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Interior window shutter/slider

keithtom | Posted in Green Products and Materials on

Newbie here. Planning a new house in zone 10-high desert of California. Going with cmu exterior wall with separate 2×4 stud wall. Double wall basicsally, instead of strapping onto cmu for drywall. Thinking of leaving a 1-2″ gap between the 2 walls and using that gap to house a sliding/pocket shutter to greatly increase the effective R value if all windows. Haven’t seen any good commercially available products and I don’t want to Mickey Mouse a diy shutter. Anybody try this? Any insights?


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not a fan of insulating thermal shutters. Lots of builders experimented with these shutters in the 1970s, but almost all were clumsy and soon abandoned. (Homeowners soon tire of the twice-a-day chore of closing and opening these shutters.)

    I don't know of any commercially available products that would work for the application you describe.

    By the way, you didn't tell us where you will install your wall insulation.

  2. nick_vk | | #2

    Apologies - I re-read the request and realized the question was about interior shutters.

    I know nothing about this product other than it is manufactured down the street from where I live. And I also don't know if that have a product that would work between your two walls, as the product I am familiar with is stored in a somewhat bulky roll above the top of the window. But for what it's worth:

  3. keithtom | | #3

    Martin, with a 2" gap, I could use 6" R19, except near windows, but that would contact the cmu. Is that an issue? Fiberglass insulation in contact with cmu? Does it wick moisture from the blocks? Eifs on exterior.

  4. GBA Editor
    Martin Holladay | | #4

    Ideally, all of your insulation will be on the exterior (as part of the EIFS). I don't know how you could possible detail your sliding shutters with interior fiberglass insulation.

  5. Expert Member
    Dana Dorsett | | #5

    Yes, fiberglass would wick moisture from the CMU, but a sheet of polyethylene or EPDM between the fiberglass and CMU would solve that problem.

    R19s are about the crummiest possible option for 5.5" of insulation, especially if there will be unimpeded 1.5" x 2" vertical channels at every stud to promote convection. R23 rock wool or R21 fiberglass would be far more air-retardent. But 2" of rigid foam (any type) filling the gap at the stud edges, or filling the whole thing with blown cellulose would be easier/ cheaper/ better.

    CA zone 10 is inside of US climate zone 3, where as little as R8 on the exterior (2" of EPS) or even no foam on the exterior with R13s in a 2x4 studwall would meet IRC 2015 code minimum, but more is better. Installing more R on the insideof the CME than the R vale of the foam under the EIFS exterior pretty much erases any thermal mass benefit of the CMU. Instead of a gap, installing the 2x4 /R13 wall tight to the CMU wall and installing at least 3.5" of EPS (R14+) on the EIFS exterior would deliver still deliver thermal mass benefit, and you'd be at more than twice IRC code min performance (it would even meet IRC code minimum in Fairbanks.) With 4" of EPS on the exterior and NOTHING on the interior (or 2" on each side) it would meet IRC code minimum in Minneapolis with nothing but air in the 2x4 cavities. But really anything more than ~R20 "whole wall" may end up costing more than the lifetime energy savings. That would be 2" of EPS on the exterior, R13s in the studwall (even though you lose most of the mass benefit.) Spending some time modeling it in BeOpt (a freebie DOE download tool) would be useful.

    Don't make it too complicated or it'll be guaranteed to get screwed up. The amount of time & money it takes to do operable shutters WELL would be better applied to higher performance windows. IRC code min would be a U0.35 / SGHC 0.25 window. It doesn't cost a heluva lot to bump that to under U0.25 double-pane with low-E on surfaces #2 & #4, even with air-filled 3/4" glass. In your location the low-E on #4 would not have any wintertime window-condensation issues the way it might in Minneapolis or Fairbanks. See the performance charts of different surface #2 options for Cardinal's 3/4" 2 Pane LoĒ +i89 insulated glass units with air fill:

    Pay attention to overhangs and shading factors. You may want to minimize the size and SGHC of west facing windows and install sufficient overhang on south facing glass to eliminate direct mid-day mid-summer sun to keep summertime cooling loads well bounded. Again, spending some time modeling it in BeOpt would be useful.

    It's not too tough to hit Net Zero Energy (meeting the upcoming CA requirement beginning in 2020) with a PV array that fits on the house with R16 on the exterior of the CMU or R8 exterior + R13 studwall, and U0.25-U0.30 windows. See Table 2 p.10 of this document, looking at the Zone 3 row:

    Those are roughly the "whole assembly-R " performance levels that make reasonable sense on a lifecycle energy cost basis, and (coincidentally) the numbers that make it pretty easy to hit Net Zero with PV on the roof if the house has a fairly simple shape/footprint. (Not necessarily for foundations that have 16 corners and roofs with 5 gabled dormers).

    If you looking fordramatically better than Net Zero or best lifecycle financial performance, it's in a difference class of construction.
    For some buying the (not too expensive) PassiveHouse tools might make sense.

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