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Community and Q&A

WRB – Ice & Water Shield or Grace enV spray-on (to breathe or not to breathe)?

Irishjake | Posted in General Questions on

Zone-6A building a PERSIST type home with 8″ of Neopor on the exterior. Should I wrap the sheathing in Grace Ice & Water or use a spray on WRB like Grace enV. The Ice & water doesn’t breathe, and the enV does. No vapor barrier on the inside and currently there are no plans to insulate in the stud cavities.

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  1. GBA Editor
    Martin Holladay | | #1

    Either product will work. There is no reason for this layer to be vapor-permeable, since the wall sheathing can easily dry to the interior. Actually, the wall sheathing stays at interior conditions (warm and dry in winter) because all of the insulation is on the exterior side of the sheathing.

    For more information on the PERSIST approach, see Getting Insulation Out of Your Walls and Ceilings.

  2. Irishjake | | #2


    Thanks for the quick response. That is what I presumed. Can you recommend a vapor impermeable spray-on membrane? I like the thought of having a spray on membrane, with no seams.

    I know you've posted about much insulation is needed on the exterior, but how can I figure out what "I"need in zone 6A (NH) with 8,000 HDD to prevent any condensation on the inside. For over a year my plans have been based on no interior side insulation, but I am considering putting in some mineral wool to the interior side to reduce the outside thickness. I'll have a HRV.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Can you recommend a vapor impermeable spray-on membrane?"

    A. The spray-on membranes that I am familiar with are all vapor-permeable -- but that's OK, because there is absolutely no reason for you to worry about the vapor permeance of this layer.

    Q. "How can I figure out what I need in zone 6A (NH) with 8,000 HDD to prevent any condensation on the inside?"

    A. This question's easy. The chance of having any condensation on the interior of your wall sheathing is zero. Your wall sheathing will be at indoor conditions -- at room temperature, just like your dining room table. It will never be cold, so it will never be a condensing surface.

  4. Expert Member
    Dana Dorsett | | #4

    The fairly low vapor permeance of 8" Neopor (without even looking up the specs I'm confident that it would be less than 0.5 perms, as any 1.5lb density EPS would be) makes the vapor redardency of the WRB irrelevant, even if there were some cavity insulation lowering the average temperature of the sheathing.

    The fact that the sheathing is at the full interior conditioned space temp makes it even less relevant, as Martin points out.

  5. Irishjake | | #5

    Dana & Martin,

    Thanks for your continued articulate information. Yes, yes I do understand that the exterior insulation will cause the interior sheathing surface to be warm and therefore stop any condensation as long as my RH is managed appropriately.

    My conundrum are this - in the Building Science Corp Document "BSI - 081_Zeroing In by Joseph Lstibureck he states on page 4
    of the document....."you can easily go to 8-inch thicknesses of continuous insulation and not get into trouble with attaching cladding when you hold things together with 1x4 furring and long screws." Where is the evidence that this is true? All the other articles and research by BSC have limited exterior insulation to 4", due to sag and deflection of the screw or lag attachment thru the furring strip.

    I also wonder with only 4" of insulation on the exterior of the walls, and 6" on the exterior of the roof in the New Zero Test house he describes in the Zeroing In document where the dew point is in the walls that are depicted there. The stud and rafter cavities are full of blown-in cellulose. Wouldn't that cause the dew point to be somewhere in the cellulose (especially with the exterior insulation being poly-iso and it's negative thermal shift degradation)?

  6. Irishjake | | #6

    Oh and so if vapor perm doesn't matter - would you go spray on or peel and stick?

  7. GBA Editor
    Martin Holladay | | #7

    While it's possible to install furring strips through 8 inches of rigid foam, you should contact an engineer to determine the type of fasteners and spacing of fasteners used to secure your furring strips.

    For more information on this issue, see Fastening Furring Strips to a Foam-Sheathed Wall.

    Q. "Would you go with a spray-on WRB or a peel-and-stick WRB?"

    A. Either one would work. So would taping the seams of your OSB or plywood wall sheathing, and protecting the sheathing with housewrap. If I had to decide between a spray-on WRB and a peel-and-stick WRB, I would probably choose the least expensive option.

  8. Dana1 | | #8

    I don't personally have the talent to reliably hit the studs with a 12" timber screw- maybe JoeL knows some real magicians who can pull that one off(?). At 4" foam thickness it's still pretty easy, at 6" it's starting to get pretty awkward.

    "The stud and rafter cavities are full of blown-in cellulose. Wouldn't that cause the dew point to be somewhere in the cellulose ..."

    The dew point it a temperature, not a location- it's a measure of the absolute moisture content of a volume of air, specifically, the temperature at which it would have 100% relative humidity and begin condensing. In a wall assembly with fibrous cavity insulation there are three dew points of relevance- the dew point of the exterior air, the dew point of the entrained air in the cavity insulation, and the dew point of the conditioned space air. When those dew points are all equal, there is no moisture diffusion taking place between them, when they differ, there is a vapor pressure difference pulling moisture from higher dew point locations to the lower dew point areas, at a rate limited by the vapor permeance of the material and the difference in vapor pressure.

    In winter in a zone 6A location the outdoor dew points are quite low, but the dew point of the cavity insulation air tracks the temperature of the coldest side of the cavity, which is typically the sheathing. The vapor permeance of wooden sheathing is pretty low (less than 1-perm when dry), so the cavity air moisture collects in the wood. When the temperature of the sheathing is below the dew point of the conditioned space air, there is a vapor pressure difference between the conditioned space and the cavity, and moisture will migrate from the conditioned space into the cavity, and accumulate in the sheathing. So the dew point of greatest concern is the dew point of the conditioned space air relative to the sheathing.

    The dew point of 68-70F 30-35% relative humidity air (typical winter air in a sorta-tight house with reasonable insulation/infiltration rates) is about 38-40F. So as long as the AVERAGE temperature of the sheathing over the winter is at least 38-40F the amount of moisture accumulation in the sheathing is limited to levels where rot & mold aren't a problem, as long as it can dry at a reasonable rate during warmer weather. Typical interior latex paints run about 3-5 perms, which is an adequate vapor retardency to limit that moisture accumulation, and offers very adequate drying rates for any moisture that finds it's way into the cavities. But if you keep the interior a 50% RH all winter there can still be problems for sheathing that averages 40F over the winter, since the dew point of 50% RH/ 70F air is a bit over 50F, whereas the dew point of the cavity air averages 40F, which generates a significant vapor pressure difference to move moisture from the conditioned space into the sheathing, and 3-5 perm paint isn't quite tight enough to be fully protective.

    In a zone 6a climate, as long as you have at least 35% of the center-cavity R on the exterior, the sheathing will stay warm enough to be protected from the moisture drives presented by 30-35% RH / 68-70F conditioned space air. For more humid interior environments (say, a pool room or sauna) you have to go much higher, or skip the cavity insulation entirely. For unvented roof assemblies the exterior R has to be 50% of the center-cavity R value or greater. (Radiational cooling makes roofs run colder than the air temp, whereas the exterior of insulated walls hew pretty close to the outdoor air temp.)

    The drawings in that linked-to article do not specify the climates for which it still works, but with 4" of polyiso on the exterior of 5.5" of cellulose, even if you derate the average performance of the polyiso to R4.5/inch (it'll do better than that) you would have R18 on the exterior to at most R20 in the cavity, or 47% of the center-cavity R on the exterior, which is PLENTY of margin for zone 6.

    On the roof they have 6" of polyiso (call it R27 if you derate it to a worse than reality R4.5/inch) , to 11.25" of cellulose which is at-most R49, so you'd have at least 39-40%
    of the total R on the exterior, which works for zone 5 (where Joe Lstiburek lives :-) ) but is a bit shy for zone 6. It can still work OK for zone 6 though, since the cellulose itself will buffer and share the moisture burden with the roof sheathing, but it would still be better to use a "smart" vapor retarder on the interior (eg Certainteed MemBrain, or Intello Plus) to limit the moisture uptake, while maintaining the full drying rate. Smart vapor retarders have vapor permeance of less than 1 perm when the proximate air is under 35% RH, but go high-perm at higher humidity. So in the spring when the roof deck is giving up it's moisture during the warm days, it lets the moisture out faster than it regains it during the cold overnight hours.

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