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External foam board when using siding and stone / brick

greenhorn2 | Posted in Green Building Techniques on


I am embarking on having a home built in lucky zone 5. I was hoping to use some green building techniques where possible and not cost prohibitive. The current exterior is planned to be stone base and LP smart siding.

I mentioned the concept of 2″ XPS foam over 2×6 walls with blown in mineral wool to my architect and he responded that that would cause issues and/or extra expense because the depth of the wall with the stone would be almost 12″ and the depth with the smart siding would be 8 5/8″ the two main issues being 1) the foundation pour is 10″ and expanding to 12″ would add significantly to the cost, 2) windows would be much more expensive since we would need extended jambs and more framing around them to get them to fit correctly.

He recommended switching to 2×4 walls with 1″ of sheathing and closed cell spray foam to achieve a similar effect without a cost increase. It seems like the R-value would be similar but would I have moisture issues?

Any advice / thoughts?

I also mentioned 4″ of foam under the slab and he recommended foam only at the edges to keep costs down as well.



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  1. GBA Editor
    Martin Holladay | | #1

    To determine whether any particular energy upgrade is worth the investment, you need to run an energy modeling program. If your architect doesn't offer this service, you may want to hire an energy consultant.

    If an energy-saving measure is important to you, you may be willing to invest in it, even if the measure has a long payback. Every homeowner is different. But you have to do the math.

    Most energy experts consider the idea of filling stud bays with closed-cell spray foam to be a waste of the spray foam, because of the thermal bridging through the studs. It almost always makes more sense to add rigid foam on the exterior side of the sheathing.

    Installing stone veneer over a wood-framed wall is a risky approach to cladding, and is often associated with wall rot. If you choose a simpler siding, you may be able to afford better energy-saving measures.

  2. greenhorn2 | | #2

    Martin, thanks for the response, as you mentioned it makes sense to add the rigid foam but you say closed cell foam does not make sense due to thermal bridging. Wouldn't having 2" of rigid foam on the exterior stop the thermal bridging (I thought that was the point) allowing the closed cell foam to work at maximum efficiency? My worry was moisture but I think closed cell and 2" of foam would effectively make that a non-issue.

    As to stone or stone veneer , if it caused wall rot then every brick/stone veneer building would have the issue and I certainly don't see that around here (and every other house is brick/stone where I am). It is installed with a 1" air gap which I assume lessens that issue.


  3. charlie_sullivan | | #3

    The 2" of external foam would alleviate much of the thermal bridging of the spray foam, but spray foam is so expensive that it's a shame not to take full advantage of it. Also, you mention wanting to build green, and spray foam, as well as XPS, use blowing agents (the gas the fills the bubbles in them) that have global warming impact >1000X worse than CO2. Avoiding that is easy for the rigid foam--just switch to EPS. But for spray foam, better alternatives are just emerging from the experimental stage to initial test installations. If you timeline is long enough, they might be available in your area eventually. But I think you'd get better bang for your buck with paying working with the large wall thickness.

    And the wall thickness issues might not be as bad as your architect says. Only the load bearing portion of the wall really needs to sit on top of the foundation wall. The window trim issue is more of a real issue, but lots of builders routinely install windows in thicker walls than your proposed wall.

    Re rot: How humid is your climate? Zone 5 includes some very dry and very humid regions.

  4. Expert Member
    Dana Dorsett | | #4

    R6/inch foam between R1.2/inch framing over 25% of the wall area (typical for 16" o.c. framing) is a waste of high-R foam, since most of the heat is being lost through the framing. A center-cavity R20 in 2x4 framing performs much worse than R20 in 2x6 framing, since the framing fraction through 3.5" deep studs is R4.2 instead of R6.6.

    It also doesn't meet the letter of code for zone 5, since code does not specify a center-cavity R for 2x4 framing without external sheeting. Its either R20 2x6, or R13 + R5 continuous. The fact that the code is silent on center-cavity R for 2x4 without insulating sheathing that there is no commercially available insulation material that would would be sufficiently high R to make a 2x4 framed wall perform as well as R20 2x6 construction, since it would have to be in the neighborhood of R45 at center cavity to overcome the performance hit of the lower-R framing fraction. That would require cavity insulatin of R13 PER INCH, which is higher R than even aerogels.

    This is not a subtle performance hit- it's HUGE hit! The architect suggesting that R23 2x6 + R10c.i. framing fraction) is somehow equivalent to R20 foam 2x4 + R5 c.i. really needs to get educated on the topic! The whole-wall performance after factoring in thermal bridging at a 25% framing fraction is ~R24 for the 2x6 wall, -R16.5 for the 2x4 closed cell wall. R16.5 whole-wall is nowhere NEAR as good as R24- count on it!

    His proposed solution meets code, but it's a 30%+ increase in heat loss over the previous blown rock wool + 2" XPS solution. His beats code min by only about R3 in whole-wall performance.

    The cheaper better compromise:

    A 2x4 /R15 rock wool + 2" of XPS wall is the EXACTLY same wall thickness of a 2x6 wall with no insulating sheathing, and should thus not be a big PITA to build. The biggest cost adder would be the longer masonry ties, but that's more than offset by the cheaper insulation. The whole-wall performance of that stackup is about R20, about R6 better than code-min.

    If you did the green thing and used 2" EPS (R8.4) instead of XPS (which has 5x the lifecycle greenhouse damage compared to EPS of the same density due to the HFC-134a blowing agent) it would still run about R18.5 whole-wall performance, still more than 20% better than code min, and measurably better than the much more expensive closed cell foam cavity fill + 1" XPS proposal. If you did 1" of polyiso next to the sheathing and 1" of EPS over that facing the brick it would even slightly outperform 2" of XPS in a zone-5 location (the EPS being necessary to keep the polyiso in it's optimal operating temp range), coming in at whole-wall performance of about R20.5. (Polyiso also uses low-impact blowing agents.)

    And that's at the same wall thickness as a 2x6 framed wall with no insulating sheathing.

    Most closed cell foam is also blown with a high-damage blowing agent (HFC-245fa), and is to be only used judiciously, and not wasting it's performance by putting it between framing studs.

  5. charlie_sullivan | | #5

    One more option for better-than-EPS performance without XPS's global warming impact is Neopor graphite-infused EPS.

    Also, I think Dana mistyped the ratio between XPS and EPS warming impact: it's 50X not 5X (

  6. user-4310370 | | #6

    Thanks for all the great responses - definitely not going with the blown in closed cell insulation in 2x4 wall. I am looking for a pro in the area now to help with recommendations (Northern Illinois if anyone has a suggestion). But at this point its likely we go back to the std 2x6 wall or the 2x4 with the 2" XPS or graphite infused EPA. However, that looks like it doubles the costs of the wall based on this paper: and that would be about 4% added to the total cost and may be out of budget.

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