Glossary of terms*

Terms listed in this glossary are indicated in tinted bold the first time they appear within the text of the book. Tinted bold text in this glossary refers to terms elsewhere in this glossary or to units in the glossary of units.

AECB The Sustainable Building Association, formerly known as the Association for Environment Conscious Building. See Resources.

AECB Silver Standard / AECB Gold Standard Voluntary sustainability standards for buildings developed by the AECB. AECB Gold Standard is a Passivhaus-plus energy standard. See

Air changes per hour (ach) See Air permeability and air changes per hour (ach), below.

Air permeability and air changes per hour (ach) Air permeability is the measure (in m³/hr/m²) most commonly used in the UK to measure airtightness. Passivhaus uses a different measure: air changes per hour. Both conventions are based on a pressure difference of 50Pa (pascals) above and below ambient atmospheric pressure, i.e. under both pressurisation and depressurisation.

The ventilation volume, Vn50, is the measure of total internal air volume, including volume that is above areas excluded from the treated floor area (TFA). For example, it includes space under staircases, loft space, and spaces used for the MVHR unit and hot water store. As such, it needs to be calculated independently of the TFA and should not be confused with Vv, which is simply the ceiling height (assumed by default in the Passivhaus Planning Package [PHPP] to be 2.5m) multiplied by the TFA.

Airtightness The degree of leakage of air through the thermal envelope. (See air permeability and air changes per hour, above). Airtightness is a property of a building.

Annual [specific] primary energy demand The measurement of consumption of primary energy – the energy consumed at source (e.g. a natural gas well) – by a building to meet all its energy needs. This includes all the energy used by every fixed or portable appliance or device within the building. The Passivhaus limit is 120kWh/m².a (kilowatt hours [of primary energy] per square metre [of usable floor area or treated floor area (TFA)] per annum. (‘Specific’ refers to the per-area measurement.) In the Passivhaus Planning Package (PHPP), a ratio, the ‘primary energy factor’, is used to convert primary energy to delivered energy (energy delivered to the building). For mains electricity this ratio is currently 2.6:1.

Annual [specific] space heat (or cooling) demand The expression used to measure the energy consumed by a building to provide heating to 20°C (or, in hot climates, cooling) of the internal space within the thermal envelope. It is measured in kWh/m².a (kilowatt hours per square metre [of treated floor area] per annum). (‘Specific’ refers to the per-area measurement.)

ASHRAE American Society of Heating, Refrigerating and Air-conditioning Engineers. See Resources.

Blown-in A mechanism for ‘pumping’ in loose insulation through flexible pipes to fully fill a cavity.

BRE Formerly known as the Building Research Establishment, but now simply as BRE. See Resources.

Breathable The ability of a material or building assembly to allow vapour through it by diffusion. In practical terms this refers to water vapour, since this is the constituent of air in a building that varies in concentration to any significant degree. A breathable (or breather) membrane is both airtight (it acts as a barrier to air as a whole) and liquid-moisture-tight but vapour-open, so will allow water vapour – moisture – through.

BREEAM Building Research Establishment Environmental Assessment Method. A widely recognised environmental assessment method and ratings system mainly used for non-domestic buildings (approx. 200,000 buildings assessed to date). It uses a straightforward scoring system, BREEAM ‘Outstanding’ being the highest award. There was a domestic new-build equivalent termed ‘EcoHomes’ launched in 2000, but this expired in April 2012, being replaced by the Code for Sustainable Homes (CSH) for new housing.

Building assembly A structural part of a building (walls, floor or roof) made up of a number of building elements. Referred to as ‘Building Element Assembly’ in the U-values worksheet of the Passivhaus Planning Package (PHPP).

Building element A single material or object comprising part of the structure of a building, i.e. part of a wall, floor or roof. An ‘opaque’ building element refers to any building element except windows and doors.

Building Regulations In the UK, these currently (as of 2010) apply in England and Wales and set standards for building construction including energy efficiency. There are 14 technical parts to the regulations (Part A to Part P). Part L relates to the conservation of fuel and power, and Part L1A relates to new dwellings. The edition of Building Regulations previous to 2010 was in 2006, and the next planned update is in 2013. The power to set Building Regulations has now been devolved to Wales, so the 2013 regulations are likely to apply only to England. For copies of Building Regulations see

Capillarity (capillary action) Movement of liquid water against gravity, through small pores or capillaries in a material.

Certified Passivhaus Designer An individual who has trained (taken a Passivhaus-Institut [PHI]-recognised Certified European Passive House [CEPH] course and PHI CEPH examination) and qualified in the principles and methodology needed to design a Passivhaus. It is also possible to gain Certified Passivhaus Designer status by designing and building a Certified Passivhaus building, although this is generally seen as a harder and potentially riskier way to become a Certified Designer. See

Chi-value (χ) Similar to psi-value (ψ), this measures the rate at which heat passes through a material that penetrates another material at a point, where the penetrating material conducts heat better than the surrounding material: for example, a metal bolt, used to mount a balcony, that passes through an external wall. In a Passivhaus chi-value is used to measure heat loss in a point thermal bridge. It is measured in W/K (watts per kelvin).

Code for Sustainable Homes (CSH) The UK’s national standard for the sustainable design and construction of new homes (residential buildings). Dwellings can be rated from Level 3 to Level 6 (L3 to L6), L6 being the most stringent and originally termed ‘zero carbon’. Zero carbon now applies to both L6 and L5, and is currently set to come into force as a statutory requirement in 2016. L3 and L4 are seen as a stepping stone between current UK Building Regulation requirements and CSH L5 and L6. L4 is planned to come into force in 2013.

Compact unit A single unit, about the same size and footprint as a large fridge-freezer, that combines mechanical ventilation with heat recovery (MVHR), hot water production/storage and the provision of small amounts of heat, sometimes via supply duct heating (see supply duct radiator).

Cooling load Analogous to [specific] heat load, this is the power needed to keep a building cool in a hot climate. Whereas [specific] heat load has been defined precisely, in terms of the power per square metre and the temperature difference being maintained (+20°C inside, -10°C outside), no similar quantified definition of cooling load has yet been finalised by the Passivhaus Institut.

Desiccant A hygroscopic substance, which is used to remove excessive humidity and thus avoid condensation in spaces such as between the panes of a double- or triple-glazed window.

Diffusion The thermal motion – movement driven by temperature – of all liquid and gas particles. The speed of the motion depends on the temperature and the particle size. Diffusion explains how particles move from a place of higher concentration to one of lower concentration (across a vapour pressure differential or gradient), but it also occurs, more slowly, where there is no pressure differential.

Ducts The pipes that run between the building’s thermal envelope and the MVHR unit and between the MVHR unit and the various supply and extract points within the building. The intake duct (sometimes referred to as the ‘ambient’ duct) takes fresh air from outside into the MVHR. The supply duct takes that air (now containing the heat recovered by the MVHR unit) to supply vents in the living room and bedrooms. The extract duct takes old air from the bathroom(s) and kitchen back to the MVHR unit. The exhaust duct takes the now-cold air back outside. See Chapter 12.

Embodied energy The energy used in the sourcing, manufacture and transport of a material. The way embodied energy is measured varies according to what you choose to include in the calculation – for example, some calculate energy only up to the point the product leaves the factory gate (cradle-to-gate). Measured in MJ/kg (megajoules per kilogram).

Energy balance A term to express the simple idea that energy must be input into a building at the same rate that energy is lost from it. Energy losses in buildings take place because heat is conducted out through the building fabric (walls, roofs, windows, etc.) and convected out via gaps in the building fabric, or where it is not recovered via any ventilation system. In order to maintain a temperature difference, enough heat needs to be provided (gained) to compensate for (balance) the losses. Heat is gained (solar gain) and lost through the windows. If the windows are designed, installed and oriented optimally, the gains can outweigh the losses. The other source of heat gain is from the building’s occupants (internal heat gains). Any remaining shortfall has to be provided by a heat source.

Energy returned on energy invested (EROEI) A ratio of the usable energy acquired against the energy that had to be expended to obtain it. We naturally tend to exploit high-EROEI energy sources first. EROEI is sometimes referred to as ‘energy return on investment’ (EROI).

EnerPHit The Passivhaus Institut’s energy performance standard for retrofits. It allows a maximum annual [specific] space heat demand of 25kWh/m².a and an upper airtightness limit of 1.0ach, if the 0.6ach target can be shown to be impracticable, and also sets requirements for individual elements of a retrofit, should the 25kWh/m².a requirement not be met. Only retrofits in certain climates, including central Europe and the UK, can be certified to the EnerPHit standard.

Fabric Energy Efficiency Standard (FEES) Developed by the Zero Carbon Hub (see Resources), FEES is the first Passivhaus-style fabric energy efficiency standard to be incorporated into the Code for Sustainable Homes (CSH) (see Ene 2 on page 40 of the 2010 Code for Sustainable Homes: Technical Guide: Previously, energy performance was defined solely in terms of reductions in carbon emissions. FEES is still under development, and at the time of writing applies only to CSH Levels 5 and 6.

Form factor The ratio of the external area of the thermal envelope to the treated floor area (TFA). Form factor is a measure of how compact the build design is. It is broadly similar to the area: volume ratio.

G-value One of the measures of vapour resistance: see vapour permeability. G-value is not to be confused with g-value (see below).

g-value A measure of the percentage of energy from the sun that passes through the glazed unit to reach the interior. In the USA, g-value is known as the solar heat gain coefficient (glazing), or SHGC-glazing.

Ground source heat exchanger (GSHX) Used with mechanical ventilation with heat recovery (MVHR) to preheat and pre-cool air via pipes in the ground, where temperatures are more constant than above-ground.

[Specific] Heat load The peak power, in W/m² (watts per square metre) of treated floor area (TFA), needed to maintain 20°C inside when the outside temperature is -10°C. (‘Specific’ refers to the per-area measurement.) The Passivhaus Planning Package (PHPP) calculates this at -10°C with overcast conditions and in sunny weather, and takes the more pessimistic of the two scenarios.

Heat main A system of insulated pipes that run between buildings, enabling the use of a large-scale heat source that would be too big for a single building. The heat main transports the heat from the point of generation to the point of use. Such infrastructure is common in Denmark and Holland, where heat from electricity power plants runs on natural gas or biomass; heat is thereby used efficiently instead of being wasted as it is in the UK.

Home Energy Rating System (HERS) A measure of (primarily) energy efficiency; mainly used in the USA. A HERS index of 100 means a home meets the code standard that is based on a standard US house; a HERS index of 70 means the home is 30-per-cent better than the code standard. The report generated advises on potential improvements to an existing property. A home-energy assessor visits the building, and tests carried out normally include a site blower test (pictured on page 125).

Hygroscopicity The property of a material to absorb, retain and release moisture from the ambient air. Materials that readily do this are often described as hygroscopic.

Indoor air quality (IAQ) The quality of air within buildings, in regard to both health and comfort. Indoor air often contains a complex mixture of contaminants and common pollutants, including smoke, volatile organic compounds (VOCs) and moulds. The level of carbon dioxide (CO2) in indoor air also relates to IAQ, and is an accepted marker for the wider mix of potential indoor air pollutants. ASHRAE issues guidelines on acceptable IAQ.

Intelligent membrane A membrane with humidity-variable characteristics, meaning that it is more vapour-open when there is a higher average relative humidity (RH) on the inside and outside of the membrane, thus allowing more drying out of the building fabric.

Internal heat gains The heat gains in a building from its occupants and the use of appliances within the thermal envelope. See energy balance.

Interstitial condensation Condensation that occurs within a building assembly, when warm moist air (generally from inside a heated building in winter) penetrates into the assembly, meets a cold surface and condenses. This can cause serious structural damage and affect insulation performance, especially if the condensation is not able to dry out.

kelvin (K) See K in glossary of units.

Lambda value (λ), also known as k-value A measure of thermal conductivity, measured in W/mK (watts per metre [depth] per degree kelvin). The inverse measurement is thermal resistivity. See also Appendix B. Lambda 90/90 (λ90/90) values are thermal conductivity values that have been calculated according to the Lambda 90/90 convention, which means that 90 per cent of the test values show a lower conductivity than the stated value, to a statistical confidence level of 90 per cent. These are the values adopted in the UK for Passivhaus calculations. Lambda 90/90 values refer to materials that are factory-produced and regularly tested. Materials that are blown-in on-site need density checks, as conductivity is strongly density-dependent. It is much harder to obtain a 90/90 value for materials that are made on-site, such as hemp and lime.

Leadership in Energy and Environmental Design (LEED) A US sustainability rating system, broad-based and internationally recognised. See

Mechanical ventilation with heat recovery (MVHR), also known as heat recovery ventilation (HRV) or comfort ventilation A whole-house ventilation system that takes out heat from the old (exhaust) air and gives it to the new (intake) air. Fresh air is delivered to living areas (e.g. living room and bedrooms) and extracted from kitchens and bathrooms. MVHR units do not supply new heat into the supplied air. However, a supply duct radiator can be used to add heat to the new air after it leaves the MVHR unit. See Chapter 12.

n50 The term for describing the air changes per hour (ach) at a pressure of 50Pa (pascals) above and below ambient atmospheric pressure, applied to the inside of a building in an airtightness test.

Net energy The remaining energy available to society after the energy needed to obtain it has been subtracted. This is sometimes expressed as a ratio and is referred to as energy returned on energy invested (EROEI). ‘Peak net energy’ (a more accurate term than the more commonly used ‘peak oil’) refers to the maximum rate at which energy can be extracted from a source; that rate being constrained by physical rather than economic factors. See

Off-gassing The evaporation of volatile chemicals (including volatile organic compounds [VOCs]) at atmospheric pressure and room temperature. Off-gassing of potentially harmful chemicals occurs in many modern, mainstream building materials, such as paints, varnishes and chipboards. Concerns are sometimes expressed about off-gassing from some insulation materials. Off-gassing becomes a problem in enclosed environments without sufficient air changes.

Parging A term (in a Passivhaus context) for plastering of rough walls, etc., to seal for airtightness.

Parts per million (ppm) Used to measure atmospheric concentrations of pollutants and other gases, including carbon dioxide (CO2).

Passive stack ventilation A means of ventilating without using mechanical fans. Utilises natural ‘stack’ effects from the temperature difference between the inside and outside of the building and from wind passing over the building (creating suction). Systems use physical vertical pipes to provide exit points at a high level.

Passivhaus Institut (PHI) The independent foundation established in Germany in 1996 to develop, promote and protect the Passivhaus standard. Known as the Passive House Institute in English-speaking countries. See Resources.

Passivhaus methodology Using the knowledge contained in the Certified European Passive House (CEPH) Passivhaus Designer course, including modelling in the Passivhaus Planning Package (PHPP), to design a building that performs to a defined ultra-lowenergy standard.

Passivhaus Planning Package (PHPP) The energy-modelling design tool created by the Passivhaus Institut (PHI) to accurately predict energy performance. It is the basis for designing and certifying Passivhaus and EnerPHit builds. See Chapter 7.

Peak net energy See net energy.

Psi-value (ψ) Similar to chi-value (χ); a measure of the rate at which energy passes through a length of material. In a Passivhaus it is used to measure heat loss in a linear thermal bridge. It is measured in W/mK (watts per [lateral] metre per degree kelvin). The psi-value of specific building elements or components is referred to thus:

•  ψspacer: psi-value of the spacer between panes in a double- or triple-glazed window.

•  ψinstallation : psi-value of the junction between a window frame and the wall.

Different conventions for measuring the psi-value are described thus:

•  ψi: internal dimensions of the junction between building elements (general convention)

•  ψe: external dimensions of the junction between building elements (Passivhaus convention)

See also U-value.

q50 The term for describing the air permeability, in m³/hr/m², at a pressure of 50Pa (pascals) above and below ambient atmospheric pressure, applied to the inside of a building in an airtightness test.

Regulated (carbon) emissions A concept in the Code for Sustainable Homes (CSH). Refers to emissions resulting from energy use to provide only heating, hot water, fixed lighting, pumps and fans. See also unregulated (carbon) emissions.

Relative humidity (RH) A measure of the quantity of water vapour in a given volume of air, expressed as a percentage of the maximum quantity of water vapour that volume can contain before it becomes saturated (after which condensation will occur).

Solar gain The amount of energy from the sun captured through glazing.

Spacer The dividing strip along the edge of a double- or triple-glazed unit that separates each pane. Warm-edge spacers are made from material or materials with a lower conductivity. The energy performance of a spacer is measured by its psi-value (y).

Specific heat capacity The amount of heat required to change the temperature of a unit of a material by a given amount. (In standard metric units, it is the number of joules required to raise 1 gram of the material by 1 degree kelvin.) Specific heat capacity is a measure of a material’s thermal mass.

Specific heat load See [Specific] Heat load.

Standard Assessment Procedure (SAP) The UK government’s recommended tool for determining the energy rating of dwellings (residential buildings), first published in 1995 and subsequently updated. It evolved from the National Home Energy Rating (NHER) scheme and was devised by BRE.

Summer bypass A control on the MVHR unit to bypass the heat recovery function so you can continue to use it to ventilate at warmer ambient temperatures.

Supply duct radiator, also known as in-line duct radiator A small radiator inserted into the supply duct just after it leaves the MVHR unit. It adds a small amount of heat into the ventilation system’s supply duct. See ducts.

Thermal bridge Commonly known as a cold bridge. A gap in insulation that allows heat to ‘short-circuit’ or bypass it. This occurs when a material with relatively high conductivity interrupts or penetrates the insulation layer. Thermal bridges exist in point and linear form. See Chapter 8.

Thermal bypass A type of thermal bridge caused by air movement in the insulation layer, for example in an unfilled cavity in a typical UK cavity wall or in poorly laid loft insulation, leading to the transfer of heat.

Thermal comfort Defined by Dr P. Ole Fanger as “the condition of mind which expresses satisfaction with the thermal environment”. Dr Fanger identified thermal comfort as being determined by: air temperature, ‘radiant’ temperatures (the temperatures of walls, floor and ceiling), air movement (draughts), temperature stratification (differences in temperature from floor to ceiling), relative humidity (RH), the insulative value of clothing (‘Clo Value’) and physical activity level (‘Met Value’). These form the basis of ASHRAE’s standards for thermal comfort. In contrast, ‘adaptive models’ describe thermal comfort as a function of physiological, psychological and behavioural factors.

Thermal conductivity, also known as conductivity A material’s ability to transmit heat, measured by the lambda value (λ). Unlike U-value, the lambda value of a material remains the same irrespective of the thickness of the material. Lambda values do sometimes vary with temperature. See Appendix B for typical values for common building materials.

Thermal envelope The area of floors, walls, windows and roof or ceiling that contains the building’s internal warm/heated volume.

Thermal mass The ability of a body of material to absorb, store and subsequently release heat (due to its specific heat capacity and its mass).

Thermal resistivity, also known as resistivity (but not to be confused with vapour resistivity – see vapour permeability) A material’s ability to resist the passage of heat. It is the mathematical inverse of thermal conductivity, and is measured in Km/W (kelvin metres per watt). Two resistivity values need to be entered for the calculation of U-values in the PHPP:

•  Rsi describes the resistivity of the static air* on the interior surface of the material.

•  Rse describes the resistivity of the static air* on the exterior surface of the material.

(*The first millimetre of air on a surface remains static even in windy conditions and therefore has insulation properties.)

Thermal store Provides the same function as a conventional hot water tank or store, but the hot water is never directly used in the taps. Instead, cold mains water is fed through a coil in the thermal store and is heated by the surrounding water. Thermal stores can work at a lower operating temperature than conventional tanks, and can also be connected to ‘uncontrolled’ heat sources, such as the back boiler of a wood burner. See Appendix A.

Transfer path A 20mm gap under a door, or a hidden 10mm gap cut into the top of the architrave and door frame, to allow air to move between supply or extract rooms and the common interconnecting spaces (in a domestic house, usually the stairwell and hallways).

Treated floor area (TFA) A convention for measuring usable internal floor area within the thermal envelope of a building. See Chapter 7, page 94.

U-value A measure of the ease with which a material or building assembly allows heat to pass through it; in other words, how good an insulator it is. The lower the U-value, the better the insulator. The U-value is used to measure how much heat loss there is in a wall, roof, floor or window, and is measured in W/m²K (watts per square metre per degree kelvin). When referring to window U-values, the following convention is used:

•  Uf – the U-value of the window frame.

•  Ug – the U-value of the glazing.

•  Uw – the U-value of the whole window. This value is often quoted by window manufacturers for a standard size and configuration of window (1230mm high by 1480mm wide, with a centre mullion, one fixed and one opening casement) and depends on Uf, Ug and the spacer psi-value.

•  Uw, installed – similar to Uw but also takes into account the losses as a result of the thermal bridge between the window and the wall, which is quantified by the installation psi-value. Uw, installed is calculated by the Passivhaus Planning Package (PHPP).

Ultra-low-energy This is not a formal standard, but a term we have chosen to use in this book to refer to buildings that require up to 40kWh/m².a (kilowatt hours per square metre [of treated floor area] per annum) for space heating and have an airtightness of 1.5 air changes per hour (ach) at 50Pa (pascals) above and below ambient atmospheric pressure. The AECB Silver Standard sets a space-heating limit of 40kWh/m².a and zero carbon sets a limit of around 50kWh/m².a in Passivhaus terms (i.e. adjusted for the difference in convention used for measurement of floor area).

Unregulated (carbon) emissions A concept in the Code for Sustainable Homes (CSH), referring to emissions resulting from energy use to provide heating, hot water, fixed lighting, pumps and fans, as well as from energy use for cooking and other household electrical appliances. In Passivhaus, the energy equivalent of this concept is annual [specific] primary energy demand. See also regulated (carbon) emissions.

Vapour barrier, also known as vapour-closed A material that is near-impermeable to water vapour, e.g. aluminium foil.

Vapour-closed See vapour barrier.

Vapour-open A material that is permeable to water vapour, e.g. a ‘breathable’ or ‘breather’ membrane (these are both airtight and liquid-moisture-tight but vapour-open).

Vapour permeability The degree to which a material facilitates the passage of water vapour through it, measured by four different values: vapour resistivity (r-value; units: MNs/gm – meganewton seconds per gram metre); vapour resistance (G-value; units: MNs/g – meganewton seconds per gram); water vapour resistance factor (µ-value; no units); equivalent air layer thickness (Sd-value; units: m – metres).

Vapour pressure differential/gradient The pressure exerted by a vapour, or gas (for example, water vapour) relates to its temperature and the concentration of the particles. Where there is a higher vapour pressure on one side of a material than on the other, the difference between the two is the vapour pressure differential. Vapour will move by diffusion across a vapour-permeable membrane, driven by the pressure difference. So water vapour will move from the side with higher humidity (high vapour pressure) to the side with lower humidity (low vapour pressure).

Watt (W) See W in glossary of units.

Window schedule A list of all the windows in a building; usually produced by an architect to communicate to other parties in the build project what windows are required.

Zero carbon – a UK energy target for housing, currently set for enforcement in 2016. It currently applies to both Levels 5 and 6 (L5 and L6) of the Code for Sustainable Homes (CSH). In broad terms, it can be considered to mean that there are no net annual greenhouse gas emissions resulting from energy use in a dwelling. Any emissions created are offset by those ‘saved’ using on-site (or possibly communal if the dwelling is part of an estate) renewable capacity, which feeds electricity back to the grid. It now includes the Fabric Energy Efficiency Standard (FEES).

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