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Old 07-09-2019, 05:52 PM   #1
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Spray Foam Roof Underside or Between Ceiling Joists


Hello,

I've searched for this answer, but can't find it, so I thought I'd give you good folks a try.

This is an 800 s.f. single story, brick veneer house on a slab in Nashville.

Would it be better to spray foam the underside of my 8-12 pitched roof or lay a floor on top of my 2x8 ceiling joists and spray foam between the 2x8 joists?

Thanks Much,
Russell
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Old 07-09-2019, 06:05 PM   #2
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Re: Spray Foam Roof Underside or Between Ceiling Joists


If you are intending to put up a ceiling then spray on top of the ceiling drywall after it is install. It will insulate and it will vapour barrier seal the ceilng.
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Old 07-09-2019, 06:31 PM   #3
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Your attic space will or should replicate the temperature of the outside air, so no spray foam there. What Joed said.
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Old 07-10-2019, 06:29 AM   #4
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Re: Spray Foam Roof Underside or Between Ceiling Joists


If you spray the roof, you must have 2" or so air space (with soffit and ridge ventilation) between roof deck and the insulation. Popular thought is to spray directly under the roof decking but if you have leaks, it stays wet and rot quickly.


Also spraying the roof is part of making the attic conditioned space. Then you need to spray the walls as well. If attic stays attic, spray the ceiling.


Urethane foam also needs to be controlled carefully. Search for spray insulation problems. Temperature is very important. Cold materials and ambient temps can leave you with uncuring foam that stinks forever. If attic is unused or light storage, fiberglass is just as good.


Air sealing the joist-drywall seam is not necessary. You have to look for OPEN spaces. You need more fiberglass but it has fairly large advantage of being able to dry out.

Last edited by carpdad; 07-10-2019 at 06:31 AM.
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Old 07-10-2019, 06:46 AM   #5
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Question is whether you should do either of what you are talking about. Is there any mechanical in the attic? Is the roof currently vented?
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Old 07-10-2019, 12:10 PM   #6
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Thanks for the replies.

I shouldn't have done it, but I ran pex up there (see attached). Pex was already up there and I didn't see a problem with it. Inspector said I could build a chase around the pex and apply spray foam, otherwise I'd have to move the Pex.

Spraying the top of the drywall seems like a good idea, but it seems like it will be tough on the person who does the spraying. And the plumbing inspector might have to get up there to verify - unless I spray the pex and get inspected and then hang drywall. Insulation man says he charges an additional $500 for a second trip.

I will also have line sets up there for mini splits and some nm cable up there. I did not drill holes for the nm cable. It runs atop the joists.

There is one (not two) gable vent(s) in the attic No ridge vent and no soffit vents. I considered installing a few panicle vents.

It's all very confusing to me. I really appreciate everyone's help.

Russell
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Old 07-10-2019, 12:26 PM   #7
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Re: Spray Foam Roof Underside or Between Ceiling Joists


A good lecture by Joe Lstiburek on the subject of ceiling or roof deck insulation if interested. He is bit long winded.
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Old 07-10-2019, 01:21 PM   #8
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Re: Spray Foam Roof Underside or Between Ceiling Joists


What is the intended purpose of this attic space ? Just be an attic ? Storage space ? Convert to living space ? The first thing that pops out to me is your VERY limited attic ventilation . Next what insulation if any is up there now ? What dimension are your rafters/trusses ? Are they 2x4 or 2x6 or ? Really need all these questions answered for anyone to be able to provide informed suggestions .
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Old 07-10-2019, 01:32 PM   #9
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Quote:
Originally Posted by Porsche986S View Post
What is the intended purpose of this attic space ? Just be an attic ? Storage space ? Convert to living space ? The first thing that pops out to me is your VERY limited attic ventilation . Next what insulation if any is up there now ? What dimension are your rafters/trusses ? Are they 2x4 or 2x6 or ? Really need all these questions answered for anyone to be able to provide informed suggestions .
He has 2x8 joist, he has water line and lines for the mini split up there.

The inspector wants the water lines to be protected.
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Old 07-10-2019, 03:45 PM   #10
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Box the water lines close to the ceiling and spray over them.
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Old 07-10-2019, 10:24 PM   #11
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Quote:
Originally Posted by RustyRealtor View Post
Hello,

I've searched for this answer, but can't find it, so I thought I'd give you good folks a try.

This is an 800 s.f. single story, brick veneer house on a slab in Nashville.

Would it be better to spray foam the underside of my 8-12 pitched roof or lay a floor on top of my 2x8 ceiling joists and spray foam between the 2x8 joists?

Thanks Much,
Russell
Do the water lines travel with the joists or thru them?


You could use foam board to keep the pipes on the warm side of the insulation and raise the floor over that area.
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Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14

Last edited by Nealtw; 07-10-2019 at 10:34 PM.
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Old 07-10-2019, 10:28 PM   #12
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Nealtw, the water lines travel through the joists.
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Old 07-10-2019, 11:45 PM   #13
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Quote:
Originally Posted by RustyRealtor View Post
Nealtw, the water lines travel through the joists.
So you still could get 3" of foam board above them

I think you need venting no matter what you do.
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Old 07-11-2019, 05:49 AM   #14
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Re: Spray Foam Roof Underside or Between Ceiling Joists


Will the codes/inspector allow you to wrap the water pipes with foam insulation ? I would think if you used the pre-cut foam weatherstripping made for water pipes and then add whatever insulation you are going to use over that it would do the job . And if you ever have a pipe problem you can lift up the insulation , remove the foam insulation and make a repair . If you spray foam it is much harder to make a repair if needed .

I also agree you need a lot more attic ventilation . With good ventilation and insulation around R40 or more you will have an energy efficient setup .
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Old 07-11-2019, 10:36 AM   #15
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Re: Spray Foam Roof Underside or Between Ceiling Joists


I like the foam board idea. If I use four 2" pieces of the pink stuff I would have R-40 above the Pex. I just need the inspector's blessing. Then I will use rolled or blown-in insulation everywhere else. And I will spray foam the inside of the brick walls and leave the roof decking uninsulated

I think Yodaman's video concludes that a ridge vent is the way to go. But I don't have overhangs, so I'm not sure how to get air into the attic - especially with gutters. It used to said, by some, that ridge vents and gable vents don't work well together. Any thoughts on that? We dont have much snow in these parts,
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