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Old 05-13-2009, 08:44 AM   #1
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Shingling, Cathedral Ceiling, Spray Foam & Venting


I can't seem to get the same answer out of two peeps on this question. I want to reshingle my old roof which will turn out to be a cathedral ceiling. I'm worried about the shingles being able to breath after I spray foam between the rafters and want to know the right method to proceed with venting or not venting. I guess I should also mention, I plan to use 2lb closed cell foam and the roof sheathing is actually 1" boards, the shingles will be typical 30-40 asphalt shingles. Thanks for all comments.

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Old 05-13-2009, 12:00 PM   #2
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Shingling, Cathedral Ceiling, Spray Foam & Venting


Are you completely filling the rafter bays, or just spraying the underside of the decking?

The reason you are getting different responses, is a combination of knowledge on the ventilation or non-ventilation of a cathedral ceiling and the different theories being dangled out in various opposing research interpretations.

Even with proper 100% continuous Balanced Intake and Exhaust Ventilation, while the results seem to have determined that would be better, it is still not as good as an open attic scenario getting properly ventilated.

Remember the vapor barrier under the warm conditioned heated side of the insulation and sealing all gaps that enter the rafter bay cavities.

Does your roof plane have any valleys or hips on it?

Are there any skylights installed in any of the rafter bays?

Are there any heat stack B-Vents that protrude through the roof where the cathedral ceiling is?

How thick will a 2 Pound Closed Cell Foam Application wind up being?

Ed

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Old 05-13-2009, 12:02 PM   #3
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Shingling, Cathedral Ceiling, Spray Foam & Venting


Are you completely filling the rafter bays, or just spraying the underside of the decking?

The reason you are getting different responses, is a combination of knowledge on the ventilation or non-ventilation of a cathedral ceiling and the different theories being dangled out in various opposing research interpretations.

Even with proper 100% continuous Balanced Intake and Exhaust Ventilation, while the results seem to have determined that would be better, it is still not as good as an open attic scenario getting properly ventilated.

Remember the vapor barrier under the warm conditioned heated side of the insulation and sealing all gaps that enter the rafter bay cavities.

Does your roof plane have any valleys or hips on it?

Are there any skylights installed in any of the rafter bays?

Are there any heat stack B-Vents that protrude through the roof where the cathedral ceiling is?

How thick will a 2 Pound Closed Cell Foam Application wind up being?

Ed



Hmmm. Wierd message when I replied.
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Old 05-13-2009, 12:15 PM   #4
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Shingling, Cathedral Ceiling, Spray Foam & Venting


I'm dealing with a simple gabled roof, the only opening will be the plumbing stack. I do have vented soffits on both eve's. I'd like to completely fill the 2x4 rafter bays which are actually full size lumber so I guess I'll basicly end up with 4"s of foam. I live in the toronto area where we get alot of snow, so also wondering if this will be an issue with ridge venting. I have read most of your advice on shingle vent 2. Thank you for your comments and advice.

Yes, I got a failure notice when trying to first look at your reply.
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Old 05-13-2009, 03:11 PM   #5
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Shingling, Cathedral Ceiling, Spray Foam & Venting


If the rafter bays are completely filled from bottom to top, no amount of ventilation will bypass that obstacle and flow.

I don't have the link available right now, but there is some good writings on "Non Ventilated Attics" done by www.buildingscience.org and I think the one I am referring to is written by Joe Lstirubek.

Also, the shingle warranty, "May" be affected with that choice. Some have a 10 year limitation on their products over that condition.

Ed
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Old 05-13-2009, 04:29 PM   #6
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Shingling, Cathedral Ceiling, Spray Foam & Venting


What if I ridge vent and then put insulation baffles or shoots right from the ridge vent into the vented soffit area.
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Old 05-13-2009, 08:33 PM   #7
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Shingling, Cathedral Ceiling, Spray Foam & Venting


Just my opinion, but I think that would be a very very good idea.

Ed
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Old 05-14-2009, 01:35 AM   #8
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Shingling, Cathedral Ceiling, Spray Foam & Venting


I have seen the insulation depressing baffles used several times in this case.
They start spraying insulation at the edges & work it up over the baffles.
The stuff dries prety quick.
This is what the inspectors wanted (not that inspectors are always right)
A ridgevent would also be needed
Dale Chomechko
DC Roofing Inc

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