**All units used in the
book are included in this glossary, and are** **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 terms in the glossary of
terms.**

**K** Kelvin, a unit of
measurement for temperature. One degree kelvin = one degree
Celsius, but zero degrees kelvin (0K) is absolute zero. 0°C is
approx. 273K.

**Km/W** Kelvin metres
per watt. The unit of **thermal
resistivity**.

**kWh** Kilowatt hour, a
unit of energy. For example, a one-kilowatt electric fire left on
for one hour would use 1kWh (1,000**W**) of energy. 1kWh = 3.6MJ (megajoules).

**kWh/m².a** Kilowatt
hours per square metre per annum. Measures energy used annually per
square metre of usable or **treated floor area
(TFA)**. This is one of the key units of measure in
Passivhaus. By defining energy use in terms of each square metre of
floor area, it allows us to make a meaningful comparison of the
energy use of buildings of different sizes. See **annual [specific] space heat demand**.

**kWp** Kilowatt peak. A
measure of the maximum power output of photovoltaic (solar)
panels.

**m³/hr** Cubic metres
[of air] per hour. The unit of measure used to describe ventilation
rates. Passivhaus ventilation calculations are based on providing
30m³ per person per hour. This rate is necessary to keep carbon
dioxide (CO_{2}) levels well below 1000 **parts per million (ppm)**.

**m³/hr/m²** Cubic
metres [of air] per hour per square metre [of **thermal envelope** area]. The unit of measure of
**air permeability**.

**MJ/kg** Megajoules per
kilogram. Unit of measure of **embodied
energy**.

**MJ/m²** Megajoules per
square metre. A megajoule (MJ) is a unit of energy, equivalent to
1,000,000J (joules). 3.6MJ = 1**kWh**.

**MNs/g** Meganewton
seconds per gram. The unit of measure of vapour resistance
(G-value). See **vapour
permeability**.

**MNs/gm** Meganewton
seconds per gram metre. The unit of measure of vapour resistivity
(r-value). See **vapour
permeability.**

**Pa** Pascal, the SI
(International System of Units) measurement of force per unit area.
1Pa is 1 newton per square metre.

**W** Watt, a unit of
power. The rate at which energy transfers (is ‘consumed’ or
‘generated’). For example, a typical low-energy lightbulb uses 15W;
an electric kettle 3kW (kilowatts, i.e. 1,000W). 1W = 1 joule per
second.

**W/K** Watts per degree
kelvin [temperature difference between inside and outside the
**thermal envelope**]. Used to quantify
**chi-value (**c**)** in point **thermal
bridges**.

**Wh/K per m²** (as
cited in the **PHPP**) Watt hours per
kelvin per square metre [of **treated floor area
(TFA)**]. 1 watt hour (Wh) is one-thousandth of a
**kWh** (kilowatt hour): for example, a
1W LED light bulb left on for 1 hour would use 1Wh of
electricity.

**Wh/m³** Watt hours per
cubic metre [of air moved]. Used by the **Passivhaus Institut (PHI)** to measure the
electrical efficiency of **MVHR**
units. 1Wh (watt hour) is one-thousandth of a kWh.

**W/m²** Watts per
square metre [of **treated floor area
(TFA)**]. Used to measure **heat
load**.

**W/m²K** Watts per
square metre [of the material/ assembly in question] per degree
kelvin [temperature difference between inside and outside the
**thermal envelope**]. Used to quantify
**U-value**.

**W/mK** Watts per metre
per degree kelvin. For linear **thermal
bridges**, this is watts per metre length of the thermal
bridge per degree kelvin temperature difference. (Used to measure
**psi-value [**y**]**.) For **thermal
conductivity** this is watts per metre thickness/depth of
material per degree kelvin temperature difference. (Used to measure
**lambda value [**λ**]**.) In both cases the temperature difference
measured is that between inside and outside the **thermal envelope**.