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Old 07-12-2019, 04:29 PM   #1
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Spray Foaming Directly Under Board Sheathing Decks?


I recently had my roof redone with metal shingles. When I went into the crawl space I noticed that a good portion of the fiberglass batts fell down. I noticed that the insulation wasn't even sufficient and that many of the batts needed replacing. I decided to rip it all out and start over. I was considering doing rigid foam insulation but decided that I wanted closed cell spray foam instead, since I will be putting a central AC unit in.

The picture below is looking up directly under my roof. The roofers just put down the blue underlayment between the roof boards and metal shingles. My concern is that the insulation will expand through the sheathing gaps and push up on the shingles...

My question is, should I put a barrier on the underside of the the board sheathing before applying the spray foam? If so, what material(s) should I use?

Side question, just confirm my research, I can skip 1" to 3" ventilation gap between the insulation and sheathing since i'm using closed cell spray foam, right?
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Last edited by kevinpv2; 07-12-2019 at 04:32 PM.
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Old 07-13-2019, 06:10 AM   #2
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Re: Spray Foaming Directly Under Board Sheathing Decks?


4" gorilla tape should work. Regular width 1.5 or 2(?) maybe ok too except I wonder if the tape comes off during spraying, foam could push into the weather barrier and create a hump. Or staple on tarpaper.


Spray foam is popular but it can hold water if there is a leak. Outdoor moisture can condense between wood decking and insulation. Either case will make the wood decking rot quickly. Listiburek is a person who thinks of nothing but good building practices. Last time I looked, his thinking is that a ventilation should be part of any roofing that's installed in pieces.


For your roof, 1.5" ventilation baffles are ok then spray but try to keep the spray within the rafters. Then cover over the rafters with 1.5 or 2" xps boards then drywall. This is if you can afford to lose up to 2". Metal roofing has those metal caps?, which should work as a ridge vent cap too. Even if you don't cut the decking for ventilation hole, there should be enough joint spaces and such for moisture to escape. As such, baffle to ridge should be sealed first with can foam or such so the ridge joints don't get sealed.
Better if you can over spray and cut off the excess. More insulation value. Still, even extra 2" xps will not give you the insulation value usually called for. XPS or closed cell foam has r5 per inch. This is from while ago but attic insulation had to be r30 or better. These days, I see that few people are calling for r60 for attic/roof. But whatever you can do, even using fiberglass with excellent air sealing + insulation + ventilation, is better than none or sloppy installs.

Last edited by carpdad; 07-13-2019 at 06:23 AM.
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Old 07-13-2019, 07:35 AM   #3
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Re: Spray Foaming Directly Under Board Sheathing Decks?


I don't understand why the rafters were insulated. Your ceiling should have been insulated, leaving the attic space to replicate the outdoor temperature with proper ventilation.

Spray foam is good as mentioned. I just question the validity of insulating rafters with fiberglas.
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Old 07-13-2019, 08:28 AM   #4
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Re: Spray Foaming Directly Under Board Sheathing Decks?


Hi Kevin, as asked why were the rafters insulated? Normally you would insulate the attic floor, ceiling below and ventilate the attic. We need to know why you are headed in this direction.

I do not recommend spray foam on the bottom of the roof drck even when it is sheathed in plywood. Fill us in.

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Old 07-13-2019, 11:02 AM   #5
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Re: Spray Foaming Directly Under Board Sheathing Decks?


Thank you Carpdad, Larry, and Bud, for responding and helping me with this project.

Yes, there is a ridge vent/metal cap on my roof. I will take your advice and seal that area first.

It wasn't until the new roof this past spring that I noticed that the insulation in the attic was done wrong. I've had the house for 7 years now, and probably should have noticed earlier, but the home inspector should have picked up on the bad/incorrect insulation job.

Unfortunately, I have no idea why the rafters were insulated in the crawl space... The house I bought has a lot of DIY work done to it and it is very noticeable with the amateur work. No roof ventilation was installed when the previous owner finished the attic.

(FYI My house was built in 1932. It's a twin with a house along one side of my house.)

So, let me give a better picture of the plans for my attic. I am going to completely redo my attic into two areas, both areas will be finished and conditioned. The livable area - which I need to downsize a bit to properly insulate, and the crawl space area. The crawl space area I will be completely insulating because I will be installing a high-velocity central AC unit system). Eventually I will be installing a mechanical ventilation system once I am able to make the house more air tight.



Current Insulation Plan - with all of your suggestions/guidance:
Tar Paper (for waterproofing) under the sheathing.(open to suggestions for other material)
Rafter Vent Baffles (Provent 14" x 4' x 1-2") after tar paper.
Spray Foam 3" (hopefully a closed cell with at least an R-7 per 1").
XPS 2"-3" Thermasheath R-6.5 per 1".
AC Install.
Insulate the floors.
Dry Wall.
Start saving up for my next project...
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Old 07-13-2019, 11:10 AM   #6
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Re: Spray Foaming Directly Under Board Sheathing Decks?


A proper ridge vent is needed for good ventilation of the attic. "Crawl Space" indicates to me an area UNDER a house. Attic is reserved for the area above your ceiling.

I don't think you understood what Bud was saying. The rafters should NOT be insulated, at all. The ceiling/floor of the attic needs insulation with the vapor barrier down, or foam, which ever you choose. Making the attic space conditioned may be detrimental, since you have machinery running in it. The attic should be vented from the soffits all the way to the ridge vent. External air needs to move from low to high and exit.
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Old 07-13-2019, 11:31 AM   #7
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Re: Spray Foaming Directly Under Board Sheathing Decks?


OP is going to convert the attic into a living space, which then makes sense.

OP: if you insulate the roof and the walls, and condition the space with heat, then you don't have to insulate the floor (ceiling for rooms underneath), except if you want some sound insulation.

Depending on how much attic space is left after the knee wall, you could or don't have to insulate it lot better. Knee wall can be built near to the eave with even 3-4' of ceiling height. This low ceiling area may get a bed or built in storage, etc. If that space is not much, it will not help with overall insulating value. I think you have to play with some numbers and which you want. Space or insulation. A small space, example, doesn't need as much heating so maybe insulation efforts can be sacrificed. In summers, you may need bigger ac than called by the numbers. If you can leave some space under the ridge and install a power attic vent fan, even better. If power fan, if it is not roof install, leave big enough access hatch. Heat above the ceiling drywall will be felt if passive venting only.
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Old 07-13-2019, 11:52 AM   #8
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Re: Spray Foaming Directly Under Board Sheathing Decks?


Some images.
Image 1. I think combining option 2 and 3 could work for you. Option 2 assumes enough rafter insulation over the living space, but you will not have as much, so might as well insulate the dead space as much as possible and also air seal.
The floor is also insulated but the way it shows may not be obvious. The floor insulation goes as far as the knee wall and floor under the knee wall is air sealed. This is new to me. Option 1 in the image 1 shows "ineffective", esp where ventilation air from soffit can travel through the fiberglass insulation. Not a problem if spraying foam. But you could save some on spray foam by air sealing the soffit to floor joist/rafter area with foam then continue with fiberglass.


Drawings don't say it but you could make the walls wide and install better than r19. Double 2x4 wall example. This can help with overall energy you'd need.


Another new to me is image 2 showing gable vents for the dead space. Depends but seems like a good idea for venting.
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Last edited by carpdad; 07-13-2019 at 11:56 AM.
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Old 07-15-2019, 10:07 AM   #9
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Re: Spray Foaming Directly Under Board Sheathing Decks?


Thank you all for the feedback and guidance!
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