Carbon neutral

Conceptually, a state whereby the CO2 generated by a process is exactly balanced by the amount of CO2 either offset or sequestered by the process. A carbon neutral building is one that either uses no fuel that generates CO2 or where its consumption of CO2-generating fuel is equally balanced by exported renewable energy. The definition continues to be debated as to the extent of direct / indirect CO2 that is included in the equation – eg CO2 generated in the construction of the building. See also ‘Zero Carbon’

Carbon sequestration

The deliberate removal or storage of carbon in a place (a sink) where it will remain. Carbon Sequestration in construction usually refers to building products derived from plant materials such as wood and hemp, where CO2 is absorbed as part of the growing process. The carbon remains 'locked' in the material for the lifetime of the building.

Carbon sink

See: Carbon sequestration.

CarbonLite Programme

The AECB's Carbon Literate Design and Construction Programme (see also: CarbonLite Programme)

Χ -(chi) value

Point thermal bridge heat loss coefficient measured in W/K

Chloroflourocarbon (CFC)

A chemical compound made up of carbon, fluorine, and chlorine. CFCs were until recently used as blowing agents in the production of insulation (as well as in refridgerators). Their use has been phased out because of their contribution to depletion of the Ozone layer.

Closed loop-recycling

A system by which a given mass of material is remanufactured into the same product.

Coefficient of performance (COP)

COP is usually referred to in measuring the efficiency of heat pumps. It is the ratio of the heat energy output of heat pump versus the amount of electrical energy input.


The use of a heat engine to generate both electricity and heat simultaneously. (See also: CHP below)

Cold bridging

A discontinued term for thermal bridging.

Cold spots

Areas either on the inside or outside of the building envelope, whose temperatures are significantly lower than the surrounding area. Usually only apparent through infrared thermographic survey they can have a number of causes including areas that have simply been shaded from the sun or, more importantly, areas of differential heat loss occurring through the building fabric. Often a cold spot on an internal surface will be matched by a ‘hot spot’ on the exterior – representing a thermal bridge where opposite, or air leakage by a more complex path through the structure when not opposite.

Combined Heat and Power (CHP)

Works based on the principle that electricity generation produces heat as a bi-product. CHP was developed as a way of capturing and using the ‘waste’ heat. CHP plants have been successfully established at a variety of locations and scales in recent years, but recent research has been concentrated on developing CHP units for domestic use. (See also: Micro CHP)


The combination of food and brown waste that is decomposed through aerobic decomposition into rich soil.

Composting toilet

A self-contained unit that treats waste using aerobic decomposition (composting). The output compost is best used for fruit trees and bushes rather than vegetables.

Cradle-to- *

Life Cycle Analysis (LCA) is often broken down into phases of lesser ambition. Where recyclable / reusable products are the subject, the entire analysis is referred to as ‘cradle-to-cradle’. For non-recyclable materials that are destined to be disposed of, the complete analysis is referred to as ‘cradle-to-grave’. Expressions such as ‘cradle-to-gate’ or ‘cradle-to-site’ refer to production from extraction of raw material and factory production; and extraction, factory production and delivery to site, respectively – though these LCAs are useful and more common, they tend not to tell the whole story.

Cross-laminated timber (CLT) panels

Pre-fabricated load bearing timber panels that are used in timber structures. Originally developed in Switzerland in the 1970s, products are now beginning to be imported from central Europe and Scandanavia. The panels are produced from softwood strips (eg spruce) that are stacked crosswise on top of each other and glued to each other to create a distinctly strong structural element. Fabricated off-site and quick to erect, the resulting panels can be used to form complete floors, walls and roofs.