Glossary

Glossary 
AECBThe Sustainable Building Association, formerly known as the Association for Environment Conscious Building.
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 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 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.
Capillarity (capillary action)Movement of liquid water against gravity, through small pores or capillaries in a material.
Certified Passivhaus DesignerAn 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.
Chi-value ()Similar to psi-value (w), 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).
DesiccantA 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.
DiffusionThe 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.
Ductshe 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.
EnerPHitThe 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: www.planningportal. gov.uk/uploads/code_for_sustainable_homes_techguide. pdf). 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.
Heat mainA 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.
HygroscopicityThe 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 (CO) 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 humidityvariable 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 gainsThe 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.
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.
Lambda valuealso 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 (290/90) 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 factoryproduced 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.
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.
PargingA term (in a Passivhaus context) for plaste 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.
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-low energy standard.
Passivhaus Planning Package (PHPP)The energymodelling 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.
Off-gassingThe 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.
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.
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 doubleor triple-glazed unit that separates each pane. Warmedge spacers are made from material or materials with a lower conductivity. The energy performance of a spacer is measured by its psi-value (v).
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.
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 comfortDefined 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 conductivityalso known as conductivity A material's ability to transmit heat, measured by the lambda value (^). 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 massThe ability of a body of material to absorb, store and subsequently release heat (due to its specific heat capacity and its mass).
Thermal resistivityalso 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:
• R, 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.)
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.
U-value 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).
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 areal 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.
Vapour barrier also known as vapour-closed A material that is near-impermeable to water vapour, e.g. aluminium foil.
Vapour-openA 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 (u-value; no units); equivalent air layer thickness (Sd-value; units: m - metres).
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).
Unregulated (carbon) emissionsA 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.
KKelvin, a unit of measurement for temperature. One degree kelvin = one degree Celsius, but zero degrees kelvin (OK) is absolute zero. 0°C is approx. 273K.
Km/W Kelvin metres per watt. The unit of thermal resistivity.
kWhKilowatt hour, a unit of energy. For example, a one-kilowatt electric fire left on for one hour would use 1kWh (1,000W) of energy. 1kWh = 3.6MJ (megajoules).
kWh/m2.aKilowatt 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 (CO2) levels well below 1000 parts per million (ppm).
MNs/gmMeganewton seconds per gram metre. The unit of measure of vapour resistivity (r-value). See vapour permeability.
PaPascal, the SI (International System of Units) measurement of force per unit area. 1Pa is 1 newton per square metre.
WWatt, 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/KWatts per degree kelvin (temperature difference between inside and outside the thermal envelope). Used to quantify chi-value (V) in point thermal bridges.