Quick advice on rim joist insulation

[snoopy7548]

Hey everyone. Today I'm planning on starting to rip out the R-30 batt insulation stuffed in my rim joists and replace it with a double layer of 2" polyiso rigid foam, for an R-total of ~26. I'll put rockwool against it as a fire barrier.

My only question is probably a simple one... I want to caulk around the perimeter of the wood framing of the rim joist prior to putting in the first square of rigid foam, which will act as an air barrier and hopefully grab onto the foam to secure it in place before I spray Great Stuff around the foam. Sort of like in this image:

I'm wondering what type of caulk is recommended for this. I've used PL300 to adhere foamboard before, since it won't eat at the material, but that is an adhesive. Can it be used as a caulk-substitute in this case, or should I go with something like Phenoseal or normal latex caulk?

Or is this overkill and I'll be OK with just pushing the rigid foam against the rim joist and Great Stuff'ing the perimeter?

Thanks!

 

[WilliamM2]

I wouldn't do it at all. If it gets extremely cold where you are, frost will form between the foam board and rim joist. Then you get mold and rot when it thaws. And you will never be able to see it, or inspect for it.

You can use caulk, and then something that can breath, like un-faced fiberglass insulation. Also not sure what caulking the sill plate to the foundation achieves, mine already has a foam seal all the way around, just as shown in you pic.

 

[Greenman]

Almost anything will stick enough to hold the ridged foam in place till the spray foam sets up.
I have to question the logic of doing this though. It doesn't seem as though you would never recoup the cost of the materials, or make any real difference in interior comfort.
Educate me a bit please, is this really worth doing?

 

[snoopy7548]

In everything I've read, it's highly recommended to either spray foam or use rigid foam to insulate and air seal the rim joists, and that batt insulation is the worst because it permits air leakage which leads to condensation and mold. Air cannot permeate rigid foam, so no risk of condensation on the rim joists, as long as it's sealed with expanding foam.

The major benefit in comfort and energy loss comes from the air sealing. I could simply do this and place the fiberglass batts back in, but like I said above they are air permeable.

I can't paste a link for some reason, but there's a good article on it at Fine Homebuilding's site.

 

[WilliamM2]

I've read those type of articles as well. All I know is, the only place I ever had trouble with condensation on the rim joist was an area of the house where I used foam board. I think the moisture came from the outside of the house, not the inside. Not sure how, the house is wrapped with Tyvek. Or maybe it did come from the inside, because you cant really seal around all the floor joists.

I don't think it's worth the effort, I never noticed any savings from insulating or sealing my rim joists. Now changing the attic from R-20 to R-55+ (16" of cellulose after settling) and air sealing the attic (wiring holes, light fixtures, etc) made a huge difference. Natural gas usage has averaged 30% less ever since. Most heat loss is through the ceiling, heat rises after all.

 

[Greenman]

I've found the first 90% of energy savings is easy to do, and cost effective. ROI shrinks at a dramatic rate when you start chasing those last minor items that waste energy.
Note that is in the Bay Area, we don't really have weather here.

 

[stormkroe]

OP, if your heart is set on this, save yourself 750% of the labor and buy a Froth Pak. It will get in every single little hole/crack (you won't the other way, no matter how diligent you are), it's fairly inexpensive for what it is, and it's also a vapor barrier (so there's no 'in between' space for moisture to collect, you'll be good as long as your exterior is breathable). Are you in a cold climate?
https://www.lowes.com/pd/Dow-FROTH-PAK-210-Foam-Insulation-Kit/1000015591

 

[Red Squirrel]

Wood is porous, so the caulk won't do much. The rest of the wood is still going to leak air, although much slower rate than actual open cracks.

I ended up just buying a spray foam kit at home depot and did mine.

spray foam kit:
https://www.homedepot.ca/en/home/p....ent-spray-foam-insulation-kit.1000718744.html



I did kinda a sloppy job tbh, it's a bit harder than you'd think, but overall does a nice seal. Don't forget to seal behind the top plate of your wall and the foundation, as that way when you insulate the walls and put vapour barrier it will all tie in as a continuous seal.

 

[stormkroe]

I did fibreglass capped with 3"+ foam for extra R and vapor barrier in my attic.

 

[Greenman]

I did fibreglass capped with 3"+ foam for extra R and vapor barrier in my attic.

I'm confused. It looks like the building envelope extends beyond those roof planes. It also appears that there is a load bearing wall resting on the left girder truss. Please explain what I'm seeing as the little framer that runs around inside my head is having a tantrum.

 

[Red Squirrel]

I'm guessing it's a "conditioned" attic. The part that is beyond the envelope is the unconditioned part. The part behind the short wall would be a closet or storage.

 

[stormkroe]

@Greenman what you're seeing is the non load-bearing side of one of my dormers. The house is very tall because we decided to spend the extra $3k to use attic trusses, and I gained another 900 sq ft for the house. The insulation is that way because I claimed just about every cubic foot of space for storage, and I didn't want it to be cold storage.
*edited to say there's a small 10/12 roof load on those side walls, just not a live load of any kind. They'll see hardly anything.

 

[Greenman]

I got it now! I'm looking into the dormer from an angle. That was killing me. I was seeing it as two internal planes of roof that were insulated, and it made no sense at all.
Thanks for clearing that up.

 

[snoopy7548]

Gonna do this in one post. Thanks for the replies. Now you guys got me re-thinking this. When I pulled away one of the batts, it looked all pink and brand new, meaning no air infiltration. The house is about 5.5 years old. My basement stays around 36-45F all winter long (mostly around 40-42F, except when the temps go negative for a week), I forget about the summer temps but probably around 55-65F.

I've read those type of articles as well. All I know is, the only place I ever had trouble with condensation on the rim joist was an area of the house where I used foam board. I think the moisture came from the outside of the house, not the inside. Not sure how, the house is wrapped with Tyvek. Or maybe it did come from the inside, because you cant really seal around all the floor joists.

I don't think it's worth the effort, I never noticed any savings from insulating or sealing my rim joists. Now changing the attic from R-20 to R-55+ (16" of cellulose after settling) and air sealing the attic (wiring holes, light fixtures, etc) made a huge difference. Natural gas usage has averaged 30% less ever since. Most heat loss is through the ceiling, heat rises after all.

How much foam board did you use? If your layer is too thin or not sealed properly, it could allow condensation especially if you covered it with a much higher R-value of batt insulation.

I am thinking about air sealing my attic floor at some point, or fully insulating my attic and turning it into a storage space. I had a home energy assessment by my electric company a few years ago and they had a local insulation company do some work up there, but they didn't really do much air sealing as far as I could tell. It has about R-38 to R-42 of insulation.

I've found the first 90% of energy savings is easy to do, and cost effective. ROI shrinks at a dramatic rate when you start chasing those last minor items that waste energy.
Note that is in the Bay Area, we don't really have weather here.

True. My basement is small though, so the cost of materials to insulate the rim joist with rigid foam shouldn't be more than $100; $150 max.

OP, if your heart is set on this, save yourself 750% of the labor and buy a Froth Pak. It will get in every single little hole/crack (you won't the other way, no matter how diligent you are), it's fairly inexpensive for what it is, and it's also a vapor barrier (so there's no 'in between' space for moisture to collect, you'll be good as long as your exterior is breathable). Are you in a cold climate?
https://www.lowes.com/pd/Dow-FROTH-PAK-210-Foam-Insulation-Kit/1000015591

Spray foam is definitely the way to go, but that is pretty expensive. Yeah, I'm in zone 5 - Massachusetts.

Wood is porous, so the caulk won't do much. The rest of the wood is still going to leak air, although much slower rate than actual open cracks.

I ended up just buying a spray foam kit at home depot and did mine.

spray foam kit:
https://www.homedepot.ca/en/home/p....ent-spray-foam-insulation-kit.1000718744.html



I did kinda a sloppy job tbh, it's a bit harder than you'd think, but overall does a nice seal. Don't forget to seal behind the top plate of your wall and the foundation, as that way when you insulate the walls and put vapour barrier it will all tie in as a continuous seal.

Nice job. It looks like you only filled the cavity with 1-2" of spray foam. Is that acceptable or should the whole thing be filled for the max R-value possible?

 

[Red Squirrel]

For me the goal was to mostly air seal. Then I put some fibreglass and whatever else I had that was already in there to add extra R value. Only thing is that the vapour barrier should be on the warm side, so this kinda goes against that, but think because in this case the foam acts as insulation too it's probably fine.