyclical wetting and drying of materials can have a detrimental impact, including encouraging general movement (expansion and shrinkage), especially in timber. Some materials’ performance characteristics will be changed as they become moist – insulating performance especially can be compromised. Some natural insulation materials will retain thermal performance when wet, and this might be a very useful quality in some situations (sheep’s wool is probably the best example of this).

Vapour permeability

All materials have a level of vapour permeability. In other words, some vapour will eventually diffuse through any material – whether natural, such as timber, or man-made, such as polythene. Given enough time and/or pressure differential, vapour will even diffuse through metal. However, this is unlikely to occur to any noteworthy degree in the course of a building’s lifetime. Materials with poor vapour permeability are therefore described as ‘vapour barriers’ or, in the United States, ‘vapour retarders’.

In the UK, materials that have a high vapour permeability are often described as ‘vapour permeable’, although this term can refer to a range of materials with differing vapour permeability levels. You may want to check what the actual vapour permeability is for a specific product. In the USA, materials can be defined as impermeable, semi-impermeable, semi-permeable and permeable, which is perhaps a more helpful means of distinction and provides a quick way to compare one material with another. Vapour permeability there is measured in grains of water vapour per hour per square foot per inch of mercury – known as US perms! (See Table 10.1 below.)

In the UK and Europe, four different units are used to measure the degree of vapour permeability of a material. To meaningfully compare materials, make sure that you have comparable units of measurement and that these relate to the thickness of material you are using. These differing and scientifically dense measures of vapour permeability may initially seem a little confusing, but you can convert between the four different measurements quite easily (see opposite) and then, simply, the lower the figure, the more permeable the material. The most important thing is to be able to compare the vapour permeability levels of the materials across your building assembly, which is not overly difficult to do.

The degree of vapour permeability in a material can be expressed in terms of the following. These values are summarised in Table 10.2 opposite.

•  Vapour resistivity (r-value) – expresses vapour permeability relative to a standard metre depth of the material. The unit of measure is MNs/gm – meganewton seconds per gram metre.

•  Vapour resistance (G-value) – effectively the same measure as the r-value but for a specific material thickness (MNs/g – meganewton seconds per gram). When assessing a material’s vapour permeability within a structure, be sure to refer to its G-value – for example, emulsion paint might have a high r-value, but since you apply it in a layer only a few microns thick, its G-value will be relatively low. (Note that this unit is not to be confused with the ‘g-value’ that relates to window glazing – see Chapters 7 and 11.)

•  Water vapour resistance factor (µ-value, pronounced ‘mu-value’) – a measure of a material’s relative reluctance to let through water vapour in comparison with air. You would need to multiply this number by the material’s thickness if you wanted to compare materials in your construction. As it is a relative measurement, it does not have any units. This value is commonly used in Europe.

•  Equivalent air layer thickness (Sd-value) – expresses a material’s vapour permeability as though it were a thickness of still air in metres. It is therefore related to a specific thickness of material.

Table 10.1 US classifications of vapour permeability (‘perms’)

Table 10.2 UK and European vapour permeability values

Unfortunately, in the UK (at the time of writing), obtaining any of these values from manufacturers and suppliers can be difficult, partly because they are often not published along with the general technical properties. You can look up BS EN 12524, which includes the hygrothermal properties (those relating to moisture and heat) for many common materials. Since it is relatively easy to convert between the r-value, G-value, µ-value and Sd-value, figures can be compared even if you are only able to obtain, say, the r-value from one manufacturer and the Sd-value from another, as follows. (Unfortunately, perm ratings are not so easily interchangeable with the UK/European measures.)

•  To convert r-value to G-value: multiply by thickness of material (in metres).

•  To convert G-value to r-value: divide by thickness of material (in metres).

•  To convert r-value to µ-value: multiply by 0.2gm/MNs (gram metres per meganewton seconds) – the UK value for the vapour permeability of still air.

•  To convert µ-value to r-value: divide by 0.2gm/MNs.

•  To convert µ-value to Sd-value: multiply by thickness (in metres).

•  To convert Sd-value to µ-value: divide by thickness (in metres).

Table 10.3 overleaf gives the typical r-value and G-value for some commonly used construction materials, with the values for still air for comparison.

The vapour resistance of your materials is important in terms of how a particular wall or roof construction is built up, since it will determine how effectively any moisture can move through it (and out of it) and also in which direction it may travel. The potential for vapour diffusion across a construction will be related to the vapour pressure differential or gradient across it. As noted earlier in this chapter, for most of the year in a climate such as that of the UK, the vapour pressure outside (cool and dry) will normally be much lower than the vapour pressure inside (warm and moist), and if your construction is ‘breathable’, vapour diffusion will act to move moisture from the inside to the outside. The diffusion will occur evenly over the whole surface area of the construction, and the speed of diffusion will depend on how vapour-permeable the materials are. Generally, it is good practice to combine materials with similar moisture-managing characteristics. This is especially true for timber-frame construction.

It is useful to note that some materials can be quite vapour-permeable but not particularly hygroscopic. Mineral fibre insulation (e.g. stone wool) is vapour-open but not particularly hygroscopic and has limited capillarity – it will resist water take-up and will dry out relatively quickly if wet. Natural fibre insulations have some capillarity, high hygroscopicity and high vapour permeability. They will take up water easily – initially in vapour form, so will perform as if ‘dry’. Once ‘actually’ wet with liquid water, they will dry out at a similar rate to mineral fibre.

Table 10.3 Vapour resistivity and resistance values*

* Typical values only – variations occur.

High-performance ‘closed-cell’ insulations are not vapour-permeable or hygroscopic and do not have any capillarity. They will block moisture ingress but equally will block moisture egress. All three mechanisms (capillarity, hygroscopicity and vapour permeability) determine the potential moisture risks for any particular construction type.

Moisture management in construction

When considering a low-energy build, the two main sources of moisture within your construction will be via rainwater penetration externally and from moisture transported by the internal air. We do not cover the third source, groundwater, here, as this will be managed in a similar manner to conventional constructions – with a capillary break (see below and right).

Protecting your building from rain and other forms of precipitation is part of normal good building practice. The level of protection is of course affected by climate (in more aggressive climates, larger roof overhangs become more common), but also by the type of construction and its ability to manage moisture. Certain structures (e.g. brick or stone) will absorb liquid moisture and then release it in due course, and are referred to as a ‘reservoir cladding’. Such structures will therefore experience less water run-off. A reservoir cladding will be capillary-open (i.e. subject to capillary action) and usually decoupled from the structural and thermal elements of the building, therefore this moisture should not adversely affect building performance. Alternatively, assemblies can be designed to protect the structure from any liquid moisture penetrating the surface (e.g. using cement render), called a ‘drainage plain’. Drainage plains are either capillary-closed or have limited capillarity – they are effectively waterproof – and are therefore sometimes referred to as a capillary break. Since a drainage plain does not allow water to enter the structure, it will have increased water run-off. Reservoir cladding and drainage plains are the two main means of dealing with rainwater ingress in walls and roofs; both are designed to prevent rainwater reaching the structural and thermal elements of the building.

Your low-energy build should be essentially airtight, so moisture carried by the internal air should only be entering your construction materials via the three material transport mechanisms we have already outlined. Rainwater (and groundwater) transport moisture in greater volumes than any source of vapour moisture. However, vapour moisture transport can still create serious problems in a low-energy build due to the increased level of insulation, especially as the potential for interstitial condensation often increases (with multiple layers within the assembly) and the opportunities for moisture to escape or dry out can be reduced.

There will also be an element of inbuilt moisture in new constructions, resulting from wet trades and exposure to the elements during construction. Accidental or unforeseen moisture ingress, such as from leaky plumbing, can also be a significant source of moisture damage; even a 1mm gap in an airtight layer can allow significant volumes of water into the structure. Preventative measures do not usually involve matters relating to the fabric design and are therefore not covered in this book. However, ensuring that moisture can escape to the outside will be helpful. Even after a repair has taken place, saturated materials, if not replaced, may need time to dry out.

The challenge as regards low-energy design is that there are currently no standard ‘robust details’ available and no standard approaches, especially for retrofit projects. Making up unique details carries the risk of omitting to take ‘something’ into consideration – and at present we would suggest that moisture is perhaps the most likely ‘something’ to drop out of the low-energy equation. How your wall, roof or floor is managing moisture is something you need to think through carefully.

Breathability (or vapour permeability)

We refer to ‘breathability’ here because this is the term most commonly used, although we are essentially discussing the potential for vapour moisture movement.

In the first instance you will have the option to choose between using breathable and non-breathable materials for your assemblies. A breathable fabric will let more moisture into your assembly but will also let more moisture out, so will theoretically reduce the risk of interstitial moisture accumulation. Conversely, non-breathable fabrics will allow much less moisture in but, should moisture build-up occur, drying will take much longer or not occur at all.

Whichever your choice, it is advisable to maintain a level of moisture (and thermal) compatibility throughout – that is, use materials with relatively similar levels of capillarity, hygroscopicity and vapour permeability. An abrupt change in these properties can cause uneven distribution of moisture across an assembly, and may allow it to accumulate in some regions. If this accumulation of moisture occurs in a material with a high moisture tolerance (i.e. moisture does not significantly affect its performance or cause degradation), then it is of limited concern – assuming it will not accumulate to saturation point. If, however, it collects in an area containing moisture-sensitive materials, particularly if they comprise structural elements, you may have a serious problem.

Breathable assemblies

The rule of thumb to remember when designing a breathable assembly is the ratio 1:5. The vapour resistance of the material on the outside should ideally be five times less than that of the material on the inside. This excludes any service voids or rain screens. The moisture-driving potential is then set up to move from the inside to the outside of the assembly. From the values shown in Table 10.3 (page 154), then, we might choose to use woodfibre board (very vapour-permeable) towards the outside of a timber-frame wall and OSB on the interior side – this would deliver a sensible vapour resistance ratio. In this example, we would also be using the OSB as our airtightness layer and taping at all the board joints (see Chapter 9). While we are using a material that will allow some vapour movement, we do not want to allow any moisture carried by the interior air to travel into the assembly. The airtightness layer ensures this, and this is one reason it is placed on the interior side.

As already stated, ensuring that the airtightness layer is contiguous is important for both thermal performance and moisture management. How much moisture the air might carry into your structure through a non-contiguous airtight layer can only be roughly estimated. The photo opposite of a retrofit assembly shows moisture in a floor condensing against an internal brick wall that is colder than the surrounding structure. The moisture source is almost certainly from a combination of moisture held in the Leca® (clay-blown insulation), likely due to poor site storage prior to installation, and vapour moisture from the internal air – the airtight barrier was not yet taped to the wall. The problem should resolve itself, since both moisture sources are temporary, but it illustrates the need for materials to be dry (protected from rain, etc., during construction and stored appropriately). Check that materials are adequately dry before final finishing layers are applied. This is particularly relevant if using a product such as hempcrete (hemp plant mixed with a lime-based binder). Materials that are applied wet and then need to dry out over time must achieve appropriate RH levels before applying finishes that will inhibit or slow down further drying. The initial airtightness test (see Chapter 9) will identify any breaks in the airtight layer.

Non-breathable assemblies

While there are numerous benefits to designing your building to be breathable (mainly allowing water moisture to escape out of your assembly), for reasons of cost, time, space or planning, this may not be practicable. Indeed, the majority of high-performance insulation materials – such as extruded and expanded polystyrene or polyurethane and polyisocyanurate foam – have limited or (in the case of closed-cell insulations) no moisture-managing capabilities. If highly vapour-impermeable insulation is installed on the outside of a masonry wall, moisture contained in the masonry will take a long time to dry out (which means it’s important to make sure the masonry is as dry as possible before applying it). However, bricks are relatively robust and will not deteriorate due to the effects of moisture in the same way that many other materials might. Greater risks occur if choosing to adopt a non-breathable material alongside more ‘organic’ materials such as timber. If choosing to use a vapour-impermeable membrane as your airtightness layer, this would need to be installed without error – any holes or rips could allow moisture in and the structure would then potentially struggle to dry out. Vapourimpermeable airtightness layers with timber-frame assemblies are commonly used in low-energy houses in cold European climates, although the level of experience in both timber-frame and using such membranes is much higher in those regions than in the UK. Unless you are very confident in your ability to detail such an assembly, in our view it is best avoided.

Interstitial condensation forming at the junction of the floor to wall assembly in a retrofit. Damp ness is visible at the base of the wall (below the plaster, where the skirting has been removed) and between the timber floor joists. The voids between the floor joists have been filled with a clay insulation (Leca®).

An example of a vapour-impermeable low-energy construction would be a rendered concrete structural wall that is externally insulated with a high-performance, vapour-closed insulating material (say polystyrene or phenolic foam). The materials’ moisture-management characteristics are then well matched. Any risk of moisture entering the wall assembly would then be through cracking in the external render or poor detailing around openings, although with such materials (unlike with timber constructions) there is likely to be little movement to exacerbate such cracking. If the insulation were to experience prolonged exposure to moisture, there would be some material deterioration and thermal performance would be compromised, but if the rendered surface is maintained, the risk is small. Ideally you would detail around openings so that if water entered the assembly it could drain back to the outside. What this construction would not do is modulate the relative humidity (RH) of the interior air (effectively it has zero hygroscopicity). However, clay plasters could possibly be applied to some internal walls if some hygroscopicity was desired.

Modelling moisture levels

There are now useful tools for measuring hygrothermal (heat and moisture) transfer – in particular, for low-energy designs, dynamic numerical computer modelling (to BS EN 15026). Tools such as WUFI or Delphin can prove invaluable if you have an assembly where you are not completely confident that moisture will be managed safely. If you are unsure about a particular build-up of materials (especially in a retrofit), then such a modelling exercise may well be an excellent investment. Once a model is set up, you can alter materials within the assembly and, with such minor adjustments to your approach, you may well avoid what could have been an expensive mistake.

For measuring internal RH levels, you can purchase a hygrometer for under £10. At this early stage of adoption of ultra-low-energy design in the UK, gathering such data for wider application is very useful.

Example constructions

In this section we look at potential moisture issues that relate to some examples of ultra-lowenergy construction. This cannot be an exhaustive list, but it does illustrate the principles already discussed, and should be helpful when applying the principles to alternative assemblies.

New build: cavity-wall construction fully filled with insulation

A cavity wall is a common example of a reservoir system. The outer leaf, often masonry, will soak up rainwater, but it is free to dry out on both sides. The sun and wind will help drive moisture through and out of the brick. The cavity acts as a separating layer and allows back-drying of the external leaf. In case moisture should penetrate the entire depth and gain access to the cavity, weep holes are built in to the brickwork of the outer leaf and waterproof cavity trays encourage moisture to escape to the outside.

Once you fully fill a cavity construction, you lose this back-drying capacity. Experience gained from fully filling traditional cavity walls suggests that using vapour-impermeable insulations on a site that is very exposed (say, a coastal location) can cause moisture problems, with water building up within the construction and deteriorated performance of the insulation. Some councils have stopped fully filling cavities for this reason.

To achieve ultra-low energy levels in a new build, you will be looking at cavity insulation of around 300mm. In order to ensure that your outer leaf is effectively managing water, it might be worth considering a rendered finish (a lime render on brick would reduce water absorption while still allowing some drying out), or you could protect the wall with an alternative drainage plain – tile hanging being another possibility. Deeper roof overhangs might be another helpful strategy (and will also provide summer shading).

The Certified Passivhaus building at Denby Dale in Yorkshire (pictured opposite) comprises stone external leaf, which is less capillary-active than a soft brick and allows use of a harder mortar joint – again limiting moisture ingress. The house also has a significant roof overhang.

If you want to retain a brick finish, perhaps select a less porous brick and then always ensure you maintain your mortar joints (the problem with this is that maintenance is not often our favourite activity!). You may also need to consider treating the external face of the bricks with a vapour-permeable capillary block (a transparent liquid coating that acts as a water repellent).

To get the required level of thermal performance, you may well need to use a mineral insulation in the cavity. This would be vapour-open but not hygroscopic, so if moisture reaches it, the water will drop under gravity. You must then ensure that any moisture at the wall’s base is encouraged to drain to the outside. If the insulation did get moist periodically, it would affect thermal performance to some degree. If in a very exposed location, this may not be the wisest construction assembly.

The internal leaf could be a silicate block, which has good hygroscopicity and would help to modulate internal RH levels.

New build: timber-frame construction

In timber constructions, a rain screen is commonly used to protect the main wall assembly from the elements. The rain screen is often in timber and sits on battens (plus counter battens) in front of the main thermal structure. The screen protects the wall from most rainwater and can dry out from both front and rear (the rear must be a ventilated void). Such a rain screen is probably best described as a reservoir cladding system, since it absorbs and de-absorbs moisture cyclically.

Denby Dale, Yorkshire: a masonry cavity-wall build constructed to Passivhaus standard.
Image: Morgan O’Driscoll Photography

If not using a rain screen, then your wall may be rendered or tiled (tiling is a form of rain screening). These renders can be watertight (a drainage plain) but still vapour-open, allowing some drying to the outside.

If using a timber-frame construction, choose insulations and finishes of similar character. Combining high-performance, diffusion-tight (vapour-closed) insulations with timber is not advisable as it could trap moisture and stop the assembly drying out, leaving the timber frame vulnerable to rot and mould. Disasters have occurred where rigid phenolic insulations have been externally applied to timber-frame constructions. This is a classic example of materials with different characteristics being paired together.

Apply the 1:5 rule (see page 156) when using vapour-permeable materials. In the summer months you can encourage some drying to the interior by using an ‘intelligent’ (humidity-variable), diffusion-open membrane as your airtight layer. This will generally become more vapour-open during the summer and add an extra potential drying route to the inside (see Figure 10.3 below). It is always essential that the airtightness layer is contiguous.

On both timber-frame walls and roofs, it may be that materials providing the usual racking strength (stiffening the timber frame) are in non-ideal locations for vapour permeability. It may be preferable for such materials to move to the inside of the frame, where they won’t inhibit drying to the outside and where higher vapour-permeability levels can be tolerated. OSB could provide this racking function as well as forming the airtightness layer, for example.

Figure 10.3 Section through a vapour-open timber-wall construction.

Retrofit: internally applied insulation with solid masonry wall

Unfortunately, this type of construction often introduces a number of points of moisture risk. If you are to insulate internally, be very careful when considering your moisture-managing strategy.

Existing solid-wall buildings – such as double-skin brick or stone-and-backfill – rely on the thickness of the wall and its high level of capillarity to manage moisture. Water will generally be driven into the wall only to a certain depth before a drying cycle will begin; in other words, there will be cyclical wetting and drying. Moisture will often dry out via the mortar joints and these will then deteriorate over time but can be repointed periodically. Repointing with cement mortars inhibits this process and can cause damage to soft bricks or stones, which end up retaining too much moisture and will break under cyclical freeze–thaw conditions (often the faces will ‘pop’ off). The first point to consider before internally insulating is therefore the condition of the external wall – is it likely to require some remedial work to make sure it is in a good state of repair?

Insulating internally will isolate the existing wall from the interior, including the heat that provides the driving force for drying in winter. If left cold and capillary-open, repeated rainfall will lead to greater accumulation of moisture, particularly if a non-breathable insulation is used. This can potentially lead to saturation of the existing structure, and if or when the temperature drops below freezing, this water will freeze, expand and widen cracks in your walls, leaving them more exposed to future moisture ingress. In dry summer conditions, south-facing elevations may become too dry, as they are no longer able to draw on moisture from the internal environment.

The Passivhaus Institut advises treating the external surface of masonry walls with a vapour-permeable capillary block when internally insulating. This will prevent rainwater ingress while still allowing drying out. However, studies have shown that the level of treatment required (recoating frequency) can exceed that recommended by the product manufacturers. Any such application will require regular maintenance to ensure it remains waterproof over time (recoated, say, every two to five years). It is also vital that this hygrophobic (water-repellent) treatment penetrates the brick or stone adequately to prevent localised moisture penetration at mortar joints. Before applying such a finish, you would also need to ensure that the quality of the existing wall is good – perhaps locally repointing mortar joints wherever they are eroded.

Rendering brick and stone walls (say, with a soft lime render) is one effective means of providing some weather protection from driving rain while still allowing the wall to dry out. Lime renders and washes are often referred to as ‘sacrificial’; they weather-protect the stone or brick but wear out over time and are then reapplied – a very sensible and robust moisture-managing strategy. Lime mortars have some capillarity, so small amounts of moisture might still be drawn into the wall but will also be able to dry outwards.

In the Passivhaus-certified retrofit in Princedale Road, West London (pictured on page 77), the internal insulating wall was separated from the external solid masonry, creating a hybrid ‘cavity’ construction (see photo overleaf and Figure 10.4 on page 163). This approach treats the external solid wall as a reservoir cladding, but does mean that the brick wall is isolated from the warmth of the interior (which would normally help to drive moisture to the exterior). By introducing pathways for the moisture to escape from the back of the wall to the outside (using weep holes through the external brick skin), suitable conditions were created for some back-drying. We understand that the external brickwork was also treated with a capillary block. The material used for the framing out (the basic structure) of the cavity might be susceptible to periodic wetting and therefore needs to be selected appropriately – not timber. The internal insulation in this case was high-performance and vapour-impermeable, so there would be no drying to the interior or transfer of moisture through the materials from the interior air – as long as the airtightness barrier was not breached. Natural insulations such as sheep’s wool or woodfibre are less thermally efficient than insulations such as phenolic foam boards, and therefore require additional depth for full Passivhaus Certification, resulting in excessive loss of floor area – which is why they were not used in this project. Even with high-performance insulation there is some floor-area loss; in the Princedale Road house this was partly compensated by the removal of chimney breasts – a moisture risk if retained. You would not want to ventilate these for energy reasons, and, if not ventilated, moisture build-up is inevitable. In this type of retrofit solution the depth of the internal insulation will change the overall proportion of the rooms, which may alter the aesthetics dramatically.

One reason why internally insulating a building to Passivhaus levels can be a great moisture-management challenge is that any thermal bridges will concentrate moisture condensation into localised areas. Making sure these bridges are addressed is then critical (see Chapter 8) and will be quite invasive. Avoiding penetration of the timber floor joists through your internal insulation is therefore important: possible details are shown in the photographs on page 139.

If retrofitting an existing building and your only option is internal insulation, you need to consider carefully the level to which you can sensibly improve the building. For ultra-low energy you need to be willing to take on a radical retrofit project, aiming at giving your building its next 60-100 years of life, not just a 10-year refurbishment.

Model of internal wall insulation at Princedale Road. From outside to inside: existing brick, metal frame to new cavity, insulation, OSB taped (the airtight layer), insulation (services zone), plasterboard. Image: Paul Davis + Partners for Octavia Housing

Figure 10.4 Retrofit section showing airtightness layer and internal insulation, following the principles applied at Princedale Road.

Retrofit: externally applied insulation

In terms of managing moisture, improving the thermal performance of existing buildings is best done externally. An external insulation system that will protect the existing structure from rainwater penetration can be easily designed. The existing structure then sits on the warm side of the construction and will be at much-reduced risk of water vapour condensing on to any of its surfaces. Thus the risk of condensation on the ends of timber joists penetrating the walls is virtually eliminated.

These types of insulation are generally referred to as either external thermal insulation composite systems (ETICS) or external insulation systems (EIS). The insulation is applied to the outside of the masonry construction, whether solid masonry or cavity wall (pictured on page 138), and finished with a render that protects it from rain penetration, acting as a drainage plain. Figure 10.5 below shows a section through such a wall. If externally insulating a cavity construction, you should also fully fill the cavity to avoid thermal bypass. Try to ensure it is fully filled without air gaps – expanded polystyrene-graphite-coated beads (InstaBead) are good in this respect, as they flow well.

External insulation will normally mean having to adjust your roof and wall junction to suit the new wall depth, unless there are already deep roof overhangs.

Fixings or small cracks in the finishes will introduce potential weak points through which driven rainwater might enter. Any moisture drawn in at these points by a capillary-active material behind may struggle to get out again if the drainage plain is vapour-closed. Similarly, if a vapour-impermeable external final finish (some paints) is applied, this may also inhibit drying. You should generally assume that some moisture will get in behind the rendered surface; thus a level of vapour permeability in the drainage plain is advisable. (As we have seen, a material can be watertight but still retain some vapour permeability.) Generally, thin render systems are used, either polymer cement or silicone- or mineral-based renders, all of which allow for some transmission of water vapour. When the sun warms such a rendered wall, some drying out can then take place.

Figure 10.5 Section through an externally insulated cavity-wall construction at the Totnes Passivhaus.

ETICS generally use high-performance insulations such as rigid phenolic foam boards, which have no capillarity, hygroscopicity or vapour permeability. When this type of system is applied to a rendered masonry wall, the possibility of any serious moisture damage is limited. There are no ‘organic’ materials to rot and the insulation is moisture-resistant. An appropriately specified cement-based render, which can still provide some vapour permeability, is therefore suitable for use with such insulations. The key is to maintain the render surface and to detail well around windows and doors to avoid undue water ingress – wind can blow rain some distance up cracks and gaps. The detrimental effect of water lying temporarily against such insulations will be minimal. Far more problematic in this scenario would be if air could enter and then circulate behind the insulation, compromising its performance (see Chapters 8 and 9).

Alternatively, a more ‘natural’ ETICS solution would be to use woodfibre boards as the insulation material and then protect this with a lime render, which has a much higher permeability than cement render – again, joining materials with similar characteristics. Any moisture that gains access will get absorbed by the woodfibre boards, which are relatively hygroscopic. With a good vapour-permeable lime-based render (which may have some capillarity as well), the moisture should be able to dry out cyclically. Lime renders also have the ability to self-heal – sealing up small cracks over time. Obviously, the ideal is to avoid any moisture penetration, but this approach allows for the moisture to be managed, limiting any potential negative impacts. It is important to note that the characteristics of lime plasters and renders can vary enormously, so make sure the recommended lime mix matches the insulation product being used.

A woodfibre board will perform less well thermally than a phenolic foam insulation, which means thicker walls and greater loading on the existing structure. Another approach, then, might be to create an external timber frame with a thin woodfibre board cover, which could then be filled with a lighter natural insulation, such as cellulose. This would reduce the loading on the existing structure. You would need to match the materials in the assembly according to their moisture-management characteristics. The problem with external insulation is that it will not always be an acceptable solution aesthetically, especially in conservation areas, so upgrading a property may be possible only by insulating internally.

Flat roofs, including living roofs

It is worth commenting on flat roofs, which require vapour-impermeable finishes as your drainage plain (asphalt, felts, etc.). If you are using breathable materials, such as timber, you need to maintain vapour permeability right to the outside. The way to approach this is to use the 1:5 principle (see page 156) and introduce a ventilated void between the watertight layer and the top of the main insulated roof assembly. This means that the watertight layer works in a similar way to a rain screen on a vertical wall. If the roof is of any great size (or you have roof lights penetrating the void), you need to be confident that air is able to freely ventilate the entire void area. Blocking drying to the outside is not a clever idea!

Figure 10.6 Section through living roof ‘vapour-permeable’ construction above main timber roof utilising a ventilated void at Totnes Passivhaus. The photograph on the right shows how it was executed on-site.

For timber flat roof constructions (see Figure 10.6 opposite), an intelligent membrane on the inside of the assembly as the airtightness layer might be of benefit. The flat roof needs to be covered with a dark-coloured material (such as asphalt), which will then absorb heat from the summer sun – dark surfaces will absorb significantly more heat energy than light ones. This warm external structure should then drive any moisture trapped within the assembly towards the inside, allowing potential drying to the interior. Moisture will always be driven away from warmer areas towards colder areas. However, this does not obviate the need for this type of assembly to have a ventilated void.

Boards providing racking strength (stiffening the timber frame) and with high vapour-permeability levels should, if feasible, be located towards the inside of the roof assembly so as not to unduly inhibit drying to the outside. With a timber-assembly living roof where there is a ventilated void, the board above the insulation could be eliminated altogether and replaced solely with a vapour-permeable membrane.

Concrete flat roofs

Where the roof assembly uses non-breathable materials, e.g. in the case of a concrete roof deck, it is possible to insulate externally using a product such as Foamglas® or other similar closed-cell-structure and zero-vapour-permeability insulations. This keeps similar-acting materials together and there is no need for a ventilated void.


The consideration of moisture management within and through highly insulated assemblies is an important aspect of achieving healthy and durable buildings. It is the potentially detrimental effect of moisture on indoor air quality (in relation to relative humidity and mould), as well as the risk of material degradation, that make this so vital to an ultra-low-energy build. The integrated approach to moisture management and low energy performance is an illustration of how the principles of Passivhaus need to be adopted as a whole.

Moisture is transported in both liquid and vapour forms, and potential water ingress into the structure needs attention both from the external and internal climates. Understanding the mechanisms of moisture transport (capillarity, hygroscopicity and vapour permeability) is key, and both the airtight layer (to the inside of the assembly) and the wind-tight layer (to the outside of the assembly), play important roles in this regard. Encouraging water vapour to move towards the external side of your construction is best practice, as is combining materials with similar characteristics.

In retrofit situations, the management of moisture is even more critical, especially if insulating internally. If carrying out an unusual detail where there may be a risk of condensation (especially if within the assembly itself, i.e. interstitial condensation), then modelling constructions in dynamic computer programmes such as WUFI makes good sense.