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Wall design

jack13624 | Posted in General Questions on

I have a new construction house and am curious if this will work for the walls. I’m in upstate NY. Climate zone 6. I have 2×6 walls with OSB. I plan on putting 2 layers of 1 inch XPS on the exterior with Tyvek or Typar over that and vinyl for siding. R-15 Roxul inside the walls with no vapor barrier.

Will this be OK for moisture and condensation?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I plan on putting 2 layers of 1 inch XPS on the exterior ... Will this be OK for moisture and condensation?"

    A. No. In your climate zone, if you want to put rigid foam on the exterior of a wall with 2x6 studs, you need to install rigid foam with a minimum R-value of R-11.25. Your plan is to install R-10, and that's not enough. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. jack13624 | | #2

    Even though I'm only going to put an r15 batt in the walls? What if I only put an r13 fiberglass batt in the walls? I'm not actually filling the 2x6 walls with the full r value.

  3. GBA Editor
    Martin Holladay | | #3

    Sorry -- you're right. If you don't fill the stud cavities all the way, then (from a building science standpoint) you can get away with less exterior rigid foam. (Needless to say, you will still have to run your idea past your local building inspector, whose interpretation of section R702.7 of the 2012 IRC may be different from mine.)

    As I noted in Combining Exterior Rigid Foam With Fluffy Insulation, all you need in Climate Zone 6 is for 36% of your wall's total R-value to come from the rigid foam layer. So you're right.

    That said, I urge you to reconsider your plan. It's always better to fill the stud bays all the way with insulation. The problem with installing batts that are too thin is that you can end up with an air space between the fluffy insulation and the wall sheathing -- and that might encourage convective loops that reduce the thermal performance of the wall.

  4. jack13624 | | #4

    Ok. Thank you. I am going to re think the whole plan.

  5. jack13624 | | #5

    Based on the 36% rigid insulation wouldn't I only need r10.44 if I had r19 fiberglass in the walls? That would eliminate the convective loop? I would only need to gain a .44 r value on the exterior. I could use a fanfold along with the 2 inches of xps. An I looking at this right? What other options to eliminate thermal bridging and condensation would work?

  6. GBA Editor
    Martin Holladay | | #6

    Fanfold insulation plus 2 inches of XPS would work.

  7. charlie_sullivan | | #7

    I think you'd be better off with 3" of EPS or 2.5" of Neopor (graphite infused EPS) because in the long term, the XPS r-value advantage will go away, and the 3" of EPS probably costs less. Plus XPS has a substantial climate impact from the blowing agent used to make the bubbles, >1000X the effect of CO2.

  8. jack13624 | | #8

    Is iso any good in my situation?

  9. Expert Member
    Dana Dorsett | | #9

    R19s are crap (for just about anything but mouse-nest material). They are not very air retardent, and only perform at R18 when compressed into a 5.5" wall cavity. If you're going to insulate the wall, INSULATE THE WALL already!

    R20 fiberglass is mid-density, reasonably air retardent and is cheap. R21s are higher density and a lot better. R23 rock wool is better still. Cost-wise it may be cheaper to go with damp-sprayed cellulose (R20, but with perfect fit, and no voids) than R23 rock wool, but it won't be cheaper than R20 fiberglass.

    XPS ends up at about R4.2/inch in 50 years, so even if the labeled R is R12, it won't likely be performing better than ~ R11 at age 25. EPS doesn't have that drift issue, and 3" of Type-II EPS (R12.6) is often about the same cost as 2" of XPS. Type-I EPS runs about R3.85/inch, is pretty cheap, and 3" of Type-I would also be enough, but it's fairly fragile in handling unless it has plastic or foil facers. The facers make air sealing with tapes a bit easier though. When the mean temp through the foam is 40F a 3" layer of Type-I EPS runs about R4.15/inch, Type-II runs about R4.5/inch, so it's really going to be OK at 3", but not at 2.5"

    Polyiso performs well below it's labeled R when the mean temp through the foam is 40F or lower, so you'll have to derate it to about R4.5-R5/inch for dew point control in your location & application, but you can reliably get there with 2.5" of foam rather than 3" of EPS, and have somewhat better shoulder season performance.

    If you're going to cheat the exterior R, by very much it's worth installing 2mil nylon (Certainteed MemBrain) under the interior gypsum board as a vapor retarder. The vapor permeance of sheet nylon varies with the relative humidity of the air in the studwall cavity. During the winter the RH will be quite low and it will behave like a Class-II vapor retarder, severely limiting the vapor migration from the interior into the wall. But in the spring when the sheathing warms up, releasing it's winter time moisture burden the nylon becomes vapor open, allowing the wall to dry rapidly into the interior, limited primarily by the vapor permenance of the paint. Standard latex primer on wallboard with a layer or two of inteior latex paint runs 3-5 perms. In winter the nylon would run well under 1perm, but over 10 perms when the humidity in the cavity is high enough to support rapid mold growth.

  10. jack13624 | | #10

    I'm trying not to go over 2 inches on the exterior if possible. That's why might thought was to decrease the interior insulation to avoid the risk of damp sheathing. Though I do have 2x6 walls and as Martin said there is a risk of convective loops. Any way to avoid the convective loop with the 2x6 walls and a thinner interior insulation. R21 is code for the walls in my area. I'm trying to do the best approach with the minimal cost at once. I plan on completing the exterior before I complete the interior.

  11. Robert Opaluch | | #11

    I'm curious why you want to minimize the amount of insulation in your walls. Leaving uninsulated space between studs especially. As you probably know, minimizing insulation increases your need for winter heating, increasing your heating bills for the life of the structure. In the event of a fuel shortage or heating system problem, you may find yourself in a very cold home mid-winter. Insulation is relatively inexpensive, requires no maintenance or replacement if installed well. Are you planning just a seasonal home? Opposed to regulations about increasing insulation? Or ?

  12. charlie_sullivan | | #12

    To answer your question about reducing convective looping, you could install batts intended for 2x4 walls, and then put an air barrier of some sort over the batt, air sealed at the edges, leaving a 2" cavity. Perhaps 1/4" plywood or 1/4" drywall. But that's a lot of work to cut and foam or caulk in place.

    Another option is to use mineral wool boards on the exterior instead of foam. Then you don't need to follow any rules about minimum thickness--the moisture problem that rule is designed to avoid simply doens't exist.

  13. user-1041981 | | #13

    For the exterior insulation, I'd recommend not using EPS, XPS, or Polyiso. You really want to look at Roxul Comfortboard. See:
    I have 2-3/4" on my house which is R11, or 3" would get you R12 which exceeds the R11.25 requirement Martin noted from the IRC standard.

    I think Martin and Dana will agree that this will let your sheathing dry outward (which foam won't) and is more environmentally friendly. There also isn't any loss in insulating value over time.

  14. 39Chev | | #14

    What was the cost of Roxul. I stopped at my Home Depot and they quoted me some crazy prices.

    I am also wanting to stay with 2" foam over 2x6 construction...only because I am using vinyl siding and with 2" foam, I can use readily available long roofing nails to attach the siding through the foam. With 2 1/2 foam, it is doubtful that you would have enough nail in the stud. It looks like Dana thinks this is doable if using Membrain on the interior. (Northern edge of Zone 6).

  15. Expert Member
    Dana Dorsett | | #15

    Per code, if you're going with less than the minimum R prescribed in the IRC for dew point control you need a Class-II or tighter (< 1 perm) vapor retarder. While you could do this on the cheap with air-tight wallboard and half-perm paint ("vapor barrier latex" primer), a 2-mil nylon vapor retarder will still allow rapid drying toward the interior, but will be reliably in the Class-II zone whenever the sheathing's temperature was below ~40F or so. That gives the assembly as a whole much higher drying capacity, making it more resilient to moisture in general.

    The more exterior R there is, the fewer hours the sheathing will be that cold, but when the sheathing is above 40F it won't be taking on moisture from the conditioned space air unless you do something stupid such as actively humidifying the place to well over 40% RH all winter. The dew point of 40% RH, 68F air is 43F- there will be at least some vapor pressure difference drawing moisture through the paint and sheet nylon when the sheathing is at 40F, but with a vapor permeance of less than 1 perm the rate at which moisture can get through via diffusion is miniscule.

    But moisture can also get in via air leakage (a lot more than can get in via diffusion), which makes air sealing that much more important when skimping on exterior R. Detailing the nylon as an air barrier isn't rocket science, but there are a lot of particulars to attend to:

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