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Musings of an Energy Nerd

Exterior Rigid Foam on Double-Stud Walls Is a No-No

This type of wall needs to dry to the exterior, so in most cases it shouldn’t have any exterior rigid foam

Double-stud walls have room for lots of insulation and are relatively easy to build. A GBA reader is considering this assembly along with a wall insulated on the exterior with rigid foam. What will work best? Image Credit: Riverdale Net Zero House.

Many green builders like double-stud walls. Double-stud walls use affordable and environmentally appropriate materials to achieve a high R-value.

The classic double-stud wall is made up of two parallel 2×4 walls with a space between them. If the framers leave a 5-inch space between the two rows of studs, this type of wall provides room for 12 inches of insulation — for example, dense-packed cellulose, blown-in fiberglass, or mineral wool.

There are variations on this theme; for example, the wall can be thinner (9 inches is possible) or thicker (perhaps 16 inches). Instead of two rows of 2x4s, some double-stud walls have a 2×6 structural wall paired with a parallel 2×4 wall.

Every now and then, an enthusiastic builder will suggest an “improvement” to the double-stud wall: namely, a layer of exterior rigid foam insulation (see Image #2, below). While this layer of exterior foam might (intuitively) seem like a good idea, in fact it is usually a very bad idea.

Encouraging a wall to dry to the exterior

Traditionally, cold-climate walls have been designed to dry to the exterior. In a cold climate, the moisture drive during the winter is from the interior of the house toward the exterior. The interior air is warm and moist, while the exterior air is cool and dry. Under these conditions, moisture will move by diffusion from the interior toward the exterior. (For more information on this process, see All About Vapor Diffusion.)

If the double-stud wall has plywood or OSB sheathing on the exterior and painted drywall on the interior, moisture tends to diffuse through the drywall and the wall insulation toward the exterior sheathing during the winter. Because the exterior sheathing will be cold, it will absorb moisture; in some conditions, a layer of frost may even build up on the sheathing.

If the wall is leaky, humid…

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  1. kevin_in_denver | | #1

    Thanks for Clearing this Up
    I've worked on a few Habitat for Humanity builds. In the Denver area at least, they've been putting on 3/4" of taped blue XPS over the OSB sheathing for years. The walls are 2x6 with fiberglass batts.

    I sure hope someone at the building dept. noticed the code requirement and has put a stop to it.

  2. GBA Editor
    Martin Holladay | | #2

    Don't expect code officials to understand building science
    Your faith in building inspectors and local building departments is probably misplaced. Code requirements are unclear on this issue, and some elements of the code actually encourage risky wall assemblies.

    If you doubt my statement, check out this article: The 2012 Code Encourages Risky Wall Strategies.

  3. Expert Member
    Dana Dorsett | | #3

    They could/should switch to unfaced EPS (response to Kevin, #1)
    The vapor permeance of 3/4" XPS is about 1.5-1.7 perms, which MIGHT be sufficient drying capacity, but it's marginal. Several fan-fold siding underlayment products are in that range, eg:

    Some are in the ~1 perm range or slightly less.

    Most of these houses using these products aren't rotting (at least not quickly), but ideally 3 perms or higher would be better in zone 5.

    If they went with 1" unfaced Type-II EPS (at a slightly higher R than 3/4" XPS) it would be something like 2.8-3 perms, which would be adequate drying capacity in Denver's dry winter air. Dropping back to 3/4" it would be over 3.5 perms.

    With 1" unfaced Type-I EPS it would be over 4.5 perms, and still higher-R than 3/4" XPS. That's a higher vapor permeance than most exterior paints. If that's a problem, every T1-11 sided house that had been painted would be rotting away, yet they seem to be surviving. And if the extra 1/4" is an issue, 3/4" unfaced Type-I it's over 6 perms, and only R0.75 less than 3/4" XPS.

    Bottom line- it's early to panic about the Habitat houses with the 3/4" XPS even though it's pretty marginal, and not best practice. ~R3.75 foam with low perm foil facers would be a pretty serious problem though.

    With double studwalls there's almost no point to exterior insulation over wood sheathing, since most of the framing is thermally broken by the insulation between the courses of studs. There is still the issue of the thermally bridging subfloor and the floor joists, which needs special detailing to maintain the performance in super high performance walls.

  4. user-4524083 | | #4

    Insulation outside the sheathing
    Martin - It seems like the way to "improve" on a double stud wall is to have a permeable air barrier in the middle of the wall, where it remains warm. I know that there are builders doing some version of this. But the science or rules on it are scarce. Using the foam rules does not really follow when the insulation on the exterior of the air barrier is permeable. Likewise, there are no rules of thumb for using roxul "boards" on the outside of say a 2x6 wall in cold areas. It would make sense that the amount of exterior insulation could be reduced with a permeable exterior insulation. The sheathing may still get condensation in the winter, but in the spring it could dry in both directions, making it safer. Rules of thumb for different climate zones using mineral wool boards would be welcome. I would think that the roxul manufacturers would fund such an experiment. Any thoughts on this?

  5. GBA Editor
    Martin Holladay | | #5

    Response to Kevin Zorski
    My article doesn't address where the best place for an air barrier is in a double-stud wall. You can certainly put it in the middle of the wall if you want -- many builders do.

    My article discusses the fact that a layer of rigid foam on the exterior side of a double-stud wall limits the ability of the wall to dry outward. You are correct that if someone wants to install mineral wool insulation on the exterior side of a double-stud wall, then the mineral wool won't slow down drying to the exterior, so it wouldn't raise any concerns.

    In general, most people who build double-stud walls put the insulation inside the walls, not on the exterior side of the sheathing. So if you like mineral wool insulation, that's where the insulation would usually go. However, nothing is stopping anyone from attaching additional mineral wool insulation on the exterior side of the wall, if that is what they want to do.

  6. user-4524083 | | #6

    Reply to Martin
    Martin - I wasn't suggesting that anyone put insulation on the exterior of a double stud wall- that would be wasteful and have no advantage. I did refer to putting mineral wool boards outside a framed and insulated wall. My point is that there is not a lot of science on using mineral wool boards in place of foam boards, and it seems as though the outside R-value could be less than what is recommended with foam, due to the extra drying potential. Thanks.

  7. GBA Editor
    Martin Holladay | | #7

    Response to Kevin Zorski
    Mineral wool insulation is extremely vapor-permeable. I wouldn't anticipate any problems when builders install mineral wool insulation on the exterior side of wall sheathing, even if the mineral wool insulation is thin.

    After all, most 2x4 walls and 2x6 walls are doing fine, even without any exterior insulation. The don't need any insulation to keep the sheathing warm -- as long as they aren't covered with any exterior vapor retarders or vapor barriers.

  8. peteramerongen | | #8

    I agree 100% that adding to
    I agree 100% that adding to exterior insulation to a double wall system is a bad idea.

    The cost advantage of the double wall system would surely be lost by adding the additional steps required for exterior insulation. Rigid board insulation is typically much more expensive than batts or blown in insulation and usually needs to be done from scaffolding.

    Properly built double walls already eliminate most of the thermal bridging. The thermal bridging reduction gained from the continuos exterior insulation would be negligible. Adding enough to keep the sheathing warm in our Zone 7 climate would need long and expensive fasteners. It is much more economical and probably safer just to increase the space between the 2 walls to get the R value you need ( and add a rain screen).

  9. Expert Member
    KOHTA UENO | | #9

    BSC's Take on Double Stud
    In re putting rigid foam on the outside of a double stud wall--John Straube has a great line about adding vapor-impermeable insulation on the outside of a wall. "It's a bit like jumping over a fence--you want to either be all the way over, or not jump at all... you don't want to land halfway over the fence." As Martin discusses in the column, adding enough insulation to avoid condensation (in a given climate) is the way to go--some of my Canadian colleagues describe sheathing protected by exterior insulation as "looking good enough that you could probably unscrew it and return it to Home Depot." Of course, agree with all of the folks pointing out that double stud + exterior insulation is way more logistical work than is worthwhile.

    BSC's preferred approaches to double stud construction include using spray foam as a flash-and-batt approach on the interior, or using taped structural sheathing mid-thickness as an air barrier/vapor retarder. Yes, the latter uses two layers of sheathing (redundancy), and might be more similar to a Larsen truss than a "typical" double stud wall.

    High R-Value Wall Assembly: Double Stud Wall Construction

    High R-Value Wall Assembly: Double Stud with Spray Foam Wall Construction

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