Rooflight insulation & airtightness
February 23, 2015
We have also now added some additional insulation around the inside of the two rooflight upstands, and taken the airtightness membrane up to the top edge. Although the rooflights both arrived already fixed to a GRP pre-insulated upstand, and both of these upstands are over-insulated externally with the 300mm deep roof insulation, we wanted to improve the thermal performance as much as we could to this often problematic detail, so we decided to insulate internally as well.
We bonded shaped timber battens onto the GRP upstand so as to give us a fixing position for the rooflight lining later, and insulated between these battens. In some places we used two layers of 18mm plywood screwed together where the thickness required was only 36mm.)
You can see in this photo that we didn't insulate between the roof joists in order to maintain as much true 'warm roof' construction as possible to eliminate the risk of interstitial condensation.
We then took the membrane up into the rooflight void and taped it to the GRP at the top.
Later we will dress the inside of the rooflight void with a Fermacell lining, fixed through the membrane into the timber supports now in place behind.