Spray foam insulation applied to the underside of a roof deck?

[stav]

On Easter we had my in laws over for lunch. As I was talking to my brother-in-law about various things, the topic turned to houses and energy efficiency. He bought a new house that has the attic sealed with the spray foam applied directly to the underside of the roof deck. I am no expert on this, but I was always told the underside of the roof needed ventilation to prevent the shingles from getting to hot or developing moisture underneath or some such. Looking at various places around the internet, I can see that it is indeed done this way, but I haven't discovered why this is ok and the ventilation is not needed with spray foam. So, knowing that all answers can be found on Woodnet, here I am. I was also wondering if it would be worthwhile to do this on an older (1950's) ranch with almost no slope to the roof?

[meackerman]

i thought it had to do with the attic now being part of the conditioned air space instead of the conditioned air space stopping at the ceiling. but I'm proly wrong on that.

[Mags®]

I've seen this a lot lately and have heard if you do this, it needs to be open cell foam to allow the substrate to breath. Also they stated if you had a leak with closed cell you would never know until you pulled off your rotted roof. With the open cell the water would penetrate and let you know you had a leak. I'm also curious to the pros and cons considering my attic is terribly insulated and this would be a viable option for me.

[jlanciani]

Google is your friend, search for "hot roof"; http://www.oneprojectcloser.com/cool-roof-vs-hot-roof/

[stav]

Very informative, but it didn't address the issue of having the air flow under the shingles as you need with fiberglass batts vs spraying the foam right on the roof deck.

At least I didn't catch it if it was there.

[EricU]

they have talked about this a couple of times in Fine Homebuilding recently. The one issue is that shingle manufacturers will not warranty a roof made without ventilation underneath. Also, code enforcement is not ready for it in a lot of places. I have seen an explanation of why it's not needed, and it seemed plausible

Basically, the one article just said to forget about it, and the builder just made a channel for air to go up and put some sheathing under that which took the foam. Seems like it works for them.

[Robert Adams]

Has anyone ever heard of a roofing manufacturer paying a warranty claim on an asphalt shingle roof?

Also spray foam isn't the only way. 4" foam on the roof before shingles is also done. Course a metal roof over foam is a far better choice.

[Curlycherry]

Robert Adams said:

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Has anyone ever heard of a roofing manufacturer paying a warranty claim on an asphalt shingle roof?

Also spray foam isn't the only way. 4" foam on the roof before shingles is also done. Course a metal roof over foam is a far better choice.

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40+ Years of roofing in our family's business and I have seen (first hand) one claim in all that time. ONE. They weasel, squirm, lie and cheat their way out of every single claim.

And plenty of roofing manufacturers do indeed allow for use on insulated panel decks. When I did my roof in 2006 on SIPS, I called GAF and they said they would allow it and even pointed me to the part of their installation guide that said it was ok. Do NOT rely on the local sales clowns at a building supply or roofing place to know, they don't. Those places are just roofers that got out of being installation monkeys. ALWAYS call the main office and ask to speak to a tech service rep.

[jamesglenn]

The answer to this depends a lot on the geographic location which dictates the type of spray foam and the best method to insulate.

Here in the northeast, we use close cell foam and a "hot roof" is not an issue because temperatures aren't nearly as high consistently as the southern states.

I have cell to the underside of roof ply in my home, however a certain thickness ('R' value) is required by code to prevent condensation in the winter since our issues are more during that time of year.

[Curlycherry]

They use closed cell foam here too. My friend just had it done to his new house two summers ago. They sprayed directly onto the bottom side of the OSB. He has floor joists (18") for roof rafters and they put in 4" of foam, added netting to the bottom of the joists and then filled the rest of the space in with cellulose.

My new roof on my addition is SIP (Structural Insulated Panels) with are 16" thick polystyrene foam sandwiched between OSB sheets. When I did it a lot of shingle manufacturers said they would not honor their warranty (they never do anyway) because of the "hot roof" issue. Many, and I do mean many people have shown that a properly insulated roof is the exact same temp as ambient temp. It is an air gap that gets warm (ie the attic space) and if there is no space, there will be no buildup of heat. Indeed my roof deck when being installed was the exact same temperature as the wall structures right next to it.

If it were me I would add 1/4" plywood to the underside and spray closed cell foam against that. That way if there is a leak the closed cell foam will let the water flow out to the eves where it could be spotted, yet only cause minimal damage. And also replacing the roof deck would not be hard because it would not be stuck to the foam.

BTW you can get gigantic saw blades for sawzalling foam away from stuck OSB and sheeting products.