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Another question regarding “Monitoring Moisture Levels in Double-Stud Walls”

JocelynVT | Posted in Energy Efficiency and Durability on

So after reading Monitoring Moisture Levels in Double Stud Walls… I am in search of a solution for our double stud walls that are just about ready for insulation. We were planning on filling the 12″ cavity with blown cellulose. But that has obvious and now understood issues with moisture and settling we didn’t know about. So what to do with my walls? Our insulator suggests closed cell applied 4″ R 28 and dense pack cellulose 8″ R 28.8. This will still result in a plywood that gets wet and dries but at least we will know the dew point stay on the outside , right ? Thoughts ?


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  1. iLikeDirt | | #1

    Settling is a non-issue if you dense-pack the cellulose, as you should. Moisture from the inside is a non-issue if you have an air barrier on the inside that prevents humid air from entering the cavity--the idea of using a layer of cheap sealed OSB as an air barrier inside the double wall nailed to the outside of the inner studs, with cellulose on both sides of it is a cool idea to improve the robustness of the air barrier and prevent the requirement for meticulous drywall detailing necessary for airtight drywall. And if you build a drainage cavity on the outside to allow any moisture that does condense on the exterior plywood to dry easily, you should be fine unless you're in Alaska or something. If you're really worried about the exterior sheathing, use board sheathing instead of plywood, which is itself hugely more moisture-tolerant than OSB. For extra protection, you can build with wide roof overhangs to prevent the walls from getting very wet in the first place--a good idea regardless of what wall approach you choose.

  2. JocelynVT | | #2

    Sorry forgot to say we live in Zone 6, Vermont.

  3. GBA Editor
    Martin Holladay | | #3

    I can't tell whether you are thinking of putting the spray foam on the exterior or the interior side of your thick wall. But either way raises issues. If you want a thick double-stud wall, just insulate it with dense-packed cellulose.

    If you want to build a flash-and-batt or flash-and-fill wall, you are restricted to a thinner wall assembly, because you need to keep the right ratio between the spray foam layer and the fluffy insulation layer.

  4. user-1072251 | | #4

    Settling should not be an issue if installed correctly. Moisture has been found in some double walls, but it typically shows up in March and dissipates by late spring/early summer. No one has seen rotting taking place. The best way to avoid problems is to build the house so that it is very tight, and install an HRV; these measures will decrease excess moisture in the house, which in a leaky house could work through the insulation and condense on the sheathing. You could also use fir plywood rather than OSB for sheathing, but sounds like you are already past that stage. Done right, you should not have a problem.

  5. JocelynVT | | #5

    Yes, the walls are framed. The insulator is proposing "Flash and Fill." What ratio are you talking about Martin? From my research (may have misunderstood) the foam had to be a minimum of R 25 to keep the dew point on the outside when the total wall would be R 40+ (foam plus cellulose). While we're at it... what about Spider Insulation ?

  6. charlie_sullivan | | #6

    Spray foam would work if you did 4" as proposed, but it's an expensive solution with high environmental impact (the spray foam uses a gas with > 1000X the global warming impact of CO2 to make its bubbles). Another option is to use a "smart" vapor retarder layer like MemBrain or Intello Plus on the inside. In your climate that might not be needed but it's a great way to add extra insurance against moisture in your sheathing. Is your sheathing OSB or plywood (e.g. CDX)?

    Spider would be similar to cellulose or a little worse--the same moisture issues occur in either, but cellulose buffers the moisture and slows down any problems.

  7. Expert Member

    A couple of comments on the comments:
    Nathaniel, I don't think you can assume that air sealing makes moisture from the inside a non-issue. The houses tested by Building Science are well sealed and no house has zero air movement through the pressure boundary.
    Bob: While it may be true that there hasn't been any failures due to rot, I think John Straub's take on this makes sense. "These elevated moisture contents are lasting for several weeks to two months, depending on the study, into April. And in April, the sheathing may be at 50 degrees. That is right in the zone where you might get a problem, though it might take years for the problem to manifest itself. It’s in the gray zone.” These are risky walls.

  8. GBA Editor
    Martin Holladay | | #8

    You're right. In Climate Zone 6, a flash-and-fill wall that is 12 inches thick, and that has 4 inches of closed-cell spray foam on the exterior side of the wall, and 8 inches of cellulose on the interior side of the wall, will work. It's just expensive and not very environmentally friendly.

  9. Dana1 | | #9

    In zone 6 if you used fiberboard or gypsum sheathing (eg GP DensGlas) and vented siding you would be able to skate by with a class-III vapor retarder on the interior (eg standard latex paint.)

    BTW: "...we will know the dew point stay on the outside , right?"

    Rong- it's an improperly framed statement. Dew point is a temperature (the temperature at which a body of air is a 100% relative humidity), and not a location within the assembly. The relevant dew point in this discussion is the dew point of the interior conditioned space air, which varies, but can be controlled during a zone-6 winter by the ventilation rates. If you keep the interior air at ~35% or lower @ 68-70F the dew point of that air is about +40F.

    But the dew point of the entrained air will follow the temperature of the sheathing, and will vary considerably over the course of a day or week. As steathing temp drops it adsorbs moisture from the air in the insulation layer, and as it rises it releases some of that moisture back. Thus whenever the temperature of the sheathing is below the dew point of the interior air it imparts a vapor pressure difference across the interior side layers, and moisture will move into the cavity by diffusion, rate-limited by the vapor permeacne of those layers.

    If you installed a layer of smart vapor retarder under the interior side wallboard it lowers the vapor permeance toward the interior to less than that of CDX plywood as long as the proximate air to the vapor retarder is dry enough. If you keep the interior at 35% RH during the winter months and have a rainscreen gap between the housewrap & siding for the CDX to dry into you'd be fine. (Certainteed MemBrain is pretty cheap insurance at about $100 per 800 square feet retail. Intello Plus is a bit more expensive, but still a heluva lot cheaper than 4" of closed cell polyurethane. Either would be "worth it" if you don't provide sufficient foam for dew point control.)

    When you introduce a low permeance insulating layer such as closed cell foam the dew point of the air inside the cavity tracks the temperature of the interior surface of that foam in winter. The low permeance of the foam limits the rate at which that moisture can dry toward the interior, so it has to be sufficiently thick to keep the average temperature at the foam/fiber boundary at or above the dew point of the conditioned space air to avoid moisture accumulation.

    The vapor permeance of half-inch CDX is about 1 perm when dry, but can be as high as 5 perms when saturated. An inch of closed cell polyurethane is typically about 0.8-1.2 perms, but at 4" it's 0.2-0.3 perms, which means the assembly can only dry toward the interior. Fiberglass-clad gypsum is highly permeable, well north of 10 perms (as is most fiberboard) which allows the assembly to dry toward the exterior even in winter, provided you don't block that drying path with closed cell foam.

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