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Old 08-04-2019, 02:11 PM   #1
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"Great Stuff" spray foam to air seal top and bottom plates and stud bays


Remodeling a corner room (two exterior walls) and I've got the walls down to the stud framing. Looking to air seal before laying in fiberglass batts. The areas I've seen mentioned as areas to air seal are the perimeter of stud bays (where the studs touch the sheathing), between doubled up top and bottom plates, and between bottom plate and concrete. I wouldn't bother sealing the entire perimeter of every stud bay, just where the sheathing meets the top and bottom plates and the studs to which the ends of the sheathing are attached.

I've seen it suggested that one should use caulk for this, but that is a lot of caulk and time. A much faster alternative is one part spray foam, like Great Stuff - not only is it much faster (using a gun, not the straw), you also get a lot more linear feet out of a can than out of a tube of caulk.

Trying to figure out which spray foam to use. Great Stuff Gaps and Cracks seems like the wrong product because the gaps in question aren't even visible, so we don't need or want much foam expansion because all that expansion doesn't serve much purpose and just ends up compressing the fiberglass batts installed afterward. Great Stuff Window and Door seems better suited since it expands less. And then there's Great Stuff Pro Wall and Floor Adhesive, which according to a review I read on Amazon, expands even less than the Window and Door stuff. So this might be ideal, but none of the marketing material I see for it references air sealing, so I'm not sure if this is the right product for my application.

Thoughts?
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Old 08-04-2019, 02:52 PM   #2
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


While some of them do say they are air sealing in some of their literature, they also imply they are not. So, it may or may not work. One thing that would work would be to put down some spray foam and then a thin layer of foam board cut to size. The friction fit of the board combined with the spray foam will provide a good air sealant.

Another option is to use a 2 part spray foam kit, it is a it more expensive but does a much better job of air sealing around the joist bays. Here is the product of which I speak.

They even make it in bigger kits so that you can do more.
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Old 08-04-2019, 04:22 PM   #3
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


I recommend and use caulking, actually the PL Construction Adhesive in the large tubes. Can foam, as you mentioned, does not do well when there is a small seam and any that bulges out into the stud bay creates an air gap, although minor, that reduces the insulation performance. The caulk can be installed where you want and pushed into place with that magic tool, your finger. A rounded putty knife if you wish. But it will outlast the house. Can foam tends to deteriorate over time. It fills larger gaps where caulk cannot.

And not I said the larger tubes. When you stop wrap the tip with foil tape and it will be good for months.

I have a foam gun and they are a pain and expensive to clean and their larger cans basically no cost advantage.

In terms of getting the best insulation value within the space you have, Roxul is a high density insulation, easy to use and gives a good fit at a bit higher r-value.

IMO, can foam is used more often because it is easier, not better.

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Old 08-04-2019, 05:12 PM   #4
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


What about using 6mil plastic stapled to the face of the studs, covering the batt insulation? Yay or nay?
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Old 08-04-2019, 06:07 PM   #5
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


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What about using 6mil plastic stapled to the face of the studs, covering the batt insulation? Yay or nay?
Nay.

Warm moist air hitting that cold plastic in the summer (AC on) will result in condensation. I guess you could argue that it can dry out to the exterior, but I imagine it will get the insulation wet, reducing its efficacy.

Might be fine for very cold climates, such as climate zone 6.
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Old 08-04-2019, 06:11 PM   #6
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


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Nay.

Warm moist air hitting that cold plastic in the summer (AC on) will result in condensation. I guess you could argue that it can dry out to the exterior, but I imagine it will get the insulation wet, reducing its efficacy.
And sealing to wall on the other side of the insulation changes that how?
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Old 08-06-2019, 08:52 PM   #7
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


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I recommend and use caulking, actually the PL Construction Adhesive in the large tubes. Can foam, as you mentioned, does not do well when there is a small seam and any that bulges out into the stud bay creates an air gap, although minor, that reduces the insulation performance. The caulk can be installed where you want and pushed into place with that magic tool, your finger. A rounded putty knife if you wish. But it will outlast the house. Can foam tends to deteriorate over time. It fills larger gaps where caulk cannot.

And not I said the larger tubes. When you stop wrap the tip with foil tape and it will be good for months.

I have a foam gun and they are a pain and expensive to clean and their larger cans basically no cost advantage.

In terms of getting the best insulation value within the space you have, Roxul is a high density insulation, easy to use and gives a good fit at a bit higher r-value.

IMO, can foam is used more often because it is easier, not better.
You make a good case for caulk.

I guess I'll get the 29oz caulk gun from HD along with 28oz tubes of PL Premium Poly Construction Adhesive (thanks for the specific recommendation).

Do you caulk the seams between all doubled up studs? How about the stud bay - do you caulk the entire perimeter or just where the top and bottom plates touch the sheathing?

Never heard of Roxul before, I'll look into it.
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Old 08-06-2019, 09:24 PM   #8
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


Doubled up studs and double top plates can sometimes hide a seam on the other side. Use your judgment but this will be your last chance to get those places.

My house was built long before I got into energy auditing and bought my infrared camera. Of course I immediately went inspecting and ouch I found a few big issues, most had to wait until I ripped something apart so I had access. One was above my front door with no access from above (cape). The ceiling there was so cold I finally just tore out some drywall. Well the rim band basically had a baseball size piece missing allowing cold air directly into 2 uninsulated cavities. Fixing that felt good.

When in doubt, add some caulking.

Bud
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Old 08-07-2019, 06:24 PM   #9
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


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Doubled up studs and double top plates can sometimes hide a seam on the other side. Use your judgment but this will be your last chance to get those places.

My house was built long before I got into energy auditing and bought my infrared camera. Of course I immediately went inspecting and ouch I found a few big issues, most had to wait until I ripped something apart so I had access. One was above my front door with no access from above (cape). The ceiling there was so cold I finally just tore out some drywall. Well the rim band basically had a baseball size piece missing allowing cold air directly into 2 uninsulated cavities. Fixing that felt good.

When in doubt, add some caulking.

Bud
Thanks. Yeah, I've got a cheap IR gun. Once I catch up on this and couple of other projects, I need to walk around the house and identify other leaks.

Just to clarify, you're applying the caulk, and smoothing it out / pushing it into the seams, and then letting it cure, correct?

Are you then applying caulk on the 1-1/2" side of the studs and attaching the drywall before the caulk sets to create a seal between the drywall and the studs?
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Old 08-07-2019, 07:07 PM   #10
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


I find hot weather more difficult to read the signatures but adding an exhaust fan or two will depressurize the house and draw the hot air in. In the attic you reverse that and blow air into the house and inspect the attic.

If you go to gluing your drywall in place, which few do, it doesn't come apart easily. Sealing the perimeter of the stud cavities plus top and bottom plates is the higher priority. Air won't move sideways if it doesn't have a place to go.

Remember, if you seal it really tight you have to plan on adding fresh air. That is why some buildings simply seal to a "good" level and stop. if you are putting in a HRV then seal it all.

Bud
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Old 08-11-2019, 08:47 AM   #11
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays




I've got these gaps at the top of the wall I need to address. For the smaller gap (green arrow), I'm thinking I can fill that in with Great Stuff Gap and Crack foam.

For the larger gap (red arrow), I'm thinking of cutting a strip of 1/2" foam board and gluing that into place at the very top of the stud bay to cover the gap. The glue would be Loctite PL Premium construction adhesive / caulk (the same stuff @Bud9051 recommended to seal seams, and which I've started using). I've noticed PL Premium doesn't have holding power when first applied, so I'll either try to cut the foam board pieces large enough that I can friction fit them into place or use tape to hold them in place until the PL Premium starts curing. Repeat for top of each stud bay.

Does this sound like a reasonably good way to seal these gaps?
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Old 08-14-2019, 11:34 PM   #12
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


they make back rod out of foam, comes in a roll, and I think it comes in diff sizes too... its round, grey and looks like a noodle coiled up
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Old 08-16-2019, 10:05 AM   #13
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


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they make back rod out of foam, comes in a roll, and I think it comes in diff sizes too... its round, grey and looks like a noodle coiled up
That's a good idea. I'll have to check if they have the right diameter to stuff into those gaps; and then caulk around the junction of backer rod and framing.
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Old 08-16-2019, 10:16 AM   #14
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Re: "Great Stuff" spray foam to air seal top and bottom plates and stud bays


I'd say stick with your cutting some foam board idea.

The backer rod comes in a bag and is coiled. It will be hard to get it to stay put in your application. It works good for items where it is sandwiched and held in place, I don't think the application you have qualifies.
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