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Community and Q&A

Can you spray closed-cell insulation over mineral wool insulation?

Joseph Isherwood | Posted in Building Code Questions on

New house design with 2×4 outer wall with mineral wool then 1 inch gap then 2×3 inner wall. gap and 1 inch of inner wall sprayed with closed cell

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  1. Expert Member
    Dana Dorsett | | #1

    Seems expensive and complicated. I'm not sure if spraying 2lb polyurethaned directly onto the rock wool won't work, but it doesn't make a lot a sense even if it does work.

    Using edge strips of 1" polyiso as the thermal break between the 2x4 and 2x3 wall and using two layers of 3.5" rock wool would deliver about the same thermal performance.

    If an intermediate vapor barrier is the rationale, a continuous layer of 1" rigid polyiso is about half the cost of 1" of spray polyurethane, at about the same thermal performance. With foil-facers it's also easy to air seal with aluminum tape.

  2. Joseph Isherwood | | #2

    Thank you for your answer. You mention complicated and not making sense plus expensive and offer alternatives which are more labor intensive, provide less thermal performance and never answer the question. The rationale is a total thermal break plus sealing all electrical boxes, plumbing etc.

  3. GBA Editor
    Martin Holladay | | #3

    We got lots of suggestions for idiosyncratic walls here are GBA. It's your house, and you can build it any way you want. I don't see any reason why you can't insulate the way you describe, other than the fact the most brands of closed-cell spray foam use a blowing agent that has a high global warming potential. Most green builders try to avoid the use of closed-cell spray foam.

    I tend to agree with Dana -- there are less expensive ways to achieve the goals you seek.

    For more information, I suggest that you read this two articles:

    Installing Closed-Cell Spray Foam Between Studs is a Waste

    How to Design a Wall

  4. Expert Member
    Dana Dorsett | | #4

    Joseph- I told you right up front that I didn't know the answer about spraying directly onto batts (sorry- I only know what I know- never seen it done.)

    Even with the additional labor cost of taped full layer of 1" polyiso it's cheaper (by about half) than an inch of closed cell polyurethane, at almost exactly the same performance. Maybe it's "expensive", by some measures, but not MORE expensive than the 1" polyurethane, and nearly identical in performance.

    Were you planning to insulate the 2x3 wall at all? Perhaps I'm not clear on your stackup description. I was assuming:

    outdoors | siding | sheathing | 2x4+ rock wool | 1" spray foam foam | 2x3 + rock wool | wallboard | indoors

    Was recommending:

    outdoors | siding | sheathing | 2x4+ rock wool | 1" sheet foam | 2x3 + rock wool | wallboard | indoors

    or, 1" foam strips at the studs, with 3.5" of rock wool for the 2x3 + foam strips.

    The hit in performance from edge-strip only with the foam is pretty small. It roughly doubles the R-value of the much more thermally conductive framing fraction, but is only a very modest increase in the center-cavity thermal conductivity, displacing R6-R6.5 foam with R4.3 rock wool, less than a 10% increase in total center-cavity conductivity.

  5. Brendan Albano | | #5

    Have you ruled out the "standard" high performance that looks something like this, from outside to inside?

    - Siding
    - Rainscreen gap
    - Rigid insulation (say 1.5" of polyiso, EPS, or rigid mineral wool, or more if desired)
    - WRB
    - Sheathing
    - 2x6 @ 24" o.c. with fiberglass, mineral wool, or cellulose
    - Gypsum

    Your wall assembly is somewhat idiosyncratic, which perhaps is because there is an unusual design requirement in this project. If you could share the design requirements that lead you to your wall assembly, other posters might be able to give you more targeted advice to help solve your particular envelope design challenge.

  6. Joseph Isherwood | | #6

    Dana- the wall assembly is 2x4 filled with mineral wool, then 1 inch gap and 2x3 wall. The gap and 2x3 wall are sprayed with 2 inches of closed cell foam. The wall comes in at R30 but more important is the total thermal break achieved including electrical boxes, a major source of leakage. The local building inspector wants documentation this application is acceptable. Your method is cheaper but does not seal all edges, seams, boxes without extra time, labor and waste. It also doesn't achieve R30. With regards to being green icynene brand has pro-eco 100% water based closed cell foam

  7. Expert Member
    Dana Dorsett | | #7

    The inch of closed cell foam is between the 2x3 studs buys almost nothing. A full 2.5" of fluff or half-pound open cell foam (which would fully seal everything) in the 2x3s will be substantially higher performance, and costs about the same as an inch of closed cell foam.

    The inch of foam filling the gap between the courses of stud can be done more cheaply and uniformly using rigid polyisocycanurate, and is higher performance than an inch of ProSeal Eco, which is only R5 @ 1". A shot of 2.5" of half-pound Icynene Classic (LD-C-50) filling the 2x3s even uses less than polymer than an inch of ProSeal Eco (which is 2.2lb density).

    At 2" ProSeal Eco is still not at class-II vapor retardency either (but it's close.) It takes at least 2.4" to duck under 1US perm.

    Your center cavity R is well shy of R30 with 3.5" (R15) rock wool, 2" (R10) ProSeal Eco, and nothing in the other 1.5" of the 2x3 stud bay depth. It's only R26. With R15 rock wool, 1"(R6) polyiso, and 2.5" of LD-C-50 (R9) or compressed R13 fiberglass or split R23 rock wool you'd be at R30, and the thermal bridging of the 2x3 framing is reduced by 60% even before the thermal break of the sheet polyiso.

  8. Joseph Isherwood | | #8

    My rvalue calcs were based on regular icynene pro not eco. I mentioned eco to try and pacify the green advisors knowing the depth would have to increase to 2 1/2 inches to meet vapor and Rvalue. Thanks for the suggestions

  9. Joseph Isherwood | | #9

    Just a word of thanks to Dana for saving me money with the suggestions posted. I have decided to spend some of the money joining green building advisor.

  10. GBA Editor
    Martin Holladay | | #10

    I'm glad that the information you found on this site is useful. (Thanks, Dana!)

    And I'd like to welcome you to the GBA community. Thanks for subscribing.

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