[Mark Wright]

Thanks Mark,

Thinking about it more carefully, we have quite deep joists (175mm), and a friendly spray foam contractor.

In a warm roof, the thermal resistance of the between rafter insulation must not exceed the thermal resistance of the over rafter insulation layer, so:

100mm of Kingspan K7 phenolic insulation above the rafters: lambda = 0.020 (90/90), r= 5.00
150mm of Lapolla FL2000 closed cell PU spray foam between the rafters: lambda = 0.030, r =5.00
U=0.116 (PHPP, 20% timber in the foam layer) Foam density 30kgm3 Mass 4.5kgm2

90mm of Kingspan K7 phenolic insulation above the rafters: lambda = 0.020 (90/90), r= 4.50
130mm of Lapolla FL2000 closed cell PU spray foam between the rafters: lambda = 0.030, r =4.33
U=0.130 (PHPP, 20% timber in the foam layer) Foam density 30kgm3 Mass 3.9kgm2

I wonder whether to believe claims that closed cell PU foam can act as a vapour control layer and air barrier…

Mark.

[Author]

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