Insulation for Pitched Roof Sarking
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Taking a ‘fabric first’ approach to a building’s design means focusing on the thermal efficiency of each element and working to achieve low U-values using a thickness of insulation that is practical to install.
Fixing insulation to the outside of the structure is the best way to achieve the ‘tea cosy’ effect that reduces heat loss, but it can prove challenging in reality - especially on pitched roofs.
New-build projects are likely to offer fewer constraints and therefore more room to accommodate insulation fixed above the rafters. Large-scale refurbishment projects, especially ones featuring a complete re-roofing, may offer a similar opportunity.
An over-rafter insulation layer can help to achieve a well-insulated, airtight building with a useful and comfortable habitable room in the roof space. Installing insulation over rather than between the rafters eliminates concerns about thermal bridging and, overall, saves on the total thickness of insulation used.
The key is good detailing. It’s important to make sure the roof insulation layer connects with the insulation in the wall to achieve a continuous thermal envelope. Any breaks and the benefit of fixing the insulation in this way are quickly lost, in the form of heat escaping through the gaps at the junction.
Of course, combining the depth of both the rafters and the insulation can result in a substantial roof construction. Getting the detailing right also makes sure the total thickness has been accounted for at design stage and doesn’t cause any nasty surprises when it comes to building the roof. Low-energy construction does not have to mean compromised aesthetics.
Saint Gobain Insulation UK's lightweight, rigid polyisocyanurate (PIR) foam insulation boards are among the most thermally efficient, commonly-available insulation products on the market. Fixed over the rafters of a pitched roof, they can help to achieve the required U-value in just a single layer.
Where the benefits of over-rafter insulation are desired but practical restrictions prove difficult to overcome, you may consider insulating between and over rafters.
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|
|Celotex GA4000||50-100mm||Width 1200mm, Length 2400mm||0.022 W/m.K|
|Celotex XR4000||110-200mm||Width 1200mm, Length 2400mm||0.022 W/m.K|