Celotex Isover

Insulation for Between and Over Rafters

Application image
U-value Calculator
Start your U-value calculation >
Use the calculator

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.

Where the benefits of over-rafter insulation want to be enjoyed while still creating a useable room in the roof space, it’s possible to insulate over and between the rafters. Due to the thermal bridging effect of the timbers, the overall thickness of insulation used to achieve a low U-value is not as efficient as over-rafter insulation only, but it remains a useful compromise solution.

To protect the roof from the potential of condensation occurring unseen within the construction, it’s important to remember that the layer of insulation installed over the rafters should, as general rule, be double the thickness of the insulation layer between the rafters.

An over and between rafter insulation solution can be a feature of a new-build project. It is also suitable for retrofit with a substantial renovation of the roof. The loft space can be used as a habitable room.

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 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. This can be supplied in a range of thicknesses to suit typical rafter dimensions the 1200mm wide rolls can be cut to suit the exact space available horizontally rafter-to-rafter (typically cut oversize by 10mm to allow for a friction-fit between the rafters). In addition ISOVER GMW has inherent acoustic benefits.

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


Insulation products
Product Name Thickness Range Sizes Lambda BBA
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
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
ISOVER Metac 50-200mm Width 1200mm, Length 3200-9300mm 0.034 W/m.K Tick Icon