Traditional roof green roof n sensible et r q q q substrate 0 where r n net radiation q sensible sensible heat flux due to convection q et.
Green roof r value.
R value of various materials.
Green roofs save energy compared to uninsulated roofs or poorly insulated roofs or even better black poorly insulated roofs.
Increased r value a measure of the resistance of a material to heat flow of the roofing system along with reduced temperatures on the roof lessen hvac loads resulting in energy cost savings.
A green roof may improve property values and marketability especially in urban areas with little green space.
For example if you have a type of insulation that has an r value of r 5 per inch of thickness you will need a depth of 6 inches of this insulation installed in your attic to reach r 30 if you live in zones 2 or 3.
For example a code may require that skylights have only an r 5.
Codes may specify a smaller r value for these components compared to the rest of the roof.
Green roofs can have lower energy cost than conventional roofs and sometimes compete well against white roofing effects are more pronounced for lodging buildings.
Green roofs reduce the temperature of the building and the surrounding air in multiple ways.
Here s a handy r value table that covers most types of roof coverings building insulation and a few other materials.
Let s imagine a roof that should have an r 30 value.
R value minke et al.
They provide shade to the top of a building evapotranspiration.
These r values are a sum meaning this should be the total r value once you add up the entire depth of insulation.
Plants need water to grow so the green roof is designed to retain some degree of moisture.
2003 evapotranspiration schmidt 2003 berghage et al.
Completely dry growing media the dirt of the green roof so to speak has a minimal r value typically about 0 5 inch.
The plants in a green roof absorb water through their roots and then use surrounding heat from the air to evaporate the water while some roofs can reach temperatures of up to 90 f in the summertime these two features can allow.
R value of about 2 5 ft f btu.
In other words go above r 20 and make green roof decisions for other reasons than energy.
You should of course see the manufacturer provided technical data sheet for the r value of specific products.
Once you have more than r 20 in a roof assembly that is you meet the code things pretty much don t matter.
Skylights and roof hatches are another component of roof thermal performance.
Neglected thermal storage moisture dependent performance and.
For a very good introductory overview article on r value see r values over at the website of.
Every construction design architect engineer a e knows a material s r value is an important factor when specifying insulation products with many regulations requiring minimum values for wall ceiling and roof assemblies.
