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