Celotex Isover

Insulation for Between and Under Rafters

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Arguably the most popular pitched roof solution, installing insulation between and under the rafters uses the depth of the structural timbers to best effect. Its versatility is the key to its popularity, whether being used to achieve:

  • an efficient room-in-a-roof in a new-build property where over-rafter insulation is not desirable
  • a refurbishment of an existing roof with head room to spare below the ceiling, or
  • a project converting a loft space into a room in the roof

It allows insulation to be installed in a watertight roof, either after the roof covering has been fixed on a new build, or without disturbing an otherwise sound existing roof covering as part of a renovation.

Modern U-value targets mean installing insulation between rafters only is rarely a viable option. Using a thermally efficient material to get the maximum benefit from as much of the rafter depth as possible means the under-rafter insulation layer can be kept to a minimum, making installation easier.

Rigid polyisocyanurate (PIR) foam insulation boards from Celotex are among the most thermally efficient commonly available insulation materials on the market. Depending on the specifics of the roof, it may be possible to fully fill the rafter depth or, in most cases leave a 25mm or 50mm air gap between the top face of the insulation and the sarking membrane.

The level of thermal performance required then dictates the thickness of insulation required below the rafters - usually anything from 25mm to 65mm of PIR foam. That can be a general application board followed by a separate plasterboard. Alternatively, using a thermal laminate featuring rigid PIR insulation pre-bonded to a plasterboard means fitting the insulation and ceiling lining in one operation, reducing storage, cutting and fixing.

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. In addition, ISOVER GMW also has inherent acoustic benefits that will help to improve the acoustic comfort of the room-in-roof area as well as the thermal comfort. Once installed, the rafters will be finished in much the same way - either with a thermal laminate or with a general application PIR board followed by a separate plasterboard.

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 PL4000 25-65mm (+12.5mm Plasterboard) 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
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon