[John]

We have an insulation contractor in the area that does the expanding, spray in place foam insulation. On houses with the roof system forming the ceiling in the upstairs, such as in a 1 1/2 story house, he recommends spraying the foam directly to the bottom of the roof sheathing. He claims an air gap under the shingles is not needed with his very special product. I find this hard to believe, as does our very big roofing material supply house. Any comments on this one? I say we may have some burnt up shingles in 5 years or so.

 

[pondman]

Yep !!!!

 

[jmorgan]

Crazy. Shingles will overheat.
Jim

 

[Grumpy]

You need a recovery board placed ontop of the roof deck, before the shinles are installed, or the shingles will be toasted.

I've used recovery boards on vaulted ceilings when there was no ventilation. It's not the best solution, but it will work. The best solution IMO, other than not coating the bottom of the roof deck, is to build up a secondary roof deck built over the primary roof deck. This can be done by laying 2x2 over the existing roof deck directly over the rafters, then laying a sheet of minimum 1/2" plywood over the new 2x2. You will have 2" of air flow under all your shingles.

 

[Grumpy]

On second thought, this insulation contractor should install some kind of paneling, perhaps 1/4" plywood on the bottom of the roof rafters before applying his foam insulation. This would be the best solution actually, but still wouldn't address the issue of moisture in the air.

Other than the moisture issue, if some kind of paneling was installed before the insulation was sprayed in place, there would be air flow under the roof from soffit to vents, just like a vaulted ceiling.

 

[jmorgan]

Good idea. Double decking will work great.
Jim

 

[AaronB.]

hmmmmmm. as I think about this, I do not think this would have an adverse effect on the roofing, in theory, as the hot, moist attic air would never come in contact with the underside of the roof deck. Would have to be permanently seamless i.e. no cracking...ever. I still do not think it is a good idea, as you only want the climate control within the building envelope. .....no purpose. Spray the top of the ceiling, I say.

 

[Teetorbilt]

Pipe coughed up this site http://www.buildingscience.com/ it's worth a looksee. There are some articles that give the pros and cons to conditioned vs. unconditioned attic spaces, insulation, etc.

 

[John]

Wow. This is what this guy has to say about the insulation under the shingles or under the deck.

"Lets now talk about durability of shingles and shingle temperature. Venting or non-venting a roof has about a 5 percent impact on shingle temperature and roof sheathing temperature and even less on shingle durability. The color of the shingle is more important than venting or non-venting. And temperature is less important than the shingle getting a sunburn. The biggest impact on shingle durability is ultra-violet light. UV is more critical than temperature. The best roof for hot, humid climates for all applications (including unvented attics) is a concrete or clay tile roof. Period."

Hope he knows what he is talking about. I wonder what the shingle manufacturers would say about this.

This is at http://www.buildingscience.com/topten/south.htm

 

[AaronB.]

Some studies say that those staements are not foar off, the MFG's say otherwise. To get a warranty, you have to listen to the MFG's.

 

[Teetorbilt]

Aaron, If you were mfg'g shingles, would you admit that they have problems? I doubt it. The spin docs. make them look like God's gift to man.

 

[jmorgan]

I posed this question to three shingle reps (GAF, Certianteed & Tamko) at lunch yesterday. They all said the same thing. With no ventillated space or heat sink below the shingles, Summer heat would buildup in the shingles too much. This is why the insulation mfr's changed from non-vented nail base board to vented nail base board....no shingle mfr would stand behind their shingles over the non-vented board.
Jim

 

[lpsonbuilders]

Vent or Not to vent

Hey yal!
I looked in to buying into foam spraying buzz. I like the benifits of the product! However when I seen them spraying it to the bottom of the roof sheeting, venting came to mind. I opinion is, a well vented roof deck is going to help the life of the shingles. I use ridge and gable vents combined with full soffit vents, good air flow. I put 1x2 furring strips on edge, stapled to the truss or rafter. This gives me my airway space 2"nom., then I use 1.5 or 2in. ridgid foam cut snug, can always foam gaps though. I'll run this from the top plate up at least 4ft. for a flat ceiling and all the way to the ridge for vaulted ceilings. But with this foam system one could use those cheesy cardboard or foam airways or propervents and that could maintain your air gap! Then I feel you may have some thing. It would be worth the time and cost, heating or cooling you deal with the same issues. Unless you live in a stable climate such as Hawaii! Note when I cut open a ridge after shingling its like a blast furance, hot air rises. Helping to keep that deck and shingles from burning up and curling up!

 

[bountyhunter]

I am starting new construction and my Icynene guy is telling me to spray in the roof line as well. In Kansas where it can be -10 or 110, I am not sure about this shingle problem you all are talking about. Should I request him to spray it on top of the ceiling instead?

 

[Grumpy]

Aaron, If you were mfg'g shingles, would you admit that they have problems? I doubt it. The spin docs. make them look like God's gift to man.

The manufacturer's base their claims that ventilation is important on a study by the University of Chicago. INfact GAF and Certainteed both site the study by the University of Chicago.

 

[puzzledlog]

Interesting topic since our home needs a major roof repair next year. (We have a cathedral ceiling with no vapor barrier - just fiberglass batt insulation) We plan on tearing the shingles and deck off, pull the fiberglass batts out then fill cavity with foam, redeck & shingle. Contractor warned us some mfgs won't warranty if no ventilation, but we would be ok since we plan on using Elk brand shingles & they will honor their warranty without ventilation. Anyone else heard that Elk honors their warranty without ventilation??

 

[Grumpy]

I highly doubt Elk, or any other manufacturer, will honor their warranty without ventilation. There is one way for you to know for sure... http://roofing.elkcorp.com/ go right to the source and ask Elk.

Most manufacturer's requires 3/4" to 2" of clearance under the roof deck to validate their warranty.

 

[puzzledlog]

Grumpy - Good advice.

I talked to technical support @ Elk. They will honor their warranty without ventilation as long as the foam used is on their approved list. They faxed me a fact sheet on spray foam insulation in general and technical bulletins that show that Icynene and Demilec brand foams are approved. They said other brands are approved - call them when we know which brand we want to use & they will advise if it has been approved. Happy to share these bulletins if anyone is interested.

 

[AaronB.]

What about this......With the foam sprayed directly to the bottom side of the deck, you are essentially creating a a warm roof system with the building envelope insulation
In direct contact with the roof deck. It is done all the time in commercial applications. This would negate the need for ventilation of the bottom of the roof deck if the R-value was great enough to stop thermal transfer. Seems you would still want to vent the attic space, or would you?

 

[Grumpy]

It is done all the time in commercial applications...

Seems you would still want to vent the attic space, or would you?

But most commercial/flat roofing systems don't require ventilation.

That question is exactly what we are discussing.


First off the shingle manufacturer get's last say so, and if they say it voids their warranty then it VOIDS THEIR WARRANTY. It doesn't matter if it's logical or not.

I personally don't think it is logical. You'd need one heck of an R value to prevent Ice damns in the Chicago climate. Also the heat failure involved with shingle curling cracking and such... I can't even guess how the foam is going to prevent that from occuring.