Spray Foam insulation under roof decking

federer

Hey everyone. some of you know me from my crazy roofing project last year. Finally getting around to insulation now! So I have a few noob questions.

1. I am supposed to put in open cell foam under the roof deck. would closed cell be better? contractor says to use open cell in case the roof leaks.

2. the attic is part of the conditioned space. so after spraying the foam, can i just drywall it up or should i add a vapor barrier?

please advise!

Windows on Wash

Open or closed does not really matter. Open will allow the water to pass through at some point whereas closed will not but that is really only helpful if you are trying to locate the leak (something that most good roofers can do from the top side).

Closed cell has a higher R-Value per inch so if you can afford the offset in cost, that is the better option.

No need for a vapor barrier in this region and the foam will that in this case anyway. Painted drywall is more than enough vapor retarder in this region.

Make sure you foamer knows what he is doing and don't go for the cheapest as a default...you will regret it. There is a bunch of suspect foamers out there that are making stuff in their garage that is untested, unproven, and crappy quality.

Make sure you are using the right stuff.

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federer

thanks for the input. well foam is pretty expensive so i think we are going with open cell. are you a general contractor in the area?

shazapple

Spray foam to the underside of the roof deck is not recommended. Blocking off your roofs ventilation will lead to overheating the shingles and lower shingle life span. Also, it typically voids or limits your shingle manufacturer warranty.

Windows on Wash

shazapple,

This is completely incorrect. You have been reading too many pdfs from the roofing suppliers.

Ventilation is for moisture control and has nothing to do with shingle temperature or life. Shingle manufacturers have been trying to prove this myth unsuccessfully for years.

Shingle data in Las Vegas (read...desert and extremely high radiant heat amounts) shows only about a 2 degree increase between a completely vented assembly and a closed up/unventilated assembly.

Don't quote me on the percentages but something like 98% of shingle cooling happens to the outside surface.

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Windows on Wash

Yes I am. Feel free to email me directly for any other questions. I can point you in the right direction.

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Msradell

I definitely agree with Shingles that this is completely untrue! All recent documentation supports this position, in addition I can speak directly from experience. Part of my home has a very low pitch roof and the rest a very high pitch. The low pitch has closed cell foam sprayed directly on the roof deck and the other section has 18" of blown in fiberglass (the two areas are completely independent). The shingles in both sections look exactly the same and neither has deteriorated different from the other.

Another advantage of the foam on the roof deck is that it's much easier to work in your attic because the temperature control is better and you're not constantly getting insulation on yourself.

AGWhitehouse

Me three...ventilation is for heat & moisture control, though mainly moisture...not shingle life. If "rubberized" material needed ventilation for longevity then flat roofs would require it too and not just sit right on the insulation boads.

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federer

thanks for the input guys. it is also my understanding that the insulation against roof deck would be ok for the shingles. in fact my roof is flat on one side and sloped on the other half. also, the attic itself is actually the upstairs space, so we have closed off all the soffit vents to prevent any ventilation with the closed roof system (i forget what it's called). essentially the entire house becomes part of the conditioned space, including the attic space, thus not requiring any ventilation.

federer

so here is the proposal from the contractor: what do you guys think?

Attic
Icynene foam 5.5"

Gable Wall
Icynene foam 3.5"
Flameseal thermal barrier

Windows on Wash

How much per sq/ft is he asking for?

I would prefer to have more on the roof deck where I can get it but if you have 2x6 construction, that is as much spray foam as you can get up there.

Maybe a layer of rigid foam to break up the thermal bridging of the studs.

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Msradell

My first question is what type of Icynene foam is he going to use? They vary from R-3.7 to >R-6.0 and obviously that would make a considerable difference in your project.

HomeSealed

I'm late to the party, but I'd also agree with the "shingle-life" argument being bunk... In fact, the larger shingle manufacturers seem to be backing off a bit on that.

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federer

you know i am not sure they specified. i assumed icynene is just open cell. i didint know there were different R values for icynene. i will try and find out! thanks

Windows on Wash

Anything near R-6 is going to be closed cell.

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