Balance Point

The outdoor temperature at which a building's heat loss to the environment is equal to internal heat gains from people, lights, and equipment.

Balancing pond

A pond designed to attenuate flows by storing Runoff during the peak flow and releasing it at a controlled rate during and after the peak flow has passed.

BFRC Rating

A measure of the overall performance of a window, rated from A-G.


The increase in the concentration of a chemical in a biological organism over time, compared to the chemical's concentration in the environment.


A chemical additive which prevents growth of bacteria or fungi. More often found in paints and floor coverings. Biocides are dangerous in large concentrations.


Decomposition of organic matter by micro-organisms and other living things.


Ethanol and diesel made from crops including corn, sugarcane and rapeseed.

Biological wastewater treatment

The use of bacteria to eat the organic material present in wastewater.


A renewable energy source, commonly used to refer to plant matter grown to generate heat or electricity. (see also: Biomass)

Bioretention area

A shallow, landscaped depression that receives runoff from impervious surfaces.


Wastewater containing faecal matter and urine. It is also known as brown water, foul water, or sewage. It is distinct from greywater, the residues of washing processes.


BRE Environmental Assessment Method (BREEAM) is a voluntary measurement rating for assessing the environmental impact of buildings. It comes in a variety of flavours according to the function of the building.

Breathable sheathing

A board which is vapour permeable enough to allow it to be used externally in timber frame construction.

Breather membrane

A membrane which allows vapour to escape whilst preventing water from entering the construction.

Breathing wall

‘Breathing’ walls allow a significant amount of water vapour (and other gases) to be absorbed and released quickly to the outside, thereby regulating the room climate and hence indoor air quality. Much debate continues over the value and physics involved.

Brown roof

Local soil and rubble forms a substrate on a low-pitched or flat roof, which is allowed to colonise naturally.

Building envelope

The outer shell that separates the the interior and the exterior environments.