New Build & retrofit: Technical Glossary

Air changes per hour (ac/h)
(volumetric), the number of times per hour that the air inside a building is changed. Units m³ hr/ m³ @ 50 Pascals.
Air permeability
defined in BS EN 13829. Units m³/m²hr at 50 Pascals or m/h @ 50 Pa.
Air leakage index
per unit thermal envelope area (the CLP preferred definition). Units m³/m²hr at 50 Pascals or m/h @ 50 Pa.
The AECB’s Carbon Literate Design and Construction Programme.
Delivered energy
the amount of energy which is supplied to final users, e.g., households, office buildings, schools, factories and cars.
Global Warming Potential (GWP)
a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1). For example, methane, nitrous oxide and sulfur hexafluoride have GWPs many times that of CO2, although CO2 is being emitted into the atmosphere in much larger quantities.
Heat Loss Parameter (HLP)
a building’s specific heat loss (in units of W/K) divided by the building’s floor area (measured internally i.e. within the thermal envelope). Units W/K.m²
Low-grade heat
normally used to mean heat at a temperature of ≤ 100°C
Mechanical Ventilation with Heat Recovery (MVHR)
a system of ventilating buildings, in which heat is recovered from the exhaust air stream to preheat the fresh air intake. Normally there are two sets of ductwork, both connected to an air-to-air heat exchanger, with the air flows in the supply and exhaust branches carefully balanced.
a low energy building standard. Passivhaus Institut (PHI) originator of the Passivhaus movement and of the Passivhaus Standard.
Passivhaus Planning Package (PHPP)
a modelling and accreditation software tool developed and updated by the Passivhaus Institut.
Primary energy
the amount of energy mined or extracted at source; e.g., from coal, oil, natural gas, uranium or wood. Includes losses within processes such as electricity generation and transmission.
Stack effect
in winter, the warm air inside a building is less dense than the cold external air. Consequently, cold air tends to be drawn in through cracks and gaps at the base of the building, with warm air exfiltrating through openings in and near the top of the building.
Thermal capacity
the ability of the constituent materials in a building to store heat, for a given rise in temperature, measured in units of kWh/K for a whole building or in Wh/K.m² to indicate the building’s thermal capacity per unit floor area.
Thermal envelope
the insulated external fabric of the building. Useful space heating energy the amount of heat actually put into the heated space.
ψ (psi) value
the heat loss per unit length of thermal bridge. Units W/mK.
λ (lambda) value
thermal conductivity of a material. Units W/mK.
Χ (chi) value
point thermal bridge heat loss coefficient. Units W/K.
thermal resistance. Units m²W/K.
thermal transmittance. Units W/m²K.
a notional additional U-value, spread uniformly over the whole thermal envelope.