Radon is a colourless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium. High levels of radon can build up when houses and other buildings are constructed in areas where the underlying geology contains Uranium 238 (typically, but not exclusively granite and limestone areas).

Rainwater harvesting

The gathering, or accumulating and storing, of rainwater.

Rammed earth

A construction technique where earth is compacted between formwork to make a homogeneous mass wall.

Reclaimed material

A material that is recovered from a building as part of the deconstruction process.


The processing of used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution (from incineration) and water pollution (from landfilling) by reducing the need for "conventional" waste disposal, and lower greenhouse gas emissions as compared to virgin production.

Reflective insulation (radiant barrier)

A material typically composed of a core material such as fibreglass or polyethylene foam sandwiched between sheets of aluminium foil. Manufacturers claim that the material can stop up to 97% of radiant transfer. However, much debate continues around both the efficacy of the claims and testing methodology.

Relative humidity

The amount of water vapour that exists in a gaseous mixture of air and water. Relative humidity is normally expressed as a percentage.


The removal of pollution or contaminants from soil, groundwater, sediment, or surface water for the general protection of human health and the environment.

Renewable energy

The energy generated from natural resources—such as sunlight, wind, rain, tides, and geothermal heat—which are renewable (naturally replenished).

Renewable resource

A natural resource that has the capacity to be naturally replenished despite being harvested (eg forests, sheep). Limits to renewable resources are determined by flow rate and such resources can provide a sustained yield.

Resource depletion

Resource depletion occurs when all of the natural resources in an area, both renewable and non-renewable have been exhausted.

Retention pond

A type of constructed wetland that is designed to contain storm water or rain run-off from a small surrounding drainage area that would otherwise flow into other areas.

Reuse, Reclaim, Recycle

What to do with 'waste'.


The water flow which occurs when soil is infiltrated to full capacity and excess water, from rain, snowmelt, or other sources flows over the land.


The capacity of a material to resist the transmission of heat. The R-value is calculated by combining the lamda value (thermal conductivity) and the thickness of the material. Hence R=t/λ, where 't' is the thickness. Units are measured in m2K/W. Used in connection with insulation, the higher the R-value, the more effective the insulation. The R-value is also used to calculate the U-value (thermal transmittance) where U = 1/R