Insulation cover to steel frames in roof

[Mark Wright]

Good day.

In an open attic roof, our engineer would like 2 steel 'gable frames' to support the steel ridge beam.

To give best head height they would ideally sit somewhere in the rafter / insulation zone.

The proposal at the moment is to have 90mm PIR above rafters, and 90mm PIR between rafters: U=0.15

If the steel frames were in the same plane as the rafters, they would only receive 90mm insulation cover.

I do realise that if there is any reduction in insulation thickness we are introducing a thermal bridge relative to the rest of the roof.

BUT! Is 90mm cover sufficient to avoid a significant condensation risk on the steel? (South UK)

Thanks, Mark.

Thanks.

[Mark Siddall]

If your only goal is to address condensation risk then, yes. 90mm PIR will do the job.

One additional thing to consider will be how the detailing prevents thermal bypass caused by any air gaps.

[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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