woodsmith
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Sealing the attic is very difficult to do effectively as you would need to be absolutely thorough sealing every gap but the big problem with adding insulation like this is condensation. Unless you have a vapour barrier fitted in the ceilings of the rooms below, warm moist air will permeate into the loft. With a ventilated loft space this is not a problem, but once it is sealed in it wants to condense on the coldest part of the loft, which is often the rafters. You can even get condensation forming inside the timber (interstitial condensation) which rots the timber from the inside out.
It is possible to add insulation to the roof but I think you would be better off using a rigid foil sided insulation board like Kingspan. You would need to ensure that you provided plenty of ventilation on the cold side of the board though, so you may need add ventilation tiles to the ridge and create ventilation at the eaves. Altogether with the cost of the insulation this would be a very expensive project, which is why insulation is normally added to the ceiling and the void is left cold
As for rolls of foil insulation, they come in many forms and I've used a couple but they appear to be next to useless. There are several houses in our village with this type of insulation and they rarely have any frost or snow on the roof in the winter.
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