Celotex Isover

Insulation for Between Rafters

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The benefits of insulating a building around the outside, protecting the structure and effectively creating a ‘tea cosy’ effect, are well publicised. When it comes to pitched roofs, however, adding a thick layer of insulation to the rafters can result in a depth of roof construction that may not suit every site, meaning over-rafter insulation (sarking) only is not a viable option. Internal restrictions, like maintaining sufficient head height, can mean that fixing the insulation between and under the rafters is not an option either.

So whether it's a new-build project or if you're creating a habitable area when renovating an existing loft space, the insulation line will need to be within the pitched (sloped) part of the roof – so insulation will need to be installed between the timber rafters to provide protection to the newly-created habitable area from external temperature fluctuations.

Celotex polyisocyanurate (PIR) foam insulation boards are easy to cut and handle. Once up at roof level, the between-rafter boards can be cut accurately to fit the rafter spacings. Being lightweight, they do not impose significant additional loads on existing structures. Where the insulation does not fill the total rafter depth, the low-emissivity foil facings can gain additional thermal benefit from the air spaces they face into.

ISOVER glass mineral wool (GMW) is another option for the between-rafter insulation either as a roll or batt. The flexible nature of GMW means that the roll or batts can be accurately-cut and friction-fitted between the rafters – thereby minimising air gaps and maximising thermal performance. In addition to this ISOVER GMW also has inherent acoustic benefits. The underside of the rafters will need to be finished with a suitable vapour control layer (VCL) and plasterboard to provide a suitable internal finish to the roof.

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.


Insulation products to consider
Product Name Thickness Range Sizes Lambda BBA
ISOVER Metac 50-200mm Width 1200mm, Length 3200-9300mm 0.034 W/m.K Tick Icon
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon