How-To Install two layers of EcoQuilt Expert Insulation Under Rafters for maximum insulation value

This video displays how to install 2 layers of Multifoil Insulation under rafters in less space, cost and installation time compared to bulky PIR insulation such as Kingspan / Celotex.
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I am a big fan of multi-foil insulations but most contractors are afraid to use it. What is shown here is the retrofit installation in an existing roof and the system is highly beneficial because the over all roof thickness is kept to a minimum, maximising the interior space when compared to traditional systems of insulation. I always specify that the joints between the multi-foil sheets are taped to ensure that the system is vapour proof to prevent interstitial condensation. Multi-foil insulation values cannot be calculated like traditional materials because they work in a different way and have to be assessed empirically, for this reason I have found some scepticism in Bldg Ctrl and clients.
In a new roof construction multi-foil (by other manufacturers) can be used as a sarking that then saves even more space because the second layer is installed like the first here and then the plasterboard is fitted to the rafters without the need for counter battens. Fixing plasterboard and battens I have found is difficult with drilled screws because the layers of multi-foil wrap around the screw preventing it from driving in. I therefore use/specify stainless steel ring shank nails that are more than adequate for the job as little loads are experienced.
Although multi-foil is an expensive sheet compared to traditional insulation, the big benefit is that it can allow smaller structural sections to be used. I have had the situation where to use traditional insulation the structure needs to be 200mm thick just to accommodate the insulation but with multifoil the timber structure need only be 75mm thick, this was the case on my own house where the existing roof rafters are 3"x2" and using standard insulation the roof extension would have meant unsightly steps in the roof internally or externally. Using multi-foil I achieved the required 'U' values and could revert back to the shallower rafters. All of that saved money, which more than offset the extra cost of the insulation..