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
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

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.

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

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.

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.

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.