CLOSED CELL SPRAY FOAM FOR EXTERIOR ROOF INSULATION

[nh-dan]

Hey Everyone,

My original plan for the roof of my post and beam build was to apply ice and water shield over the entire roof followed by two staggered layers of foil faced polyiso foam boards then 2x4 vertical then horizontal to form a grid for the metal roofing.

I'm having trouble sourcing the polyiso foam at a reasonable price now and someone mentioned using closed cell spray foam on top of the roof.

Has anyone tried this or know of anyone who has?

If I decide to go this direction I would still put down the ice and water shield then add vertical boards to create channels for the foam then install the horizontal 2x4s on top of the channels for the metal roofing.

Any ideas would be appreciated
Dan

[Hilltop366]

I don't know enough about the spray foam to comment on it but putting the vertical boards on the inner deck will allow thermal bridging compared to on top of the sheet foam.

Perhaps a thiner layer of foam sheet before the vertical boards and spray foam would reduce the thermal bridging?

[nh-dan]

yes I was a bit concerned about the thermal bridging. I'll look into maybe putting down a thin layer of foam board first. I know I can get some 3/4" foil face at a reasonably price

[peakbagger]

Might be worth signing up at Heatspring.com and looking at this course Unvented Roofs without Spray Foam: The Rest of the Story (heatspring.com)
There is some pretty interesting information on moisture migration through closed cell foam and collecting up under the final roof membrane. The short version is even if there should not be moisture up near the ridge under the membrane, it appears to get there anyhow so plan on it happening and put in ridge venting. The work was done in New England and it was quite convincing that that bit of extra work is worth it.

[kantuckid]

In my area the metal roofing mfg sells that product your chasing. Mansea Metal with locations in KY & OH.