Isolating walls from foundation

[Inspector Gadget]

Interesting situation in our office this morning. A client is installing wood-frame walls in a basement, and is balking at the idea of isolating 2x4 walls - with 5.5" R20 insulation - from the foundation.

Code reference:



This leads to an interesting challenge that we thought about: what if the client built the interior stud walls so that neither the wood nor the insulation contacted the foundation? It could be argued that 9.13.2.5(1)(b) would no longer apply.

Thoughts?

 

[bill1952]

Is there an air vapor barrier planned? Assuming masonry foundation wall with no insulation and probably not first class damp proofing in the ground side, I'd put a air vapor barrier - 10 mil at least - on wall, and then build wall inside that with anchors at floor and first floor joists, not through air barrier. Any moisture that gets into wall will dry to inside. Troweling mastic to wall before pressing air vapor barrier into it would be an improvement.

(I'm assuming he doesn't want to use foam - rigid or spray - or I'd probably start there - 2" foam fully adhered and seams sealed. Put studs inside that or just glue drywall to the foam.)

 

[tmurray]

Yes, we typically had people offset the walls to avoid having to place moisture protection between the concrete wall and the wall framing.

 

[Yankee Chronicler]

To fit 5.5" R-20 insulation into a 2x4 wall without compressing the insulation (which would make it no longer R-20), the studs will need to be offset from the foundation wall by a minimum of 2 inches anyway. At that point, my concern would be the insulation wicking moisture out of the foundation, which would further compromise the insulation value.

 

[Inspector Gadget]

To fit 5.5" R-20 insulation into a 2x4 wall without compressing the insulation (which would make it no longer R-20), the studs will need to be offset from the foundation wall by a minimum of 2 inches anyway. At that point, my concern would be the insulation wicking moisture out of the foundation, which would further compromise the insulation value.

Exactly. For the cost of tyvek/vapour barrier/a layer of XPS - doesn't make sense.

 

[mark handler]

Watch the mold and mildew grow......

 

[bill1952]

Exactly. For the cost of tyvek/vapour barrier/a layer of XPS - doesn't make sense.

Where is the vapour barrier in the wall? Where is the tyvek (or are you calling that a vapor barrier?

If you need R20, two layers of 2" XPS with joints offset and sealed, and adhere gwb to foam. No framing except maybe a pt sill at bottom. Less drywall finishing. No room or opportunity for condensation leading to mold.

 

[Inspector Gadget]

Where is the vapour barrier in the wall? Where is the tyvek (or are you calling that a vapor barrier?

If you need R20, two layers of 2" XPS with joints offset and sealed, and adhere gwb to foam. No framing except maybe a pt sill at bottom. Less drywall finishing. No room or opportunity for condensation leading to mold.

1) Would still require vapour barrier on the warm side. Canadian codes allow for the interior of wood walls against foundation walls to "breathe" upwards. I'd use vapour barrier on both sides, myself. Or, like you say , go with foam.

 

[bill1952]

1) Would still require vapour barrier on the warm side. Canadian codes allow for the interior of wood walls against foundation walls to "breathe" upwards. I'd use vapour barrier on both sides, myself. Or, like you say , go with foam.

I would never recommend a vapor nor air barrier on the inside of a wall against a foundation wall. Seems like recipe for mold and rot. In "breathing up", where does make up air come from? Do you put vents at inside bottom of wall?

The only way to use hygroscopic material like fiberglass against a foundation wall is with a great vapor barrier against the wall, like Tu-Tuff, preferably embedded in mastic. Or, flash and batt, with studs spaced out an 1", 2" of spray foam, and then batts.

But the wall has to be allowed to dry inward.

 

[steveray]

Interesting situation in our office this morning. A client is installing wood-frame walls in a basement, and is balking at the idea of isolating 2x4 walls - with 5.5" R20 insulation - from the foundation.

Code reference:
View attachment 11706


This leads to an interesting challenge that we thought about: what if the client built the interior stud walls so that neither the wood nor the insulation contacted the foundation? It could be argued that 9.13.2.5(1)(b) would no longer apply.

Thoughts?

Correct

 

[tmurray]

I would never recommend a vapor nor air barrier on the inside of a wall against a foundation wall. Seems like recipe for mold and rot. In "breathing up", where does make up air come from? Do you put vents at inside bottom of wall?

The only way to use hygroscopic material like fiberglass against a foundation wall is with a great vapor barrier against the wall, like Tu-Tuff, preferably embedded in mastic. Or, flash and batt, with studs spaced out an 1", 2" of spray foam, and then batts.

But the wall has to be allowed to dry inward.

Can you point to the code section that specifies this?

9.25.3.1. requires an air barrier to prevent air leakage from the building into the wall cavity.
9.25.4.1. requires a vapor barrier to prevent moisture diffusion from the building into the wall cavity.

 

[bill1952]

Can you point to the code section that specifies this?

9.25.3.1. requires an air barrier to prevent air leakage from the building into the wall cavity.
9.25.4.1. requires a vapor barrier to prevent moisture diffusion from the building into the wall cavity.

IRC? Isn't this a single family dwelling?

What I see in 2018 IRC in 1404.3 Vapor Retarders, is that below grade and basement walls are excepted. So help me out, where in IRC does it say any kind of barrier on warm side of below grade walls?

 

[classicT]

IRC? Isn't this a single family dwelling?

What I see in 2018 IRC in 1404.3 Vapor Retarders, is that below grade and basement walls are excepted. So help me out, where in IRC does it say any kind of barrier on warm side of below grade walls?

This is for Canada.

Code reference:

 

[bill1952]

This is for Canada.

So my Polly on the wall or foam does this, correct?

I guess I'm missing the part that says you have to have a warm side vapor barrier on these same walls. Thankfully you don't in US.

 

[Inspector Gadget]

So my Polly on the wall or foam does this, correct?

I guess I'm missing the part that says you have to have a warm side vapor barrier on these same walls. Thankfully you don't in US.

Yeah, that's elsewhere in the Code. 9.25.4.3, for those of you playing along at home.