The tiny particles of solid or liquid suspended in a gas or liquid.


A thin coat of plaster applied to masonry to seal the surface. Often used to ensure airtightness.

Pascal (Pa)

A unit of measuring air pressure.

Passivhaus standard

Is a construction standard for all buildings which emphasises high levels of insulation and airtightness, minimal thermal bridging, use of solar and internal heat gains and tightly controlled ventilation. 

Passivhaus Planning Package (PHPP)

A modelling and accreditation software tool developed and updated by the Passivhaus Institut.

Passive cooling

Technologies or design features used to cool buildings without power consumption.

Passive solar design (PSD)

A design strategy that optimises a building's form, fabric and orientation to maximise solar gain from autumn to spring, whilst minimising it during the warmer part of the summer. At the same time, daylighting is maximised at all times. Passive solar design has long been the key element in developing low-energy building solutions in the UK – often characterised by large integrated conservatories. The alternative Passivhaus approach, with its emphasis on energy conservation, will likely interrupt this particular evolutionary design branch.

Passive solar technology

Devices and design methodologies that use sunlight for useful energy without use of active mechanical systems. Passive solar technologies include direct and indirect solar gain for space heating, solar water heating systems, use of thermal mass and phase-change materials for slowing indoor air temperature swings, and solar chimneys for enhancing natural ventilation. 

Passive stack ventilation (PSV)

Ventilation systems based on the 'Stack Effect'. This is the movement of planned air paths through the dwelling as a result of internal and external temperature differences and wind induced pressure differences.

Payback period

The number of years it takes to recoup an initial investment.


A measure of the ability of a material to transmit fluids or gases.

Permeable paving

Provides a structural pavement whilst allowing water to pass straight into the pavement construction for temporary storage and dispersal into the ground or for collection. It is one of the main techniques for Sustainable Drainage Systems (SUDS).

Persistant organic pollutants

Organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes. Because of this, they have been observed to persist in the environment, to be capable of long-range transport, bioaccumulate in human and animal tissue, biomagnify in food chains, and to have potential significant impacts on human health and the environment.

Pervious surface

A surface that allows inflow of rainwater into the underlying construction or soil.

PFA (pulverised fuel ash)

Is a by-product from burning coal for the production of electricity. PFA is used as a cement substitute in the manufacture of concrete. 

Phase Change Material (PCM)

A substance with a high heat of fusion (or ‘specific melting heat’) which by melting and solidifying at certain temperatures, stores and releases heat. Where integrated with a construction element component (eg gypsum board), a PCM provides a very effective form of thermal mass.


Hydrocarbons used to make resins and glues. Very toxic and may outgas.

Photochemical oxidation

The formation of reactive chemical compounds, such as ozone, by the action of sunlight on certain primary air pollutants. These compounds may be injurious to human health, ecosystems, materials and crops.

Photovoltaic (PV)(solar) cell

A module incorporating a semiconductor that generates electricity when exposed to sunlight. 


An air compartment connected to a series of ducts. For example, a ceiling plenum is the space above the suspended ceiling and below the floor above that is used as part of the air distribution system.

Porous surface

A surface that infiltrates water to the sub-base across the entire surface of the material forming the surface, for example grass and gravel surfaces, porous concrete and porous asphalt.

Post-consumer recycled content (%)

An end product that has completed its life cycle as a consumer item and would otherwise have been disposed of as a solid waste.

Post occupancy evaluation (POE)

The evaluation of opinion about buildings in use, from the perspective of the people who use them.

Potable water

Drinking water


Vitreous siliceous materials which react with calcium hydroxide to form calcium silicates that have cementitious properties. Use of pozzolans can permit a decrease in the use of Portland cement when producing concrete 

Pre-consumer recycled content (%)

A product that contains materials recovered from the waste stream of a manufacturing process. The recovered materials are not normally those reused by the original process.

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.

Purge ventilation

Rapid ventilation achieved by opening windows and doors in order to remove moisture and odours. Can also refer to overnight ventilation of hot internal air.

Ψ– (psi) value

The heat loss per unit length of thermal bridge, measured in W/mK