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Mass Walls, R Values, and Continuous

jar546

jar546

CBO
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Oct 16, 2009
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Not where I really want to be
For sake of argument, let's use the 2024 IECC, although I believe the verbiage in the note for mass walls is the same in the 2018 & 2021 IECC too. Here is the question:

1) Does the insulation (let's assume foam board) have to be continuous, whether inside or out?
2) In the case of PT furring strips on the flat being used on the inside in the case of a 3/4 R value requirements, would the R4 foam board be allowed to be placed between the furring strips, and be compliant?
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There is no mention of 'cavity' on mass walls. What are your thoughts after you answer the questions?
 
Other entries in such tables use "ci" where continuous insulation is intended or required. Since there is no mention here of "ci," I have to conclude that furring strips are acceptable.
 

R303.1.1​

An R-value identification mark shall be applied by the manufacturer to each piece of building thermal envelope insulation that is 12 inches (305 mm) or greater in width.

That is not possible if you are cutting your foam into strips to fit with gaps. I don';t believe it is designed to be pieced together in 14-1/2" strips.
 
jar546 said:

R303.1.1​

An R-value identification mark shall be applied by the manufacturer to each piece of building thermal envelope insulation that is 12 inches (305 mm) or greater in width.

That is not possible if you are cutting your foam into strips to fit with gaps. I don';t believe it is designed to be pieced together in 14-1/2" strips.
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Just commenting on this last point only (no opinion on the OP), all that section would mean is that the installer would have to cut the insulation into two narrower width pieces. Which seems pointless, but complies with the text above.

Cheers, Wayne
 
wwhitney said:
Just commenting on this last point only (no opinion on the OP), all that section would mean is that the installer would have to cut the insulation into two narrower width pieces. Which seems pointless, but complies with the text above.

Cheers, Wayne
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This is the reality. Tell me how cutting foam board to fit here with all of this thermal bridging meets the intent of the energy code.
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The simple answer is that Table R402.1.2 in the 2021 IECC does not specify continuous insulation for Mass Walls. The Table clearly discusses continuous insulation for other cases. So if it meant continuous insulation, it would say continuous insulation.

Now, I'm not so familiar with the IECC, so there may be some requirements elsewhere that would modify the above. But as the writers of Table R402.1.2 chose not to specify continuous insulation there, there's a fairly high hurdle to overcome.

Cheers, Wayne
 
wwhitney said:
The simple answer is that Table R402.1.2 in the 2021 IECC does not specify continuous insulation for Mass Walls. The Table clearly discusses continuous insulation for other cases. So if it meant continuous insulation, it would say continuous insulation.

Now, I'm not so familiar with the IECC, so there may be some requirements elsewhere that would modify the above. But as the writers of Table R402.1.2 chose not to specify continuous insulation there, there's a fairly high hurdle to overcome.

Cheers, Wayne
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Yes, I see what you mean. However, if you do it that way, such as the photo I provided, you cannot pass any inspections for wiring. There is no way the job in the photo can comply.

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jar546 said:
However, if you do it that way, such as the photo I provided, you cannot pass any inspections for wiring. There is no way the job in the photo can comply.
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Looks like your quote is from the 2024 IECC TABLE R402.5.1.1; the comparable section of the 2021 IECC is worded a bit differently.

Still, it seems like you could comply with the line item aobut "plumbing, wiring, or other obstructions" by installing board insulation against the block up to but not over the wiring, and then carefully filling the channel with the wiring with gun spray foam (and then trimming the overflow flush with the battens). That would "fill the available space."

Cheers, Wayne
 
wwhitney said:
Looks like your quote is from the 2024 IECC TABLE R402.5.1.1; the comparable section of the 2021 IECC is worded a bit differently.

Still, it seems like you could comply with the line item aobut "plumbing, wiring, or other obstructions" by installing board insulation against the block up to but not over the wiring, and then carefully filling the channel with the wiring with gun spray foam (and then trimming the overflow flush with the battens). That would "fill the available space."

Cheers, Wayne
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Yes, but not practical CI, then furring strips over that is the only way. The better builders understand that and do it that way.
 
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