Why you can’t just insulate between the rafters.

K18 over K7

In this blog post we are looking at insulating a pitched roof and why just putting insulation between the rafters wouldn’t be a viable solution.

We’ve looked at pitched roofs before but this was a specific query which came from some feedback on our U-Value calculator where we were asked why we didn’t give the option of just insulating between the rafters on a pitched roof.

Our U-value calculator, and the advice we give, is that you should always insulate between and above the rafters (warm roof) or between and under the rafters (cold roof). We don’t give an option for just putting insulation between the rafters.

But why not?

Building Regulations
From Part L of the Building Regulations (England and Wales) or Section 6 in Scotland, we recommend that the U-value of a pitched roof, insulated at rafter level should be 0.18 W/m2.K for a domestic refurbishment. For a new build this should be 0.11 W/m2.K in England and Wales and 0.15 W/m2.K in Scotland.

If a ventilated roof is being constructed then there is a requirement to have a 50mm fully ventilated air gap between the insulation and the sarking felt, so as 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 100mm to 200mm. This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm. This generally would not be a sufficient thickness to achieve the U-Values required to meet Building Regulations and more insulation would have 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 blog post on insulating a pitched roof at rafter level.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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