Measures the degree to which glazing blocks heat from sunlight. The G-value is the fraction of the heat from the sun that enters through a window. G-value is expressed as a number between 0 and 1. The lower a glazing’s G-value, the less solar heat it transmits. 


A waste treatment process where waste is heated to produce a combustible gas that can be burned in excess air to generate heat. 

Geothermal energy

Is heat derived from the earth. See also ‘Ground Source Heat Pumps’


Permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain.

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.

Green Guide to Specification

Now published online, the BRE’s ‘Green Guide to Specification’ rates a large directory of construction elements according to their environmental impact. The ratings are based on Life Cycle Assessments using the BRE’s ‘Environmental Profiles’ methodology. The ‘Green Guide’ is closely integrated into the BREEM and Code for Sustainable Homes assessment tools. Some of the ratings are much derided by many in the design community. (www.thegreenguide.org.uk)

Green Register

The Green Register is an independent organisation promoting sustainable building through training and events. (www.greenregister.org.uk/)

Green roof

A roof of a building that is partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane.

Greenhouse gases

Gases in the earth’s atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The main greenhouse gases in the atmosphere are water vapour, carbon dioxide, methane, nitrous oxide, and ozone.


A term used to describe the practice of companies disingenuously spinning their products and policies as environmentally friendly.


Wastewater generated by domestic processes such as washing and bathing.

Ground source heat pump (GSHP)

A system that extracts heat from the ground, upgrades it to a higher temperature and releases it where required for space and water heating. 


Water that is naturally located beneath the ground surface. It supports ecosystems as well as providing a source of drinking water. Under threat from water demand, pollution and climate change.