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Community and Q&A

Air Sealing from the outside

BruceAF | Posted in Energy Efficiency and Durability on

Looking for some input. We are starting a project soon in Cambridge, MA. The house is a large structure built in 1869. Air sealing the envelope is a part of the program. We are stripping the two (2) layers of existing siding to expose the sheathing. We are not removing the existing decorative trim (cornice, large corner boards, window trim, etc) so there is not an opportunity to add much rigid insulation. We are presently discussing the approach to air sealing the sheathing. The architect suggested fanfold insulation with tape. We are leaning more toward a spray on product such as EnviroDri though I am not sure as of yet if this product can be sprayed effectively on to what I assume will be 1x sheathing. Another alternative might be adding Zip wall sheathing system and leave it at that. I would like to do something more than wrap the structure in house wrap.

The walls section is (from the inside): lath and plaster, 2×4 balloon frame filled with cellulose (blown in some years ago), 1x sheathing board. I would like an effective air barrier with a high perm rate to allow the walls to dry seasonly. I am not sure yet how much we can add to the sheathing, but given that there are presently two layers of siding (clapboards covered with shingle) I would say there is at least 1-1/4” minimum. The new siding will be cement clapboard type siding.

Any input would from the gurus who frequent the site would be much appreciated. Thanks….

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  1. GBA Editor
    Martin Holladay | | #1

    I'm confused by your statement that you want "an effective air barrier without a high perm rate to allow the walls to dry seasonly." Don't you mean WITH a high perm rate, not WITHOUT a high perm rate?

    There are several options:
    1. Thermoply with taped seams
    2. Zip System sheathing with taped seams.

    Your 2x4 walls in climate zone 5 could receive a layer of exterior rigid foam, as long as the foam had a minimum R-value of R-5. So you could install 1-inch-thick foil-faced polyisocyanurate with taped seams if you wanted.

  2. BruceAF | | #2

    You are correct... Should have said 'with high perm rating. I made correction above.

  3. albertrooks | | #3

    Bruce, I make no claim to be a guru master.

    If you'd like a safe air-barrier, I really suggest that you stick with a high perm house wrap membrane.

    Lay it with staples and then tape the staple heads. Use a good membrane with an accompanying good tape. While I may be wrong, I think its safer on a irreplaceable structure to keep the layers as permeable as they originally were. It's made it this far...

    There are a few really good membranes and tapes. I'll suggest ours and our competitors. Don't use an average house-wrap for a permanent air barrier. The ones I'll recommend have a fleece layer on both sides that protects the "control" layer.

    The good folks at 475:

    Ours: There is a decent video on how to hang it. Not that you really need instructions.

    The spray on products probably won't keep up with the building movement. Gaps opening up a could be a problem. Hanging a membrane on this age of a structure is a safe, no voc, no urea, way to go. It's clean and simple.

    If you need additional insulation, adding mineral wool onto the continuous, gravity layered and taped membrane will maintain the same level of permeability as you go out. It's hard to attach with the wrong fasteners, not hard with the right ones.

    We've got nothing that old on the West Coast. Well not much anyway. Help it last another 143 years. :)

  4. BruceAF | | #4

    Albert - thanks for you response. I went to a Siga workshop a few weeks ago sponsored by New England Passive House. I should have mentioned that I was also considering this approach as well. I tend to agree with your statement about keeping the exterior as permeable as possible. I am not very familiar with the mineral wool sheathings but will look into it. As I stated above, I am not yet sure what I am working with until I do some exploratory demo to look at the existing conditions. I will post what I find out in the coming weeks.


  5. user-1137156 | | #5

    I strongly disagree with the fan fold XPS, way to little moisture permeability, about 1 perm, hence no drying to the outside! Frankly this is a good place to use Tyvec & taped seams about 28 perms). Then if you have room under siding add rigid mineral wool NOT FOAM insulation. 8# density Roxul mineral wool gives r4/inch and excellent permeability (near 50 perms). Adding a near vapor barrier outside wood is an invitation to seasonal condensation moisture problems with rot and fungus!

  6. GBA Editor
    Martin Holladay | | #6

    You wrote, "Adding a near vapor barrier outside wood is an invitation to seasonal condensation moisture problems with rot and fungus!"

    Your statement is only correct if the vapor barrier (or vapor retarder) has no R-value. If you add sufficient R-value -- enough to keep the sheathing about the dew point in winter -- you won't have any moisture problems.

  7. BruceAF | | #7

    We are honing in on our final program to address our air sealing strategy. I, however, still need to convince the architect (and client) that adding 1/2" foam board to the exterior (zone 5) is not advisable. I am leaning toward sheathing the house with huber zip wall and tape as required or applying a high performance air barrier (maybe Siga product). Unfortunately, the 1x sheathing has large gaps preventing use of any type of liquid applied product.

    It is obvious to me that allowing moisture to dry to the outside is mandatory if not using minimum of 1" rigid foam (min R-5) on the exterior. However, I have been wondering what the drying potential to the inside is for old plaster and wood lath walls. I have been unable to locate any data on this (if it even exists).
    Any thoughts.....

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