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
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.

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