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Basement floor and wall details

fpds2014 | Posted in Energy Efficiency and Durability on

Re: FHB Feb/Mar 2005 article “The Stay-Dry, No-Mold Finished Basement” and 4/7/14 GBA Q&A “What’s the Best Basement Flooring System?”

Project: Converting an existing basement to living space in a 1900 house in Portland, Oregon.
See attached sketch for detail.

Questions about how to finish the basement while minimizing mold potential:

1. Floor assembly: Head-height is a significant concern in our basement finishing project where every inch counts (starting with 6′-7″ from slab to bottom of 1st floor joists). Rather than using the floor assemblies cited in GBA & FHB articles mentioned above, would like to use: 1/2″ EPS rigid insulation, taped & sealed, then covered with one layer 3/4″ t&g plywood (doubling as finish floor when stained & sealed). Will this be a safe system to use in Portland, OR to avoid mold issues and also relatively warm for thermal comfort?

2. Perimeter wall assembly: Foundation wall is only 42″ above slab height. Previous owners insulated the pony wall above foundation with rigid insulation & installed drywall flush with inside face of concrete (foundation wall below is exposed & painted). We plan to install 2″ rigid EPS to the face of the concrete wall, then trim the bottom of the existing drywall from the pony wall, and install 2″ EPS on the top of the concrete wall, so that the EPS is continuous from floor to the existing rigid insulation at the pony wall. All EPS taped & sealed. Then at the lower half of the wall, install a 2×4 furring wall with mineral wool batt insulation in front of the EPS. (Attached detail hopefully clarifies this.) Will this approach work to isolate the cold concrete from the interior air and provide a safe (mold-free) & warm space inside?

3. There is a secondary portion of the basement that we would like to enclose (i.e. wall off with insulated walls), but may not have the funds to completely finish. If we leave the concrete floor bare but finish the walls per this same detail, will that be problematic (for mold)? I realize that we’ll have a huge thermal bridge over the entire floor in this area, but it’s for storage and laundry, so would not be as critical for thermal comfort.

Thanks for your input on this!

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  1. Expert Member

    i'm sorry if this is a bit off-topic and involves more work you didn't anticipate or probably want, but the first thing I would do with a 1900 house with a partially buried basement is to secure the framed walls to the stem walls below and floor system above with seismic anchors. Those old houses hop right off their foundations in a shake.

  2. GBA Editor
    Martin Holladay | | #2

    Without running any calculations, I'd feel more comfortable with 1 inch of rigid foam rather than 1/2 inch above your concrete slab. But I understand that compromises sometimes have to be made in a renovation project like this.

    Your plan to install EPS on the interior side of the concrete wall sounds fine to me.

    During the summer, the uninsulated portions of your concrete floor may be damp. But if you don't leave any cardboard boxes on those floors, and don't install carpeting, you can learn to live with such a floor.

  3. Dana1 | | #3

    Average outdoor dew points in Portland are well below the deep subsoil temperatures, so as long as there is reasonable ventilation a half inch of EPS is going to be enough. A vapor barrier is essential though.

    The R15 IRC 2012 or 2015 code minimum performance for zone 4C foundations could be met with 1" of wall EPS + 2x4/R15 studwall. If you're turning the studs sideways, 2" of EPS + 1.5" of rock wool between furring falls short of that performance level. Going with 3" of polyiso and 1x furring through screwed to the foundation makes it though.

    As long as the humidity is controlled to 60-65% RH in the unfinished part of the basement there is no serious mold risk. About 90% of the time that can be achieved with ventilation alone in your climate, but a room dehumidifier wouldn't be working overtime even if it were unventilated.

  4. fpds2014 | | #4

    Thank you all so much for your helpful responses!

    Malcolm, yes, I agree seismic strapping is critical (especially now there's lots of talk about the 500-yr Pacific NW earthquake that's overdue). Fortunately, we completed this during a prior renovation.

    Martin, I appreciate your expertise on this. We may revise the floor system to 3/4" or 1" EPS under 1/2" plywood, under 1/2" engineered wood, and probably forego the acoustic ceiling above to gain back some of the head height.

    D. Dorsett, would you install the vapor barrier between the concrete and EPS, both at the floor and wall? Would this be a plastic membrane?

  5. Expert Member
    Dana Dorsett | | #5

    Installing a vapor barrier at the floor is more critical than the wall, especially when the foam is only half inch. Half-inch Type-II EPS has a vapor retardency above 5 perms, whereas a plywood or OSB subfloor would be lower, until the moisture content reached mold & rot levels. Six mil polyethylene has a vapor retardency of about 0.05 perms.

    With the wall it's something of a judgment call. With a foot or more of above-grade exposed foundation on the exterior and reasonable roof overhangs to limit direct wetting (not that it ever RAINS in Portland :-) ), a poured concrete foundation can dry toward the exterior. If there's only 6" of exposure and 6" roof overhangs installing an interior side vapor barrier would raise the moisture content of the concrete enough to compromise a wooden foundation sill, and would require installing a capillary break between the concrete & foundation sill. With an inch or two of EPS and a latex painted wall there is still reasonable drying capacity toward the interior despite the vapor retardency of the foam & paint, due to the large surface area of a wall, and no direct wetting from the interior.

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