Hello, please advise what is the most thinnest insulation on the market now so that I can use it for the loft roof (dormer and slope roof on the other side), where rafters are 10cm. So 5cm will be air gap, and I only have 5cm to play with and would like to avoid reducing the head height as much as possible. Thank you!
You won't achieve 0.18U with 50mm of insulation, unless you used a Vacuum Insulation Panel (which are still very new, very expensive, and rather impractical for roof situations).
PIR is the most common material, with a lambda of around 0.021 at the very best, 100mm of this would give 0.21U. If you can afford the extra, Phenolic foam will give a slight reduction on the U value, but only slight.
Consider 50mm within the rafters and 50mm below, which would of course give around the same U value, but since the rafters would be covered, you will have reduced the effect of thermal bridging of the rafters themselves, which would give a better all round insulating effect.
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