3. Energy Conservation improved thermal performance
Green roof systems are recognized as providing greater thermal performance and roof insulation for the buildings they are laid on. This can vary depending on the time of the year, and the amount of water held within the system.
Cooling [summer] Poorly protected and insulated roofs can lead to substantial overheating of spaces beneath them. This can lead to the need for increased air-conditioning. A green roof not only acts as an insulation barrier, but the combination of plant processes [photosynthesis and evapotranspiration] and soil processes [evapo-transmission] reduces the amount of solar energy absorbed by the roof membrane, thus leading to cooler temperatures beneath the surface.
Research by Nottingham Trent University has shown the following:
Mean daily temperature 18.4C (65 F)
Temperature beneath membrane of normal roof 32C (89.6 F)
Temperature beneath membrane of green roof 17.1C (62.7 F)
A study conducted in Chicago, USA, recently estimated that building energy savings to the value of $100,000,000 could be saved each year if all roofs were greened, as the need for air conditioning would be reduced.
Thermal Insulation [winter] Green roofs can help to reduce heat loss from buildings during the winter when root activity of plants, air layers and the totality of the specific system create heat and thereby provide an insulation membrane. However the efficiency of green roofs as thermal barriers is dependent on the amount of water held within the system. Water retention can increase the amount of heat lost through the system and therefore any efficiency gains are dependent on daily conditions. It is therefore difficult to provide accurate figures on the net effect of green roofs on energy efficiency during the winter months.
The study at Trent University on the temperatures under membranes of standard roofs and those under green roofs also showed that green roofs appear to have a positive effect in winter.
Mean Temperature 0C (32 F)
Temperature under membrane standard roof 0.2C (32.36)
Temperature under membrane green roof 4.7C (40.6 F)
This shows that green roofs do have the ability to affect the temperature and insulation properties of roofs, though this is variable due to the daily conditions of the green roof. The potential for the cooling and thermal insulation properties of green roofs can have costs benefits for building owners/managers.