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Techniques/materials to improve the air-tightness of a 1920s-era bungalow in a hot & humid climate (Zone 2A)?

rhallen645 | Posted in Green Building Techniques on


I am looking for strategies to improve air tightness during a major gut/deep energy retrofit of a 1920s-era bungalow in a hot & humid climate (Zone 2A). The existing house has an un-insulated 2×4 frame, interior & exterior shiplap and exterior brick, all of which we want to save as part of historic features/character to the house. We’re planning to insulate the wall cavities with blown-in cellulose insulation and perform a blower door test to identify gaps, however we expect the walls will be quite leaky. We’d rather not have to remove the brick & shiplap since that’s likely to significantly increase costs.

Ideas & articles references to reading material greatly appreciated,

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  1. Expert Member
    ARMANDO COBO | | #1

    One option is to take all drywall out, cut and fit 1/2" rigid foam (1" better) insulation against the interior side of the outside sheathing, seal & calk, insulate the rest with dense-pack cellulose and install new drywall. This way you allow the outside sheathing to dry when it gets wet.
    I forgot to mentioned, this technique was widely used in NOLA for historical restorations after Katrina.

  2. GBA Editor
    Martin Holladay | | #2

    In addition to the walls, you'll want to pay attention to the attic and the basement (assuming the house has a basement).

    For more information, see:

    Air Sealing an Attic. (See also the articles and links listed in the "Related articles" on this page.)

    Air Sealing a Basement.

  3. rhallen645 | | #3

    Armando: I like the suggestion to combine rigid foam insulation on the outside with dense pack cellulose on the inside. The only problem is most of the house has shiplap on the interior of the 2x4s that we weren’t planning on removing… I’m assuming we’ll have to take some (most?) of the shiplap off in order to get the rigid foam insulation into the cavity. Once we take the old shiplap off, will be able to put the shiplap back after putting the rigid foam insulation back into place? Also, do some rigid foam insulations have a bit more “flex” such that we might only have to take a couple of the pieces of shiplap off, allowing us to “bend” the rigid foam insulation without breaking it before getting it back into position? Finally, are there any articles from post-Katrina restorations that describe this technique?

    Martin: House is pier & beam (no basement or slab foundation). The majority of the house has the original wooden floor that we’re saving, so the only way to access the sub-floor is from the crawlspace underneath. Our current plan is to spray 2” of cellulose mixed with adhesive…however we’d welcome other thoughts.

    All: For the walls, might the airtight drywall approach (in conjunction with the rigid foam insulation + cellulose outlined by Armando above) work in this situation? All the articles I’ve seen on airtight drywall so far discuss its use in cold climates…is it also a good approach in hot & humid climates?

  4. GBA Editor
    Martin Holladay | | #4

    If your house has a crawl space, many of the tips provided in my article on basements ("Air Sealing a Basement") still apply.

    I don't recommend the use of cellulose insulation in crawl spaces. Because of the risk of moisture in most crawl spaces, rigid foam or spray foam make more sense than cellulose in this location. For more information on crawl spaces, see Building an Unvented Crawl Space.

    The Airtight Drywall Approach works in hot climates as well as cold climates. However, if you are planning to cover the interior shiplap boards with drywall, then that fact opens up many more options for insulation -- since there is no need to preserve the shiplap boards.

  5. Expert Member
    ARMANDO COBO | | #5

    I would think that not removing the interior shiplap will make very difficult for anyone to insulate and seal properly, and you really need a vapor impermeable insulation against the interior of the outside shiplap. I suppose one could spray close sell foam, but again, I would be concern of anyone’s ability to do a good job with this application, as close cell foams need to be spayed evenly and in 2” layer max. There are some open cell injectable foams that are fairly liquid and injected through holes on the wall, either from the outside or inside, but I don’t think that would be a good idea in the long run, as it would have a high risk of moisture getting inside the wall cavity. I guess it all boils down to cost, and what risk you are willing to live with.
    I was doing a job in NOLA during the Katrina days, and had to travel back there for awhile after the storm, and I witness this technique being applied. I thought it was brilliant, but I don't know of any articles. Maybe searching the web you'll fine something. Google anyone?.

  6. Expert Member
    Dana Dorsett | | #6

    How much air space (if any) is there between the exterior side ship-lap & brick?

    Is there any tar/kraft/rosin paper on the exterior side of the ship-lap, facing the air-gap?

    How much roof overhang, and how many stories tall?

    Is there window flashing that extends to the exterior of the brick?

  7. LucyF | | #7

    We are in the process of insulating an open crawlspace in a house on piers. It is a pain, but a necessary one. Obviously air sealing comes first. We're using Roxul mineral wool insulation in the joist bays and a vapor retarder membrane - DA membrane from 475 building supply. I talked to the people at 475 and they recommended this membrane because most of the year the humidity here in SC is higher outside than inside.

    Another advantage to the Roxul is that the pests (which we have in abundance in the South) - insects, mice don't like it.

    In terms of insulating the walls, what about installing a smart membrane on the interior of the side of the outside sheathing to provide air sealing? That is flexible and you might not have to remove all the interior shiplap. Maybe you could air seal around that then install the cellulose.

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