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What needs to be pressure treated?

user-1137156 | Posted in General Questions on

I’m exploring the use of a wood basement for my “dream house. One possibility is a double wall with the outer wall framed 12″ OC and an inner wall framed 24” OC. The outer wall would, of course be sheathed with ‘foundation grade’ pressure treated plywood and framed with similarly graded lumber. The outer wall will sit on footings and outside the perimeter of the basement’s slab floor. The outer wall may extend higher than the inner, to the bottom of the sub floor. The inner wall will sit on the basement floor slab and support the floor framing and the inner of double walls on the main level. My question is how much of the inner basement wall needs to be made of pressure treated material? My thought s were to insulate the outer wall with mineral wool bats before erecting the inner wall and to sheath & seal the exterior of the inner wall, before it is erected with plywood to be the primary air barrier.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jerry,
    First of all, you can't insulate the interior of a basement wall with mineral wool batts. Because mineral wool is air-permeable, it allows warm, moist interior air to contact the cold plywood of your foundation wall. The result will be condensation, moisture accumulation, and mold.

    You need to insulate your wall with rigid foam or spray foam.

    At a minimum, the bottom plate of your inner wall should be pressure treated. The studs and top plates don't have to be pressure treated.

    Make sure that your foundation hole has a good layer of crushed stone and adequate drain pipes leading to daylight. The key to a long-lasting wood foundation is good drainage.

  2. user-1137156 | | #2

    Martin,
    Thank you for answering my question. I can use untreated studs and top plates on the inner wall but the bottom plate and SHEATHING need to be treated (the sheathing extends to the concrete). That sheathing on the outer face of the inner wall will be glued to the plates and studs and the wall will be set on glue beads. Corners of the inner wall will be glued and caulked. The air sealing, it seems to me, makes this no different from an above grade use of mineral wool. Does mold grow on PT lumber?

  3. GBA Editor
    Martin Holladay | | #3

    Jerry,
    You wrote, "The air sealing, it seems to me, makes this no different from an above grade use of mineral wool."

    I'm sorry, but it's different. Above-grade walls dry to the exterior. When the sheathing gets damp at the end of every winter, it can dry out. The plywood sheathing on a foundation wall can never dry to the exterior, which is why you don't want to use mineral wool.

  4. rockinroger | | #4

    Martin, while we're on the subject, most homes in New Orleans are raised on piers. There is usually about 16" clearance from grade to floor joists. Hence no insulation. Fiberglass batts have been tried with generally ugly results. spray foam would be the way to go, but it's very costly due to the working conditions. The old joists are true 8". I was wondering if 2' mineral wool slabs could be used at the bottom of the joists & then covered with a thin 4'x8' non-combustible sheet. Does this make sense or is spraying the only way to go?

  5. user-1137156 | | #5

    Martin,
    For starters a minimum of 8" of the wood basement must be above grade according to the wood basement design specifications so there is drying to the outside for some of the wall. Second foam reduces the ability to dry to the interior. Third,your admonition is completely contrary to the design guide for wood foundations.

  6. GBA Editor
    Martin Holladay | | #6

    Roger,
    The usual way to insulate an exposed floor (if you don't want to use spray foam) is to install a continuous layer of foil-faced polyisocyanurate on the underside of the floor joists -- with or without fiberglass batts between the joists. The key to making this approach work is to pay attention to airtightness by sealing the polyiso seams and sealing the polyiso at the perimeter.

  7. GBA Editor
    Martin Holladay | | #7

    Jerry,
    You are right that many people insulate wood foundations the way you propose. For the reasons I stated, I don't recommend it. Of course, you should make your own decision based on all of the information available to you.

  8. user-1137156 | | #8

    Martin,
    In "Permanent Wood Foundations Design and Construction Guide" by the Southern Forest Products Association. is the following" "For best thermal performance, fill wall cavities with insulation. When insulating between studs in the below grade portion of basement foundation walls, provide a space of at least 2" between the insulation and the bottom plate."
    Do you know why they would insist on the gap at the bottom?

  9. GBA Editor
    Martin Holladay | | #9

    Jerry,
    Maybe the 2 inch gap is provided to allow a pathway for the water to drip out? (I'm being facetious, of course.) But that section of the wall is like to be the coldest, and the dampest, and the most likely location for mold to accumulate -- especially when the wall is insulated with air-permeable insulation.

  10. user-1137156 | | #10

    Martin,
    It would b the coldest in summer but certainly not in winter as that is the furthest underground. Deep underground temperatures are almost constant through the year. Is it possible the gap's purpose is to create a deliberate convective loop through the air permeable insulation to facilitate moisture equilibration? Isn't pressure treated lumber immune to mold growth?

  11. GBA Editor
    Martin Holladay | | #11

    Jerry,
    Thanks for your correction; you're right that during the winter, the top of the basement wall will be colder than the bottom.

    Mold can grow on fiberglass batts, as is apparent to anyone who has remodeled a basement with batt insulation against the walls. (Although the mold doesn't feed off glass fibers, it apparently grows on dust that collects on the fibers.) I haven't encountered enough mineral wool insulation in the field to know whether mold will grow on mineral wool.

    Mold won't hurt pressure-treated plywood, as far as I know, but many homeowners are nervous about mold, and they don't want it growing in their walls.

    Your "convective loop" theory is interesting. But if you are correct about this recommendation -- and if the recommendation includes a design for a wall insulation system that includes a convective loop -- it would be an admission that moisture accumulation was a potential problem. Obviously, a convective loop hurts the R-value of the wall assembly, and in many ways defeats the purpose of the insulation. Which raises the next question -- why not just use rigid foam, which doesn't need the convective loop?

  12. GBA Editor
    Martin Holladay | | #12

    Jerry,
    If the plywood is pressure-treated, there is no need for it to dry. Just like pressure-treated lumber that is rated for ground contact, the pressure-treated plywood can stay damp for its entire expected lifetime.

  13. user-1137156 | | #13

    Martin,
    I agree that my "convective loop" "theory" makes sense to address potential moisture accumulation in the plywood. I've seen another wood basement cross section, shown in the IRC section dealing with wood foundations. In the IRC cross section the insulation only fills the inner portion of the cavities specifying a 2" air space between the plywood and the insulation. The IRC section would most definitely create convective loops in the air space. This may, in fact be necessary to assure drying of the plywood. If impermeable insulation were used it would reduce the ability of the plywood to dry unless a deliberate air gap were created, prohibiting spray foam and eliminating any thermal advantage of foam which is far more costly than mineral wool.

  14. user-1137156 | | #14

    Martin,
    "If the plywood is pressure-treated, there is no need for it to dry." Doesn't that statement bring us full circle to : There is also no need for impermeable insulation in a cavity surrounded by pressure treated lumber and plywood? Some moisture will accumulate in the plywood even with impermeable insulation. Providing for drying seems prudent. The choices become impermeable insulation and a tall air gap or permeable insulation and a bottom gap as either will allow seasonal drying.

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