| radon | an odourless gas that passes from some soil types into buildings and may cause cancer. |
| rainwater harvesting | the facilities for collection and re-use of rainwater can be designed into a new building relatively easily. at the simplest level, rainwater can be collected in a water butt for garden use - subject to careful positioning of down pipes. more advanced rainwater harvesting systems can provide water supply for a range of domestic uses, including personal washing, laundry and toilet flushing. |
| rammed earth | a wall-building technique, by which a mixture of earth, water, and usually a small amount of cement, is very forcibly tamped inside formwork. the resulting wall has high mass, so it works well in hot climates. it often needs no exterior or interior covering, thus saving materials. |
| reed bed filtration | a reed bed filtration system is a sewage treatment system, which is constructed after a septic tank to improve the cleanliness of septic tank discharges. some industrial effluents can also be treated by reed bed systems. the reeds planted in gravel supply, via their roots, oxygen to bacteria that live in the gravel. the bacteria clean the sewage allowing treated, clean water to be discharged to a watercourse without harm to the environment and wildlife. |
| renewable energy | the use of energy from a source that does not result in the depletion of the earth's resources whether this is from a central or local source. |
| renewable resource | a resource that is capable of being replenished through natural processes or its own reproduction, generally within a time-span that does not exceed a few decades. technically, metal-bearing ores are not renewable, but metals themselves can be recycled. |
| renewables | energy producing technologies using natural resources in an inexhaustible manner, e.g. wind, solar, wave, tidal and hydro. |
| retention pond | a pond where runoff is detained (e.g. for several days) to allow settlement and biological treatment of some pollutants. |
| rsl - registered social landlord | a term describing not-for-profit organisations that provide social housing and are registered with the welsh assembly government. examples include housing associations, housing trusts, housing co-operatives etc. |
| rubble masonry: | the term describes many different types of masonry, the main types being random rubble (irregularly shaped stone elements, typically as it comes from the quarry) either coursed or uncoursed, and squared rubble (more regularly shaped stone), either coursed or uncoursed. much of the older housing stock in se wales is partially made from rubble masonry. |
| runoff | water flow over the ground surface to the drainage system. this occurs if the ground is impermeable, is saturated or if rainfall is particularly intense. |
| r-value (rsi) | a unit of thermal resistance, the opposite of thermal conductance. the higher the r value, the greater the insulating quality. the si units for thermal resistance are km²/w. r-values are commonly used to characterise thermal insulation materials in buildings. in this context, the unit is often written as rsi (for r-value système international), and a specific value such as 5.53 may be indicated as rsi-5.53, but may also simply be written as r5.53. one tenth of an rsi is known as a tog. |