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Community and Q&A

Insulating a P.W.F. (Permanent Wood Foundation)

Mi_Maverick | Posted in General Questions on

I purchased a house with a PWF a few years ago in southern Michigan. I am interested in finishing the basement now, and would like some advice on insulation options for the walls. The pwf is constructed of 2×6’s on 12″ centers with 3/4″ exterior plywood. From what I can see the outside is wrapped with a some type of poly / plastic wrap, and I don’t believe there is any exterior foam type insulation. I would say most of the walls are completely below grade, but certainly some portions that are not – if that impacts the insulating plan. The floor is a concrete slab, it does have drainage below with a sump-pump.
Please let me know if there are any other pertinent details that are needed…
Thanks in advance.

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  1. Dana1 | | #1

    In southern MI start an inch of closed cell polyurethane foam (not more) directly on the plywood, followed by blown fiberglass or compressed fiberglass or rock wool batts. The inch of polyurethane provdis an air-impermeable exterior R sufficient for protecting the above-grade portion from interior moisture drives in winter, but still sufficiently open to allows a reasonable drying rate to relieve ground moisture buildup on the below grade sections.

    Alternatively,on the above-grade portion (starting about 18" below grade, and up) you could go with 2" of closed cell polyurethane which would leave the correct depth to finish it out with R15 batts , and R21-R23 batts (no foam) on the portion that is more than a foot below grade.

  2. charlie_sullivan | | #2

    I think that adding insulation to the inside of the basement walls is risky regardless of how you do it. The earth against the poly well below grade will be cool all year, and if you allow the basement to be warm and moderately humid, any moisture that gets into the wood will want to stay there rather than drying. Dana's considerations address keeping the above-grade part dry, but I think the below-grade part is also a concern. If you keep the basement dehumidified, and you have no issues with water leaking through the wall, it can be OK, but I tend to be conservative about that issue.

    So my inclination is to want to excavate and insulate the exterior. But that could be expensive, and could also lead to tearing up the plastic, and it might be hard to restore that as good as it was to start.

  3. user-1137156 | | #3

    There are several options shown in PWF design guides. The most straight forward is mineral wool bats filling the spaces between studs with a 2" minimum air gap at the bottom.along with a mandatory air tight interior layer (drywall). . While I have never seen the reasoning behind this recommendation I assume the 2" air gap creates a deliberate convection loop that serves to equalze moisture and allow drying to the interior.when conditions permit. Because of the heavy pressure treatment, the wood in a PWF can safely remain wet. . .

  4. Mi_Maverick | | #4

    Thank you all for the information.
    I agree that I am certainly looking to avoid excavating around the foundation if at all possible!
    I have a few questions, just to clarify a few things as I don't want to misinterpret something and make a avoidable costly mistake.

    D. Dorset - with regards to the 1" of poly-foam - this would be "best fit" between the studs and against the plywood, would I then seal each cavity with a spray foam around the foam edge - as the fit will not be perfect?

    Jerry Liebler - when you refer to the 2" air gap at the "bottom" - does bottom refer to 2" from the bottom plate of the wall (from the floor), or a 2" gap between the insulation and plywood sheathing? i.e. like putting 3.5" of insulation in a 5.5" deep cavity.

  5. user-1137156 | | #5

    The 2" is from the top of the bottom plate to the bottom of the insulation. Above the 2" the mineral wool completely fills the cavity(s), from drywall to plywood.. FWIW The PWF design guide also gives guidance for the use of rigid foam cut from boards and placed between studs. For this situation the recommendation is that 1/2" of air space exist between the plywood and foam. While Dana's method may work it is not discussed in any literature that I've seen on PWF..
    Link to site where free download of "PWF Design and Construction Guide" can be done (after establishing free account)

  6. GBA Editor
    Martin Holladay | | #6

    The developers of the Permanent Wood Foundation approach want builders and homeowners to believe that this type of foundation is every bit as good as a concrete foundation. There is a certain amount of boosterism and marketing in all PWF literature.

    The danger with interior insulation is that warm, humid interior air will come in contact with cold (or frozen) pressure-treated plywood. When this happens, frost or liquid water can accumulate on the interior side of the plywood, and this water can puddle on the bottom plate.

    The air space and gaps are a work-around, but they aren't perfect solutions. The best way to insulate this type of foundation is on the exterior, using rigid foam. That method keeps the pressure-treated plywood warm.

  7. user-1137156 | | #7

    Unfortunately Martin is advocating a VERY expensive solution to a problem that simply does NOT exist. PLEASE do not excavate around your basement and install rigid foam!
    The statement Martin makes "The danger with interior insulation is that warm, humid interior air will come in contact with cold (or frozen) pressure-treated plywood. When this happens, frost or liquid water can accumulate on the interior side of the plywood, and this water can puddle on the bottom plate " Is theoretically correct but the likely-hood of pooled water is extremely low and so far as I can tell has never been observed even without any interior air barrier (drywall)...

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  8. Mi_Maverick | | #8

    Jerry- this condition with warm air contacting the cold pressure-treated plywood... isn't that what is happening today in my unfinished basement? If so, I have never noticed any frost or liquid accumulating on the walls. I know this very mild winter right now may be a bad example, but I did not notice it in the previous very cold winters.
    Even if this condition did occur on a rare occasion - a material like rock or stone wool is not negatively effected (long-term anyway, i.e. mold) by getting damp. I know it will "lose" some insulating properties if damp... but I would think it would dry relatively quickly? And if any water did accumulate, it would end up on the bottom plate, and eventually in the drainage below the concrete floor... which makes sense why you leave a 2" gap.

  9. GBA Editor
    Martin Holladay | | #9

    If your basement walls are uninsulated, then the interior face of the wall is warm, because there is no insulation between the interior air and the wall.

    Once you install insulation on the interior side of the wall, the temperature of the wall drops, encouraging condensation.

  10. user-1137156 | | #10

    Literally hundreds of thousands of PWF basements have been built and insulated following the design guidelines. The combination of mineral wool and air gaps along with interior air barrier (drywall) have proven durable and problem free. There is considerable resistance to PWF by traditional builders, who "invent" all manner of objections and ways to make PWF less desirable and more costly (suggesting exterior foam is required for example)... .

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Over the past few years you have posted a wide variety of building assemblies you identify as being "what I'm using", which you suggest others should use too, only to abandon them for something else as your plans change. Wouldn't it make more sense to wait until you have actually built some of them and seen how they perform before so vociferously defending them?

  12. user-1137156 | | #12

    You are absolutely correct in that I've posted many ideas and concepts as I searched for what I wanted to build.. If I said I'm using something I apologize, I should always have said "I'm considering" .However, in the case of PWF which I was slow to adopt as a desirable energy efficient concept I have seen a successful example.. About 45 years ago I built a house, using the conventional techniques of the time, in Michigan. My next door neighbor was building with a PWF, very radical at the time. I saw first hand that PWF worked, and has endured without issues. When I first mentioned PWF here the criticism was near hostile so I did much further research elsewhere.. From that research I'm now strongly convinced that PWF offers the most cost effective energy efficient "green" foundation for most residential construction. PWF however does not tolerate many typical bad practices in drainage and backfill..

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