Help need advice! XPS between rafters under metalr

[DonL]

OK guys I need some advise

I had a bad open cell foam job done and need to remove it.

I have a lot of XPS sheets so I want to insulate

under the metal roof (unvented, old house 1920s)

using XPS between rafters and use "great stuff" spray to seal it up.

If I am very meticulous will this work well?

Thank you for your help!

[Marc]

I'd be concerned with the ability of the polystyrene material to tolerate high temperatures. Here's one such formulation rated for temps up to 221 degrees.

Marc

[DonL]

thanks Marc. It says the MSDS it's good for up to 75 deg. C. My metal roof is almost white and hardly ever goes above 50C outdoor surface temp. Under the roof the planks also insulate a little. My laser temp shows 39C this summer in uninsulated spots under roof. I think possibly it could up to 55C under roof but I have never seen it get that hot. The color I believe has a lot to do with it. My black stairs facing west easily reach 65C on surface.

My major concern as a newbie was the void behind the XPS board. Is a void behind the insulation a problem for condensation?

I thought if I am very careful and seal it up nicely it should be OK

I was wondering if you guys have seen such an insulation job or whether voids were a problem.

With my current open cell spray foam there are many voids which appear to be causing condensation.

I think a more impermeable material should be better.

Or, to play it safe as you say Marc I could buy better sheets with higher temp rating for the first layer and then use cheaper XPS over it?

As long as the void isn't an issue, I am relieved