Inspiration Birmingham 2020

The design specification for the attic roof was to insulate to achieve a U value of 0.12 W/m2K. To achieve this we proposed to insulate between and under the roof rafters utilising the full depth of the rafters (as the roof was re-tiled we were able to specify a breathable sarking membrane removing the necessity to provide a 50mm ventilated cavity below the sarking). This was achieved using 100mm of phenolic insulation (Kingspan K7 board) between the rafters and 92.5mm Kingspan K17 insulated dry lining board under the rafters (80mm phenolic insulation board plus 12.5mm plasterboard). To hit the 100mm we had to fix counter battens to most of the rafters to extend their depth. Unfortunately, additional roof rafters had been fitted alongside the original rafters (probably when the loft was converted in the 1980s.) These caused lots of problems when it came to cutting and fitting the insulation as can be seen in the image above.

The old velux windows in both attic bedrooms and landing have been replaced (new U value 1.4W/m2K). To reduce cold bridging and heat loss between the window frame and roof structure a Velux insulation collar was fitted. The underfelt collar can be seen in the image above. Note also the Kinspan K17 insulation board fixed under the roof rafters

The image above shows the Kingspan thermal board fitted prior to a plaster skim being applied. To minimise the risk of interstitial condensation occuring it is important to include a vapour control layer on the warm side of the insulation. A benefit of using Kingspan K17 is that the vapour control layer is built into the product.