# Spray Foam Roof Underside or Between Ceiling Joists



## joed (Mar 13, 2005)

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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## chandler48 (Jun 5, 2017)

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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## carpdad (Oct 11, 2010)

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.


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## Windows on Wash (Aug 30, 2011)

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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## RustyRealtor (Apr 13, 2009)

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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## Yodaman (Mar 9, 2015)

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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## Porsche986S (Dec 10, 2017)

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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## Nealtw (Jun 22, 2017)

Porsche986S said:


> 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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## joed (Mar 13, 2005)

Box the water lines close to the ceiling and spray over them.


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## Nealtw (Jun 22, 2017)

RustyRealtor said:


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


 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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## RustyRealtor (Apr 13, 2009)

Nealtw, the water lines travel through the joists.


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## Nealtw (Jun 22, 2017)

RustyRealtor said:


> 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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## Porsche986S (Dec 10, 2017)

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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## RustyRealtor (Apr 13, 2009)

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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## Nealtw (Jun 22, 2017)

RustyRealtor said:


> 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,


 If you had a new roof in the near future, there are drip edge vents smart vent that go just under the first shingles. I have seen a row of box vents but that doesn't look good on the front of a house. Maybe some gable vents but low triangles close to the soffet at the corners. Just above the insulation.


We have been building houses with in 6 ft of each other. That puts the gutters 5 ft apart then we can't have vented soffet. so they wrap a skirt roof around the front and back and that is where the venting comes from.
I'm not sure it works but that is what they want. What ever you do, you want more low than what you have up high.


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## Yodaman (Mar 9, 2015)

There are shingle vents that could be installed for intake vent, but like Neil said, it is best if done during a re-shingle project.


There are several MFG's that make them. Lomanco, and Certainteed come to mind.






Combined with pan vents or spinner vents for exhaust could solve your issue.


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