Perplexed by all this talk of u-values and blower door tests? Our sustainable building glossary will help you get to grips with the key terminology.

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Term Definition
Air source heat pump

This type of heat pump captures heat from outdoor air and uses it to provide space heating or hot water using electricity to boost the temperature if needed. The warmer it is outside the more efficient an air source heat pump is and the less electricity it requires to achieve the desired temperature.


The degree of air leakage or air infiltration a building has. Making a building airtight essentially means eliminating draughts. Ideally we want to have total control over how much air we're letting in to the building through designed ventilation systems, rather than cold air entering (and warm air escaping) uncontrolled through unwanted or unseen gaps.

Airtightness is typically measured in two units: air changes per hour (ACH) and air permeability (m3/hr/m2). For a typical building, there is usually little difference in the two figures. The smaller the airtightness figure the better. Under Irish building regulations new homes must have an air-tightness of 10 m3/hr/m2. The rigorous Passivhaus standard demands an airtightness of 0.6ACH or less.


A stable, non-reactive gas that is often used to to fill the space between window panes in double and triple glazed windows because it is an effective insulator


See building energy rating (BER)

Blower-door test

This is used to work out a house's airtightness. A fan mounted to an external door is used to pressurize or depressurise the interior of the house, forcing air in or out through any gaps or cracks. The house's airtightness is determined by measuring the force needed to maintain a certain pressure difference between the inside and outside of the house


This is the Building Research Establishment's Environmental Assessment Method, a UK system used to assess the environmental impact of non-domestic buildings. It considers a range of criteria including energy consumption, water, materials, waste, transport, ecology, pollution and health. It has four levels: pass, good, very good and excellent

Brise soleil

A permanent structure designed to provide shade from the sun. In the northern-hemisphere these are often placed on a building's south-facade to help prevent glare and overheating. Some innovative approaches to brise soleil include planting deciduous climbers to provide extra summer shading and more passive solar gain in winter.

Building energy rating (BER)

The rating system used to measure the energy efficiency of Irish buildings. BERs range from a G for poor efficiency to an A1 for best efficiency

Building envelope

The exterior shell of the building, including the external walls, windows, floor and roof.

Cavity wall

A wall with inner and outer masonry layers (eg block or brick), with a cavity in between. The cavity serves as a way to drain water out of the wall. Cavities can be insulated to improve their ability to keep heat in the building, but it's important to use moisture-resistant materials like polystyrene bead for cavity insulation, particularly in wet areas.


See combined heat and power (CHP)

Code for Sustainable Homes

The BRE's environmental assessment tool for dwellings. As with BREEAM, buildings are assessed on their overall environmental performance, resulting in six levels of scoring.

coefficient of performance

This measures the energy efficiency of certain heating and cooling appliances, such as heat pumps. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, so a heat pump with a COP of 4 puts out four times as much energy as it uses. The higher the COP, the more efficient the device.


See 'combined heat and power'

cold bridging
combined heat and power (CHP)

A technology that generates both heat and electricity from the same plant. Also known as cogeneration

Condensing boiler

A condensing boiler can re-capture some of the heat normally released in the form of hot gases, and use it to heat up water returning from your central heating system. This means that it requires less energy to produce a given amount of heat, and is therefore more efficient.


Using sunlight for indoor lighting needs


See Dwelling Energy Assement Procedure (Deap)

Dew point

The temperature below which water vapour in the air will start condensing to liquid. This is important in buildings, because anywhere there is a dew point, there is a risk of condensation and mold growth. For example, improperly installing insulation on the inside of a wall can create a dew point between the insulation and the wall.

District heating

A type of heating system in which heat is piped from a large central heating system (such as a boiler) to multiple units (such as houses or apartments), rather than each unit having its own separate heat source. Often financed via energy service companies (ESCos), district heating systems tend not to become less viable in very energy efficient buildings, given that the low space heating demand means smaller bills payable to the ESCos

dry lining
Dwelling Energy Assement Procedure (Deap)

A software programme used to calculate the Building Energy Rating (BERs) of buildings

embodied energy

Energy required to extract, manufacture process, transport, and install a product


This is the Passive House Institute's standard for retrofit projects. It demands airtightness of 1.0 air changes per hour and space heating demand of 25kWh/m2/yr (as opposed to 0.6 air changes per hour and 15 kWh/m2/yr for the original passive house standard).

Exhaust air heat pump

This type of heat pump extracts heat from waste air leaving a heating system and uses it to provide hot water or space heating, using electricity to boost the temperature if needed.

expanded polystyrene

A type of rigid foam used for insulation, typically for external insulation

External insulation

Insulation applied to the outside of a building


A chemical sometimes found in building products. Classified as a 'known human carcinogen' by the UN International Agency for Research on Cancer

FSC certified wood

The Forest Stewardship Council is a non-profit group that certifies forests that are managed sustainably


Wastewater from baths, sinks, dishwashers, washing machines etc - essentially all a house's wastewater except that from toilets and macerators/food grinders

ground granulated blastfurnace slag

A steel-industry by-product that is used instead of traditional portland cement for 'eco' and 'green' cements and concretes. Because GGBS is essentially a by-product of an existing industry, it's considered to have a much lower embodied energy than portland cement

ground source heat pump

Heat pump that uses the ground - which has a fairly consistent temperature - as a heat source. Ground source heat pumps take heat from the ground and deliver it to internal spaces, using electricity to boost the temperature if needed

heat pump

A device that takes heat from one location (such as the ground, air, or water) and brings it to heat another (such as the inside of building). If the temperature from the outside source isn't enough to heat the building, electricity is used to boost it to the required temperature. The efficiency of a heat pump is measured by its coefficiency of performance (COP). See 'air source heat pump' and 'ground source heat pump'.

heat recovery ventilation

A technology that ventilates a building while also helping to heat it. HRV systems typically extract warm, damp air from 'wet' rooms like kitchens and bathrooms and use it to heat cool, fresh incoming air, which is then usually piped to living spaces such as living rooms and bedrooms


A method of construction that uses part of the hemp plant, along with part of the lime plant, to construct solid, insulating walls


Heating, ventilation and air conditioning

Intelligent vapour check

A type of membrane, often used in timber frame construction and timber roof structures, that becomes more or less permeable to water vapour depending on ambient conditions. Typically in winter it prevents water vapour from getting it but becomes more vapour permeable in summer to allow water vapour to diffuse out and building components to dry

internal insulation

Insulation applied to the inside of a building, sometimes called dry lining


A joist is one of the horizontal construction elements, typically made of timber, that support a ceiling, roof, or a floor. Insulation materials are often installed between the joists, which run parallel to each other.


Leadership in Energy and Environmental Design is an environmental rating system for buildings developed by the US Green Building Council. It addresses location, water efficiency, energy, materials, indoor environmental quality and design. There are four levels of certification: Certified, Silver, Gold and Platinum

life cycle assessment

An examination of a material or product's impact (typically on the environment, but also on people/society) throughout its life cycle, from the extraction of raw materials through to its disposal or recycling

low emissivity coating

A thin metallic coat on a window that lets most light in while blocking heat.


Type of construction in which individual units such as blocks, bricks or stone are bound together with mortar

mechanical ventilation heat recovery

Also known as heat recovery ventilation. This is a system that ventilates a building while also recovering heat from extracted air. It's typically installed as a centralised whole building solution, but decentralised systems are emerging too, including single room ductless systems. MVHR systems typically extract warm, damp air from 'wet' rooms like kitchens and bathrooms and use it to heat cool, fresh incoming air, which is then usually piped to living spaces such as living rooms and bedrooms.


See 'mechanical heat recovery ventilation'

Party wall

This is a wall shared between two properties. Coincidentally, in buildings with poor acoustic properties it can be the focus of much tension when neighbours throw parties

Passive House Planning Package

A software programme developed by the Passive House Institute that's used to design and test buildings aiming to meet the passive house standard. It's often used as a design tool for low energy buildings even if the architect or builder is not specifically aiming to meet the standard

PEFC certified wood

Like the FSC, the Programme for the Endorsement of Forest Certification certifies forests that are managed sustainably

Performance gap

The difference between how a building is designed to perform and how it subsequently does in reality once built. The term usually refers to energy consumption but can refer to other aspects of building performance too


A technology that uses energy from the sun to produce electricity


A type of masonry block with a lower embodied energy than traditional concrete, made from clay that is heated to about 1000C. Also known as teracotta block

Psi values

This is the 'linear thermal transmittance', the rate of heat flow per degree temperature difference per unit length of a thermal bridge. It is measured in W/mK, and is used to calculate the heat loss or gain through a thermal bridge. Under Irish and UK building regulations, the Psi-values for all non-repeating thermal bridges are multiplied by the measured length of each bridge before a Y-value for the building can be calculated, expressed in W/m2K.


Thermal resistance or R-value is a measure of a material's ability to resist heat flow. A high R-value is desirable when choosing insulation for your home. R-values are commonly quoted in North America, whereas U-values are used more often in Europe. A U-value of 0.27W/m2K equates to an R-value of 21, while a U-value of 0.15 W/m2K equates to an r-value of 38. See U-value for more information

Relative humidity

This is the amount of water vapour in the air relative to the amount the air can hold at the current temperature. Healthy relative humidity is generally regarded as being between 40% and 60%. High relative humidity can lead to condensation, dampness and mould.

Seasonal performance factor

The ratio of useful heat energy output from a heat pump to the electrical energy input (including compressor, circulation pumps and electrical immersion, if present) averaged over an entire heating season

single leaf

A single leaf wall consists of just one layer of a building material. Single-leaf walls of hollow concrete blocks were the most common form of construction in the greater Dublin area for decades. They are different to the cavity wall construction common throughout the rest of the country, which consist of two layers of masonry with a cavity in between.

solar gain

This refers to the heat and light energy that a building receives passively from the sun. Designing a building so that the rooms that are used most during the day face south means they will get light and heat from the sun, reducing the need for mechanical heating systems and electrical lighting

Space heating demand

The amount of active heating input required to heat a building usually expressed in kWh/m2/yr. It is often calculated using building energy software applications such as PHPP, Deap or Sap

Strip foundation

A strip of concrete running under all of a building's load bearing walls. This will normally include the external walls, and possibly some of the internal walls.

Surface to volume ratio

This is the total external surface area of a building relative to its volume. A lower surface to volume ratio is generally more energy efficient, as it means there is less surface area from which heat can escape the building.

thermal bridging

A thermal bridge occurs when heat or cold transfers across an external surface of a building. This can cause heat to escape from the building or cold to enter. Thermal bridging occurs when a thermally conductive material (ie a material with low resistance to heat flow) can penetrate or bypass the insulation layer. For example, insulation is often placed between timber joists in roofs, however the joists themselves may conduct heat and reduce the effectiveness of the insulation by acting as a thermal bridge along which heat can be transferred out and lost. Thermal bridging can greatly reduce the effectiveness of insulation, so it's crucial to minimise thermal bridges during the design of a new build or refurbishment.

thermal bypass

On this one we?ll defer to Mark Siddall, writing here for "Thermal bypass is heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. Defined in this manner thermal bypass includes convective loops, air infiltration and wind washing. In this context [...] it should be recognised that the term thermal bypass is being applied to largely unfamiliar, and often unregulated, heat transfer. Furthermore it is an acknowledgement that air movement can lead to a significant increase in the heat loss when compared to predicted values. This means that even when the architect, and builder, thinks that a design has addressed the performance requirement it is very likely that it has not."

You have been warned...

thermal mass

The capacity of a building material to store heat. Materials with a high thermal mass absorb heat, store it and then release it later on. This can help to smooth out extremes in temperature inside a building, helping to maintain a comfortable internal environment and reduce the need for heating. Heavyweight construction materials like concrete and bricks have more thermal mass than lightweight materials like timber. Thermal mass is particularly important in climates where there is a large difference between daytime and night-time temperatures, which isn't the case in the temperate Irish climate, though thermal mass can still be beneficial in Irish building - it's of most benefit when buildings are well insulated and occupied consistently throughout the day.

Thermostatic radiator valves

Self-regulating valves, typically attached to radiators or other water heating systems, used to control the room temperature automatically based on what temperature the TRVs are set at

The U-value of a material or construction element is the rate of heat loss through that material, taking account of both thermal conductivity and thickness. The lower the U-value of a material, the less heat can pass through it and the better it is at insulating. U-values are measured in watts per metre squared kelvin (W/m2K). Homes built to the passive house standard in Ireland or the UK typically includ a wall U-value of 0.15 W/m2K or better (More moderate U-values may be possible in buildings with more compact forms, which are inherently more efficient . A 2016 analysis by Passive House Plus of data from SEAI's National BER Research tool revealed that the average U-values for new Irish homes have been dramatically improved as a consequence of tightening building regulations - with respective average figures for walls, roofs and floors of 0.17, 0.13 and 0.14 W/m2K, though the backstops stipulated in the regulations are less ambitious. U-value requirements in building regulations in the UK lag some way behind at present.