Foam Insulation Over Vaulted Ceiling

DaveM

Homeowner has a vaulted ceiling over the living room/dining room area (24x24) with exposed rafters. The rafters are 4-feet on center with tongue and groove 2x6, 1/2 inch plywood and asphault shingles. In the process of quoting a re-roof, the homeowner asked if it was possible to add insulation during the process. I have heard of folks using 2-inch foam over the current sheathing and using 2-1/2 inch roofing nails to secure the shingles over that, but this doesn't sound sturdy enough to me. Is there a standard method for insulating over the current sheathing?

Thanks!

Grumpy

I'd just use a recovery board. It is foam inisulation with a plywood deck factory adhered to it, with air pockets and everything in the plywood deck.

No matter what... I would want my roofing nails to go through some kind of wood, not through foam then wood. This means if you do place another insulation board you should also place another plywood.

Honestly I hate working on houses with this type of roof system.

dsh

You can find a lot of good info on this website. Think there is another article on this site that had details on screwing the foam down with plastic washers. Don't have time to look for it right now.

buildingscience com resources roofs unvented_figures. pdf

Don't have enough posts so won't allow me to post the url. Above is kinda butchered, but you can probably figure it out.

Cole

http://buildingscience.com/resources...ed_figures.pdf

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AaronB.

I have gone over hundreds of roofs in my career with T&G then foam then the roof covering. Never had a problem related to anything other than heat sink.

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jmorgan

We can never do this. The heavy snow & ice load around here causes the roofing nails to bend in the foam, from the shear forces. You can see the "waveyness" in the course lines after time. We have to add sleepers and a new plywood deck. We put the foam in between the sleepers and lay the plywood on top. This is a common retrofit around here with older condo complexes.
Jim