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Which would you recommend for exterior insulation in climate 5 Habitat for Humanity home?

lucyna99 | Posted in Energy Efficiency and Durability on

I specifically mention Habitat for Humanity, as this means, the work is executed by volunteers and quality control may be an issue. Will the taping and flashing be done exactly right..? Which solution would offer a most “fool-proof” option? I specifically worry about reverse flashing created by somewhat incorrectly installed horizontal tapes. Read couple of articles on that…

So, the house is two story, slab on grade, with most likely vinyl siding. 2×6 construction and OSB sheating, unless we opt for some SIP or other system. I decided that I would not be able to trust ZIP installation. The two high runners right now are 1” polyiso sheets and high density rock wool board. Could someone chime in on pros and cons?


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  1. GBA Editor
    Martin Holladay | | #1

    In Climate Zone 5, the minimum R-value for continuous exterior rigid foam installed on 2x6 walls is R-7.5. Even though some brands of polyiso advertise an R-value of R-7.5 per inch, I think that (a) the claim is exaggerated, and (b) the polyiso performance declines with cold weather. I suggest a minimum of 1.5 inch of polyiso or 2 inches of EPS for 2x6 walls in Zone 5.

    The standard answer to your question is, "A Habitat project should use donated materials whenever possible." Contact builidng materials manufacturers and distributors to see what can be donated. Don't reject Zip sheathing or Zip R sheathing -- either product is worth considering, especially if the materials are donated.

  2. lucyna99 | | #2

    Hi Martin,
    Does the minimum R-7.5 for exterior come from the dew point considerations..? And would polyiso require furrings before the siding is installed? If there are installation details somewhere in your library, could you please point me to them?

  3. GBA Editor
    Martin Holladay | | #3

    Start by reading this article: How to Install Rigid Foam Sheathing.

    Yes, the reason that you need the rigid foam later to meet certain minimum R-value requirements is to avoid moisture accumulation in the sheathing. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. Expert Member
    Dana Dorsett | | #4

    The R7.5 is straight out of the IRC code, chapter 7:

    It's not just a good idea- it's the code minimum necessary to be able to avoid interior vapor retarders and potential moisture traps, meeting the "Continuous insulation with R-value ≥ 7.5 over 2 × 6 wall" exception in TABLE R702.7.1.

    If you're going to cheat the R-value, rock wool sheathing under vinyl siding still works in zone 5 without interior vapor retarders. Rigid rock wool is highly vapor permeable and won't slow drying toward the exterior. Vinyl siding & OSB sheathing qualify as the "Vented cladding over wood structural panels" exception in TABLE R702.7.1 .

    Achieving good wall flatness is far easier with rigid foam than with rigid rock wool, due to it's much higher resistance to compression by the furring. With rigid foam if the structural sheathing & framing is flat, the siding plane will be too, pretty much automatically. With rock wool there is likely to be some tweaking of the tightness of individual screws to get it all into a reasonably flat plane.

  5. lucyna99 | | #5

    Hello Martin and Dana,
    I believe we are leaning towards 1.5 inch of polyiso on the exterior. Since we are going to use vinyl siding, please confirm that this CAD library detail is correct for the flanged window at sill
    What I notice is that there is no rainscreen/vented gap between vinyl and rigid foam in vinyl case. Is that correct? I understand from the articles that Martin directed me to that the WRB (Tyvek) would be installed on the outside of foam, since our windows are "outies".
    We are also looking for a CAD detail that has the raised heel truss in vented attic in order to accommodate insulation on top of top plate (per Energy Star requirement). Do you have that available? I could not locate it...

  6. PAUL KUENN | | #6

    Good day Lucyna!

    Vinyl siding does not need a rainscreen as it is loose and has lots of air traveling behind it. You can easily nail it directly over the insulation w/o furring strips if only using 1.5' polyiso as long as you meet the recommended nail depth into the studs.

  7. Expert Member
    Dana Dorsett | | #7

    Paul has it right- vinyl siding is inherently ventilated, and meets the code definition of "vented cladding" even without a rainscreen.

    Bumping up to 1.5" polyiso is a good solution, as long as they get the flashing details right on the outie window mount.

  8. GBA Editor
    Martin Holladay | | #8

    I don't have any reason to doubt that the CAD detail you linked to (vinyl siding at a window sill) will work.

    Vinyl siding doesn't have a flat back, so there is always an air space between the vinyl siding and the housewrap. That's why you don't need to create a special air space (rainscreen gap) behind vinyl siding. It comes with its own rainscreen gap. For more information on this issue, see All About Rainscreens.

    Here is the link you requested to a detail showing raised heel trusses:

  9. lucyna99 | | #9

    Hello, I would like to ask a follow up question. 84 Lumber that will be framing the house wanted to skip OSB and install metal bracing + 1.5'' polyiso with 2x6 stud framing. I know from Martin's article that it is a legitimate construction option with rigid foam, but... what are the good practices now with installing vinyl siding over that? How to setup to be sure siding is nailed to studs..? With OSB present, there would be some forgiveness / flexibility as to where the nails end up, but not for us now... Second question: how to best achieve air tightness with this construction technique? We understand that all seams of polyiso need to be taped, but corners, transitions... - anything to pay close attention to?
    We also tried to convince 84 Lumber to go for 24 oc construction, but got a no on the account they are less familiar with 24 oc...

  10. Jon_R | | #10

    An interior side air barrier (of equal leakage) is better than exterior. And both will reduce leakage.

  11. lucyna99 | | #11

    Hi Jon,
    Could you elaborate? I know we should not enclose the wall section with two vapor barriers. With polyiso, we already have a vapor barrier on the outside. So, what would be the materials that you'd recommend for interior..?

  12. Jon_R | | #12

    For interior side air sealing, you could use air-tight drywall or taped smart membrane (eg, Intello).

    I know of a house with just foam (no wood sheathing) and vinyl siding. Seems to work fine.

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