The general advice we give is that you should always insulate between and above the rafters (warm roof) or between and under the rafters (cold roof). On our U-value calculator, we don’t give an option for putting insulation between the rafters. But why not?
Building regulations
The Approved Documents L (for England & Wales) and Technical Handbook 6 (for Scotland) give guidance on how to meet the energy efficiency requirements of the Building Regulations and Standards.
Within these documents, there are U-values that should be met in order to comply with the relevant Regulations and Standards.
If we look at refurbishing a pitched roof at rafter level, in England, Wales and Scotland the maximum (worst case) U-value that should be achieved is 0.35 W/m2K.
However, where possible, you should look to meet the area-weighted values below:
England & Wales - 0.16 W/m2K
Scotland - 0.12 W/m2K
Click here for further information on the energy regulations in England, Wales and Scotland.
If a ventilated roof is being constructed, there is a requirement to have a 50 mm fully ventilated air gap between the insulation and the sarking felt, in order to avoid condensation. If you are constructing an unventilated roof without the use of counter battens, space would need to be left for the breathable membrane to drape, both of which would impinge on the space available for the insulation.
Rafter depth
Common rafter depths range from 100 mm to 200 mm. This limits the maximum thickness of the insulation between the rafters to between 50 mm and 175 mm. This generally would not be a sufficient thickness to achieve the U-values required to meet Building Regulations or Standards and more insulation would need to be added.
Adding another layer of insulation, either above or below the rafters gives another key advantage – it reduces the effect of cold bridging through the timber rafters. Obviously, heat will be lost through the rafters at a faster rate than it is lost through the insulation. The amount of heat loss through cold bridging can be minimised by using another layer of insulation either above or below them.
So if you are insulating a pitched roof, we would recommend having insulation either above or below the rafters. For more details about these methods of construction and the U-values that can be achieved, take a look at our article on insulating a pitched roof at rafter level.
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