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Question DetailsAsked on 1/4/2012

What criteria do I need to consider for a new roof where the rafters are spray foam insulated?

New custom house - we have heard some roofing companies won't warranty roofs with spray foam directly underneath.

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1 Answer


When the rafters are spray foam insultated, thus negating any ventilation, this is what is known as a "Warm deck" design. The term warm deck refers to the fact that the substrate (deck) is warm (insulated). By insulating the rafters, as I said you are negating ventilation, therefore the concern of the utmost importance is the warranty of the roofing manufacturer. I am assuming this is a shingle roof. I would HIGHLY recommend contacting the shingle manufacturer of your choice and inquire about their warranty limitations in regards to warm deck designs. They may warrant the design, but with limitations, usually in terms of warranty term. A 30 year shingle may only now be warranted for 15 years.

An expensive investment, but one that will not negate ventilation thus keep the shingle warranty intact and ensure the shingle roof lasts as long as intended, is the installation of a false deck. This can be done a number of ways. In the past we have laid 2" boards secured into the existing rafters over the existing roof deck. We would then sheet over this with plywood. Another easier option is a product known as Atlast Cross Vent. The false deck ensures proper ventilation beneath the shingle roof.

Attic ventilation is important with asphalt/fiberglass shingles for a number of reasons, but the way I explain it to my customers is like this: Think of your shingle roof like a candle. A candle is meant to burn from one end and shingles are meant to take heat from only one end. When you burn a candle at both ends it lasts only half as long, and when a shingle roof takes heat from both ends it too will only last half as long. This is certainly 100% true when it comes to non-vented attic spaces of the cold deck design (a cold deck design is a traditional shingle roof where the ceiling is insulated and the attic space ventilated). There are some claims this burning of both ends will not happen with spray foam insulated decks but in my opinion there is no real world data yet to prove or disprove these claims. We have accellerated data but only time will tell.

To tell the truth I am on the fence about spray foam insulated roof decks. I haven't decided if I am for or against this practice. Traditional roofing practices tells me not to do it, but as I said before only time will tell. I'm going to give it a few more years before I decide if I like it or not. ventilation aside, there are numerous other concerns that I have in regards to the spray foam insulated roof decks, but that may be a topic of an another discussion.


Answered 8 years ago by ReliableAmericanRoof

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