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Roof Retrofit: Insulating Eaves and SIPs

zetetic | Posted in General Questions on

We are in Climate Zone 7 (>9000 heating degree days) and see up to 13 feet of snow on the ground, and 300 lb/sqf of snow load on a 6:12 pitch roof.

Our current roof is R4 (not a typo).  It was built in 1968 and consists of 4×6 T&G that serves as both a cathedral ceiling AND the roof sheathing.  We get impressive ice dams – to be expected.

We plan on insulating the roof to R49 or better for Zone 7.

The eaves are 48″ long.  Is there any need or benefit to insulate the eaves?  Will we still see ice dams once we increase the roof insulation from R4 to R49, but leave the eaves uninsulated?

We are currently thinking of the following roof stack inside-to-outside:

* Conditioned interior (existing)
* 4×6 T&G ceiling and roof sheathing (existing)
* Grace Ice & Water (air / vapor barrier barrier)
* Raycore R52 SIP with 2×8 rafters embedded 24″ OC
* 5/8″ plywood sheathing
* #30 felt
* Composite shingles

Yes, I realize the SIP is sandwiched between two impermeable layers, and I did read:

Sandwiching Roof Sheathing Between Two Impermeable Layers

At the end of the above article, it states “sandwiching roof sheathing between low-permeance layers is still relatively safe, as long as the sheathing is dry on the day that the spray foam is installed.”

From what I’ve read, it seems better to dry to the outside if you can because you can more easily achieve a more air tight structure.  That said…

Would this stack be better:

* Conditioned interior (existing)
* 4×6 T&G ceiling and roof sheathing (existing)
* Raycore R52 SIP with 2×8 rafters embedded 24″ OC
* 5/8″ plywood sheathing
* Grace Ice & Water (air / vapor barrier barrier)
* #30 felt
* Composite shingles

We could increase the thickness of the roof by another 4 inches and use Rockwool Comfortbatt instead of the Raycore SIP.  This design would cost less than the Raycore SIP, but the extra thickness will look chunky.

Besides cost and roof deck thickness, is there any reason to choose mineral wool over the Raycore SIP (polyurethane)?

I know this is a 3-part question actually.  If you got this far, thanks for reading, and thank you even more for any feedback.

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