VB/Insulation sanity check

[PCustoms]

Pulled down a 2x4 framed wall, insulation came out perfectly clean and got gently put aside

New wall is 2x6. Thinking about stuffing some 2" XPS foam in the cavity, against the exterior sheathing, sealing with canned foam and then putting the batt back in.

Paper facing would need to be removed, correct?

 

[billconner]

I would just slash it with a matte knife OR put batts in first, staple to sides of studs, and put 2" foam on inside. I would not worry about the paper against foam.

 

[PCustoms]

I would just slash it with a matte knife OR put batts in first, staple to sides of studs, and put 2" foam on inside. I would not worry about the paper against foam.

Foam on inside against paper would work, but seems it would defeat the purpose of have the VB/sealing on the outside side of the wall...

 

[billconner]

In a heating climate, vapor barrier is usually on inside. Or simply, it's on the moist side - interior for heating climate, exterior for cooling climate, and ground side below grade. At least that is what I've concluded after looking at the issue for over 40 years.

 

[PCustoms]

In a heating climate, vapor barrier is usually on inside. Or simply, it's on the moist side - interior for heating climate, exterior for cooling climate, and ground side below grade. At least that is what I've concluded after looking at the issue for over 40 years.

Usually is the key word here.

As a frame of reference, I had my attic foamed and then bulked up with blown in several years ago. I had some vague recollection of dew points and condensation and the minimum thickness of the foam.

Did some light reading to confirm my memory:

https://profoam.com/document-files/a2c2b44343b53a14fe7a363a03c3f432109ff1f4.pdf

Basically what this says is you can use hybrid insulation (foam and bats/loose fill) if you sufficiently insulate the cold side of the cavity to prevent condensation in the wall:



I believe when I had the spray done the minimum was 1.5", we went almost double that. With XPS sheet it looks like 2" is needed, which is what I picked up last night.

Dug into the 2021 energy code a little bit, a little crazy what that has added for requirements IMHO. I've heard some people around here complaining about the continuous exterior envelope, seems late to the game (winters here are getting warmer) with potentially catastrophic moisture control issues...

 

[billconner]

I don't think rigid foam inserted will work same as spray foam. It just won't seal. Maybe if you used foam in a can to seal the rigid to framing.

Yeah, codes don't change quickly or keep up with climate change.

 

[u2slow]

The paper face is a vapour retarder. Not a barrier like poly or rigid foam (w/ spray-foam for crack-sealing)

What you're describing is moving the vapour barrier to the outside of the wall. Decide if that is appropriate for your climate zone.

Edit: you can also get rockwool type insulation in 2" thickness.

 

[Firebrick43]

I don't think rigid foam inserted will work same as spray foam. It just won't seal. Maybe if you used foam in a can to seal the rigid to framing.

Yeah, codes don't change quickly or keep up with climate change.

Factory foam is less permeable than any spray foam. If the edges are foamed with a gun it does just fine.

Usually is the key word here.

As a frame of reference, I had my attic foamed and then bulked up with blown in several years ago. I had some vague recollection of dew points and condensation and the minimum thickness of the foam.

Did some light reading to confirm my memory:

https://profoam.com/document-files/a2c2b44343b53a14fe7a363a03c3f432109ff1f4.pdf

Basically what this says is you can use hybrid insulation (foam and bats/loose fill) if you sufficiently insulate the cold side of the cavity to prevent condensation in the wall:



I believe when I had the spray done the minimum was 1.5", we went almost double that. With XPS sheet it looks like 2" is needed, which is what I picked up last night.

Dot into the 2021 energy code a little bit, a little crazy what that has added for requirements IMHO. I've heard some people around here complaining about the continuous exterior envelope, seems late to the game (winters here are getting warmer) with potentially catastrophic moisture control issues...

You are correct in where the foam need placed to properly place where the dew point happens and no vapor barrier/retarder inside. I did this in my only two exterior stud walls in my icf house which are small ones between the kitchen and sun room/enclosed porch. Although it is offset 2x4 on a 2x6 top/bottom plate.

 

[PCustoms]

I don't think rigid foam inserted will work same as spray foam. It just won't seal. Maybe if you used foam in a can to seal the rigid to framing.

Thinking about stuffing some 2" XPS foam in the cavity, against the exterior sheathing, sealing with canned foam and then putting the batt back in.

@Firebrick43 thanks for your post, not sure why I was having a brain cramp, forgot I had done the same exact thing when I turned a patio door into a window (and had the foam).

Aside from being $60 a sheet went super fast yesterday. Ripped down on the TS, wedges it tight and hit the edges with a can. Need to put the pellet stove chimney thimble back in then do the last 2 cavities.

 

[coldh2o]

Why the switch from 2x4 to 2x6 walls? Was it structural, or just for the insulation value?

Too late now, but I would have put full sheets of foam on the exterior wall before reinstalling a 2x4 wall. Would have been a tighter envelope, and provided a thermal break for the studs. Plus less labour and cost of lumber.

 

[billconner]

Why the switch from 2x4 to 2x6 walls? Was it structural, or just for the insulation value?

Too late now, but I would have put full sheets of foam on the exterior wall before reinstalling a 2x4 wall. Would have been a tighter envelope, and provided a thermal break for the studs. Plus less labour and cost of lumber.

How do you deal with windows and doors then? When I've seen this it reminds me of a jack-o'-lantern several days after Halloween, all recessed. Of course you can remove and reinstall the windows and doors, and extend the interior jambs.

 

[PCustoms]

Why the switch from 2x4 to 2x6 walls? Was it structural, or just for the insulation value?

Too late now, but I would have put full sheets of foam on the exterior wall before reinstalling a 2x4 wall. Would have been a tighter envelope, and provided a thermal break for the studs. Plus less labour and cost of lumber.

Structural/cosmetic. I wanted more support capturing the end of the beam for the house (a couple 2x4 looked anemic), and also wanted the wall wider to better match to the poured wall, as block our poured would be at least 8" wide. It also gave me a nice lip at the top plate to capture strapping for my ceiling.

Not overly concerned with the amount of Insulation, given this is a walk out basement with an overhead door 2' away this will be the most insulated portion.