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Community and Q&A

Wood-walled basement?

user-1137156 | Posted in Energy Efficiency and Durability on

From what I’ve read it appears one can build a basement’s walls out of wood. The wood is heavily pressure treated and specifically graded for use in foundations. Typical walls use 2×8 s on 16″ or 12″ centers and either 1/2″ or 3/4″ treated plywood. If the stud cavities are filled with air permeable insulation an interior air barrier must be used. The bottom of the walls is restrained by the basement floor which may also be made of PT wood or ,of course concrete and the top of the walls are braced by the first floor framing and sub floor.

I priced material cost for a 9 foot wall using 12″ OC & 3/4 plywood with cavities filled with Roxul and an inside sheathing of Thermoply. It comes to about $3.50 /sq ft of wall. This is what the concrete for an 8″ wall would be if concrete was $141/yd. Which it ain’t up to yet but any way to get the same r value (r21) with concrete will add enough to make the material for the concrete system more costly. To top it off the wood version is made of mostly renewables and has lower embodied energy ie it’s greener.

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't ask a question, so maybe you've already got everything figured out.

    Here's some feedback, however: be wary of the advice to use air-permeable insulation on a below-grade wall. The PT plywood used for the foundation will still be cold, and there is no interior air barrier yet invented that will prevent warm interior air from reaching the cold plywood.

  2. user-1137156 | | #2

    You are certainly correct I didn't ask a question. Sorry about that. Before making the post I searched here for wood foundation and wood basement and was surprised to find nothing. I guess I'm worried that what seems so good might have unintended consequences. Are you aware of any? Your point about condensation possibility I recognize but. I can't see a difference between above grade walls and below grade. In either case if permeable insulation is used an air barrier is vital to prevent damaging condensation within the wall. Shouldn't air barrier construction, details and materials that work above grade work just as well below grade? If I do use a wooden outer basement wall I'm fairly sure I'll still use an inner stud wall as well to support the floor framing. It seems to be the only way to reduce the thermal bridging in the "band joist" area. But with a wood exterior basement wall it seems quite practical and economically reasonable to have r40 basement walls as well by using mineral wool. A well detailed air barrier is certainly worth doing. No doubt some, hopefully very small, amount of condensation will occur whenever the outside temperature is below the indoor dew point at least in the below grade case it's on pressure treated lumber that is really toxic to mold etc. All an air/vapor barrier can do is dramatically slow the condensation process,because you are correct there is no ideal air/vapor barrier.

  3. GBA Editor
    Martin Holladay | | #3

    GBA discusses wood foundations in the "Basements" article of the GBA Encyclopedia: "Foundations made from treated lumber can be less expensive than building with concrete or concrete block. Despite their novelty among many builders, these foundations have a long history of reliability. Regular carpentry crews can erect them, and wood foundation walls can be set on a gravel base rather than a concrete footing. All of that helps make construction simpler, and sometimes faster, than a concrete foundation. Also, framed walls are easier to insulate and wire than concrete. Structural insulated panels with a treated-wood exterior are a similar option."

    Lots of information on these foundations is available on the Web:

    From the NAHB research center: Wood Foundations.

    From the Forest Products Journal: Comparing in-place costs of wood and masonry foundations.

    Wood foundations are code-accepted; see R402.1.

    You wrote, "I can't see a difference between above grade walls and below grade." Here's the big difference: above-grade walls can dry to the exterior. That's what happens every spring, when damp wall sheathing dries out after absorbing moisture all winter.

  4. user-1137156 | | #4

    Let's assume I'm going to use a wood foundation wall system made of PT 2x8s. & exterior PT plywood and I insulate on the inside of this wall with foil faced ISO . I have a choice, do I stuff the stud cavities with mineral wool bats or not? Adding the bats will cost about $1.15/sq ft and will add r21 to the assembly.. To get the same r value using ISO will cost about $2/sq ft. Like most basements some of the wall will be above grade everywhere but it may be as little as 1/8 of the wall height.. If I insulate with only foam on the inside and don't pay attention to air sealing won't the sheathing get even wetter through the winter? In other words air sealing is equally necessary for ANY insulation! Making a portion of the insulation of an air permeable material changes nothing in the moisture problem! Am I wrong in my conclusion that I may as well exploit the economy of mineral wool but once insulation is added an air barrier detail needs to be used? And further that only air permeable insulation should be used in the stud cavities otherwise the lower portion of the sheathing may never dry out.

  5. Expert Member
    Dana Dorsett | | #5

    If you don't let the air-permeable insulation dry toward the interior there's a good chance that it will have significant wetting and (temporary) loss of R-value during the colder seasons. Putting an air-barrier that is at least somewhat vapor retardent between the foundation & studwall is advisable, but there is still risk of condensation on the air barrier if there are any air leaks or high vapor permeance of the interior-side air barrier.

    The better solution is to put a layer of air-impermeable semi-vapor-permeable insulation between the foundation and batt-insulated studwall that has sufficient R-value that the studwall-side of the insulating air barrier has limited condensing potential. A relatively cheap way to hit R21 whole-wall this way (that works for ALL of the lower 48 of the US) is to use 2.5" of EPS (seams sealed) between the foundation and a 2x4 studwall with unfaced R13 rock wool, with the only interior-side vapor retarder being latex paint on the gypsum. That way all of the wood is nearly half-way inside the thermal boundary, the only region with even a remote condensation risk is at center cavity on the above-grade portion, and the condensing surface is EPS, which isn't affected by the moisture. The foundation can still dry toward the interior through 2.5" of EPS (which runs about 1.5-2.5 perms at that thickness) so won't end up saturated with ground moisture. Even cheap low-density Type-I roofing EPS is just fine as the air-impermeable insulation in this stackup.

    EPS increases in R-value as temperatures fall, and assuming the wintertime basement air has a dew-point under 45F (usually is unless the house is extremely air tight and under-ventilated.) When it's cold enough outside that the warm side of the EPS is 45F, a 2.5" layer of Type-I EPS will be approaching R11.5. A simple studwall with a framing fraction of about 15% with R13 batts come in at R10+, using R1 for the combined-R of the gypsum + foundation (on the other side of the EPS.), total stackup is ~R21 when it counts the most, a bit less in the summertime.

    And that stackup is 2" thinner than the 2x8 studwall.

  6. BULLHONK | | #6

    There are products out there that produce such a system and will give you a warenty to such . I have worked with this product before in cold wet climates and it was fantastic.

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