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Double Stud Wall & Basement Insulation Review – Climate Zone 5A

Illinois_Prairie | Posted in Energy Efficiency and Durability on

I’ve read practically every article on this site and many others related to 2×4 double stud walls and moisture. I want to have a wall that is low risk for moisture issues without introducing any more expense and complexity than necessary. The following seems attainable with my budget, and doesn’t seem to cause much builder concern for construction. From exterior to interior:

LP Smartside Siding
1/4 rainscreen composed of either a mesh or furring strips with an insect screen at the bottom
Tyvek Drainwrap
CDX Plywood, seams taped
blown in fiberglass described as a R48 blown in fiberglass system
Membrain on the interior wall

The walls will be about 11 1/2 inches from outside stud edge to interior stud edge. If it helps, the walls will be under a 2′ overhang with a hip roof and no gable. Is this a low risk wall?

Below grade the proposal is for 2″ of foil faced polyiso glued to the basement walls and taped, foil facing inward, plus 2×4 framing with fiberglass batts between the studs, but will the foil faced polyiso provide enough of a moisture barrier to stop the fiberglass batts from getting musty?

No one seems interested in subslab insulation, I’ve been told how it doesn’t matter, it will make my concrete crack, it might make my teeth rot and give me bad breath too from their descriptions. How significant will omitting this detail be? Manual J, Manual S modeling if it is accurate seems to indicate that it won’t make much of a difference, but my reading seems to indicate that it will increase moisture levels in the basement.

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  1. Expert Member

    I'm afraid I'm not much help with either of your questions.

    Just a couple of small comments on your rain-screen:

    - 1/4" furring strips mean that parts of the cavities will often end up blocked, as it's almost impossible to get any WRB to stay perfectly flat on a wall. I'd suggest considering something thicker. 1/2" or 3/4".

    - I think Tyvek Drainwrap and other textured WRBs are typically used as a substitute for a rain-screen. I'm on sure what benefit they give when there is already a drainage cavity.

  2. GBA Editor
    Martin Holladay | | #2

    Malcolm's right -- you want ordinary housewrap, not Drainwrap, if you have a rainscreen gap.

    I'm a little suspicious of the claim that blown-in fiberglass will give you R-4.17 per inch -- that seems high to me -- but it's not too important. You'll have a high R-value. I'd probably call it R-43, not R-48.

    The basement wall insulation will work.

    For a new house in Zone 5, I would always include subslab foam -- to reduce the chance of summertime condensation and lower the chance of musty odors. It's rare these days to find a concrete contractor who doesn't know how to do subslab foam. You must have found a crusty old guy in his 60s.

  3. Illinois_Prairie | | #3

    Thank You Malcom and Martin both, I’ll avoid the DrainWrap. Drainwrap was originally intended as the rain screen. It seemed that DrainWrap would permit water to leave the wall, but it also seemed as if it wouldn’t provide enough space for moisture to leave the outside of the wall. Is that a valid concern?

    Is the reason that Obdyke Home Slicker (locally Menards carries Aqua-Vent) and similar products can get by with being as thin as 1/4” that they force the House wrap to set flat when the siding is installed, or would I need a thicker variant of that as well? The thicker it is, the more difficult it will be to install, but if insisting on this detail up front saves me rotten plywood 20 years from now, I’m all for it. I just need to know what to insist on so I know, and if that’s 1/2” or greater furring strips OR 1/4” or greater mesh. With that minimum having been met, you be comfortable with that wall assembly if it were your house?

    I too shared your skepticism on the fiberglass R value, and told the individual doing the HVAC calculations it was an R40 wall. As for the concrete slab, well, I guess I can’t have everything if I want to get the house built.

  4. STEPHEN SHEEHY | | #4

    Shane- installing subslab insulation is very simple. Just compact as usual, put down the foam,then poly, then the slab. Our slab has zero cracks, which I attribute to good compaction and control joints. You'll regret not doing it. You might want to use the basement some day and without insulation under the slab, you'll get condensation. Look for reclaimed foam sheets ( xps or eps).

  5. Expert Member
    RICHARD EVANS | | #5

    PHIUS lists dense pack fiberglass as having an R-Value of 4.2 per inch. This can vary of course depending on density. But I wouldn't be surprised if it was that high. (Katrin Klingenberg used dense pack FG for her Urbana Passivehouse).

    Last weekend, my wife and I added 1500 sq feet of 6" of EPS (2 layers of 3") under the slab for our new build (zone 6a). Here are a couple of takeaways:

    1.) If I had not added the foam, then heat loss calculations showed me that I would have needed to heat the basement with the mini split. The foam was cheaper than the mini split- even without the cost of running the thing over time.

    2.) Putting foam down is technically easy but something that our concrete contractor didn't want to touch. He generally doesn't like foam under the slab either because most contractors only put down ONE layer of XPS and call it a day. The idea is that XPS is tongue and groove and doesn't need to be layered up. As a result, the single layer of foam is very unstable and can even float upwards during the pour. Our concrete contractor compared this phenomenon to "swimming in corn flakes". We learned ourselves during installation that staggered layers is key for a flatter, less bouncy surface to pour the slab. I would never trust just one layer.

  6. Expert Member

    There is a hierarchy of effectiveness when it comes to rain-screens. At the top would probably be a 3/4" gap vented top and bottom. At the bottom, crinkly housewraps. Things like Homeslicker fall somewhere in the middle. To me they are most appropriate behind materials like cedar shakes that are difficult to install on conventional rain-screens. Vertical furring is cheap and easy to install. I'd go that route if I were you.

  7. Andrew_C | | #7

    RE - "swimming in cornflakes"
    I thought that this only happened if the insulation was incorrectly put on TOP of the vapor barrier, instead of putting down the insulation first and then putting the vapor barrier over the top so that it's in direct contact with the concrete as intended. If someone made this mistake, it should have been obvious and never repeated.

  8. STEPHEN SHEEHY | | #8

    Rick: the guys from our general contractor put down the foam, single four inch sheets. Maybe because we used heavy poly over it, we had no issue with the foam moving around. We taped or foamed around pipe penetrations. The concrete sub had no issues at all.

  9. GBA Editor
    Martin Holladay | | #9

    My experience with one layer of rigid foam (and ordinary 6 mil poly on top of the rigid foam) was that the concrete pour went smoothly, without any floating problems or other problems. It's not that complicated.

    The "swimming in cornflakes" simile evokes images of soupy concrete (instead of concrete without too much water, which is what you want). If your contractor likes soupy concrete, it's one more reason to avoid the contractor.

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