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

Wufi is a software application that allows for realistic, dynamic modelling of the movement of heat and moisture in walls, roofs and other building elements.

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.
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

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.

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 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.

Synonyms - Thermal bridges
Surface to volume ratio

Surface area to volume ratio is a measure of how compact a building is. In passive house design, it is often expressed as the ‘heat loss form factor’, which is the ratio for the external surface area of the building to the treated floor area.

The higher the figure, the less compact the building, meaning there is more surface area from which heat can escape, making it more difficult to meet the passive house standard.

Synonyms - surface area to volume ratios
Structural insulated panels

Structural insulated panels (SIPs) are prefabricated, structural building panels comprising insulation – typically polyurethane – sandwiched between two layers of OSB.

Synonyms - SIPs
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.

Stack effect

The movement of air in and out of buildings due to differences in the temperature or moisture content of air. For example, the opening of high-level windows can allow warm air to rise out of a building, creating a ‘stack’ or ‘chimney’ effect that pulls cooler outdoor air in through open windows down below, helping to provide ventilation.

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

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

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.

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

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.


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

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.

Primary energy Renewable

Those fiendishly clever souls at the Passive House Institute have deigned to make a complicated subject even more complicated. To be fair, they have a point. Primary energy (see previous entry) is useful, but it's problematic too. Take renewable energy sources such as wind, solar and hydro or tidal. Primary energy figures give these sources a score of 1.0, meaning its regarded that 1 kW of, say, wind energy used in the building, took 1 kW of wind to generate it. (The implication is that waste of renewable energy doesn't matter, which troubles us). But of course, the reality is more complicated. In reality, these energy sources have one big limitation: intermittency. And because, say, the wind isn't always blowing or the sun isn't always shining sufficiently to generate electricity - either via the grid on microgeneration - exactly when people need to have a shower or turn on the heating, there are inefficiencies to contend with. In addition to grid transmission losses, this includes storage losses. So for this reason, the Passive House Institute now has two primary energy metrics that can be used: primary energy, and primary energy renewable (PER). PER takes account of the additional renewable energy required to be generated to provide the renewable energy required for specific uses in a building.

For a better explanation, click here.

Primary energy

Primary energy is the energy at source, whereas delivered or final energy is the energy used in a building. In the case of grid electricity, primary energy is the energy used at the power station to generate electricity. There can be significant energy losses in the generation process, added to additional losses transmitting energy through the grid. The gas, oil and biomass burned directly in buildings with boilers or stoves are typically assumed to have a primary energy factor of 1.1, meaning 1.1 kW of energy is required at source to generate 1 kW of fuel for the building. (Though we wonder about the primary energy of oil in particular, given the fact that cruder, harder to access oil sources are now becoming more common - which may require more energy to extract and refine.)

It's also worth bearing in mind that 1 kW of fuel does not necessarily mean 1 kW of heat. A properly operating high efficiency boiler may be more than 90% efficient, meaning 1 kW of fuel delivers more than 0.9 kW of heat, whereas a properly operating heat pump may generate 3 or 4 kW of heat per kW of 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


A technology that uses energy from the sun to produce electricity

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

PEFC certified wood

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

Passive House Plus standard

The ‘passive house plus’ standard is a new certification category designed to recognise the production of onsite renewable energy by passive buildings. It requires a minimum of 60k kWh/m2/yr of renewable energy generation, along with a maximum renewable primary (PER) energy demand of 45 kWh/m2/yr. PER is a new energy factor developed by the Passive House Institute designed for a future where electricity grids are powered entirely by renewables. It is designed to replace traditional primary energy demand in the long term. In this case, a higher PER demand, 56 kWh/m2/yr, was allowed because the dwelling produces much more (83 kWh/m2/yr) than the renewable energy generation target.

The even more advanced ‘passive house premium’ standard requires generation of 120 kWh/m2/yr and a maximum PER of 30 kWh/m2/yr. Meanwhile under this new system, the traditional passive house standard is rebranded as ‘passive house classic’, and has a max PER of 60 kWh/m2/yr, with no renewable generation required.

This new classification system is operational alongside the traditional passive house standard, with its maximum primary energy demand of 120 kWh/m2/yr, and no requirement for renewable energy production. The targets for space heating demand (15 kWh/m2/yr) and heat load (10 W/m2) remain the same under both certification systems.

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

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


From the end of 2018 all new public buildings in EU countries must be nZEBs, while the same rule will apply to ALL buildings from the end of 2020. Each country is free to come up with its own definition of an nZEB, within certain parameters.

Ireland is proposing that for dwellings, nZEBs must have an energy performance co-efficient (EPC) of 0.3 or less in the Deap software, which is used to calculate building energy ratings and demonstrate compliance with Part L of the building regulations (which deals with energy consumption).

Because Deap uses Ireland’s 2005 version of Part L as a reference, this means that a house with an EPC of 0.3 uses, in theory, 30% of the energy of a house built to the 2005 regulations.

Synonyms - Nearly Zero Energy Building

See 'mechanical heat recovery ventilation'

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.


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

low emissivity coating

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

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


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


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.

internal insulation

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

Synonyms - insulation
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


Heating, ventilation and air conditioning


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

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

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 loss form factor

Heat loss form factor is a measure of how compact a building is. It is the ratio of the surface area of the thermal envelope to the treated floor area. The higher the figure, the less compact the building, meaning there is more surface area from which heat can escape, making it more difficult to meet the passive house standard.

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

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


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

Green oak

Green oak is oak that was felled within the last 18 months. Because it is still drying, it has a high moisture content. But according to Oakwrights it is stronger, more durable and easier to work with than dried oak, as well as being less expensive.

FSC certified wood

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


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

Form factor

Form factor is the ratio of a building’s total surface area (the walls, roof and ground floor) to its treated floor area. The smaller the form factor, the more efficient the shape of the building and the less surface area from which heat can escape. A form factor of under 3.0 is considered ideal for a passive house.

Fabric first

Fabric First, the principle of designing a building fabric that keeps in heat — through a well insulated, airtight structure that is free of thermal bridges — before thinking about ‘bolt-on’ technologies like renewable energy.

External insulation

Insulation applied to the outside of a building

expanded polystyrene

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

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.


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).

Energy Performance Coefficent

The Energy Performance Coefficent (EPC) and Carbon Performance Coefficient (CPC) measure the energy and carbon efficiency of buildings under the Irish building regulations. Here we compare the EPC and CPC for this dwelling to Ireland’s proposed nZEB standard and to the current (2011) and past versions of Part L of the Irish building regulations. The lower the fraction, the better the score - so for instance a house with an EPC of 0.4 is 60% better than the 2005 regs.

Synonyms - Carbon Performance Coefficient, EPC, CPC
embodied energy

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

Dwelling Energy Assement Procedure (Deap)

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

dry lining
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

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.

Demand controlled ventilation

Demand controlled ventilation is a control strategy that modulates ventilation rates based on demand. While typically associated with mechanical extract ventilation, it can be applied in other applications too, notably including passive stack ventilation and mechanical ventilation with heat recovery. As such there is some discussion over whether demand controlled ventilation should be referred to as a ventilation system, or a control approach for various ventilation systems.

Typically DCV systems modulate ventilation rates based on relative humidity levels, though it is also possible to utilise sensors to track other pollutants, such as CO2.

While ventilation systems should be principally thought of in terms of indoor air quality rather than energy performance, it's estimated that a typical demand controlled mechanical extract ventilation system (DCMEV) would reduce ventilation heat loss by approximately 50% compared to an MEV system with no demand control.  

Synonyms - Demand control ventilation, DCV, demand-controlled ventilation

See Dwelling Energy Assement Procedure (Deap)


Using sunlight for indoor lighting needs

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.

combined heat and power (CHP)

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

cold bridging

See 'combined heat and power'

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.

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.


See combined heat and power (CHP)

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.

Building envelope

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

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

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.


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

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


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