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What is the best way to superinsulate a walk-out basement?

NY_ART | Posted in Green Building Techniques on

I need an expert’s opinion of my current and future game plan for my Upstate NY – Zone 6, 1968 back-split renovation.

Q1: Can I safely add 4″ of exterior poly iso, since I will already have 3″ of closed cell on the interior of the wall?

Q2: How can I improve this plan?

See plan details below.

My first project will be to insulate and finish the lower level (41ft X 12ft). Two sides of the space are partially exposed (between 1.5 to 3.5 feet below grade). Third wall faces a semi-conditioned garage.The fourth wall backs up to a ventilated dirt floor crawl space. All walls are block (~4ft) and 2×4 framing (~4ft + 12″ joist space for 2nd floor).

My Plan:
[Stage 1]
Crawl space – Insulate two exterior walls + wall facing garage with 3″ closed cell spray foam. Remove ventilation and existing insulation between the overhead floor joist.Wall facing lower level will be left un-insulated.
Also, install 6 mill ply on floor. Eventually poly will be augmented by concrete floor.

Lower Level – Frame out using 2x4s all exterior block walls (two sides).
The third wall facing garage is already 2X6. Fourth wall facing crawl space will be left uninsulated.
Insulate all three walls with 3″ closed cell.
Insulate concrete floor with 1″ rigid foam + plywood & finished floor.

Rear cantiliever (1.5ft x 71ft) will be opened and insualted with 4″ of closed cell

[Stage 2]
Future plans for the house include adding 2 layers of 2″ polyiso to the exterior of the building including the lower level. This will also involve residing the bouse with either fiber cement or reusing existing cedar siding.

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  1. GBA Editor
    Martin Holladay | | #1

    Q1: "Can I safely add 4 inches of exterior polyiso, since I will already have 3 inches of closed cell on the interior of the wall?"

    A. Yes. It's common to install both sides of a concrete wall; that's the way that insulated concrete forms (ICFs) are designed.

    Q2: "How can I improve this plan?"

    A. That's a fairly general question. Any plan can be improved. For example, you could include more than 1 inch of rigid foam above your existing concrete floor.

  2. NY_ART | | #2

    Thanks for the response. Regarding adding polyiso to the block portion of the wall, I am not too worried about that section. Where I am concerned is the 4 feet of above grade wood framing/sheeting.

    Assuming that I seal the inside cavity of the wood walls with 3 inches of closed cell, do you foresee any problems with adding 4" of polyiso to the outside? I believe that normally we want the wall to be able to dry to one side or the other. Am I creating a problem for myself?


    PS: Re: the floor isulation. I only have 8feet of head room from the existing concrete floor to the bottom of the 2nd floor beams. With double 5/8 sheetrock (for sound proofing) + 1inch of rigid isulation + finished flooring, I am very close to my minimum 7'9" headroom. Is there a good way to warm the floor with less insulation? possibly more effective than the R5 rigid insulation?

  3. GBA Editor
    Martin Holladay | | #3

    If you want to install rigid foam on a wood-framed wall, choose which side you want to install it on, but don't install it on both sides. You want the wall cavity to be able to dry in at least one direction.

  4. 415irwin | | #4

    Would it be best to allow drying to the outside? Given that the rest of the basement will be covered with exterior foam perhaps the above ground portion should be allowed to dry to the outside. In a past article or discussion on this site I believe it was said that the path for much of the moisture in a basement wall is through the top of the cement wall. Is this correct?

  5. GBA Editor
    Martin Holladay | | #5

    Again, I'm confused. Are you talking about the above-ground portion of a concrete wall or are you talking about a wood-framed wall?

    If some of your concrete wall is insulated on the exterior with rigid foam, then all of your concrete wall should be insulated on the exterior with rigid foam. If you stop your insulation at grade level, it hardly does any good at all -- because your concrete wall will lose a tremendous amount of heat from the exposed above-grade portion of the concrete wall.

  6. 415irwin | | #6

    I am talking about the above groung wood framed portion of a basement wall.

  7. GBA Editor
    Martin Holladay | | #7

    On a wood-framed wall, the best location for rigid foam is definitely on the exterior, not the interior. That way the foam will keep your stud bays warm and will lessen the chance of moisture accumulation in your sheathing and studs.

  8. Expert Member
    Dana Dorsett | | #8

    First, you can't insulate the exterior below grade with polyiso- it is hygroscopic and will slowly saturate if buried. You CAN insulate the interior with poly iso.

    If you have 3" of 2lb foam in the stud bays you can still use poly iso on the outside, but you have to be selective about the facers used. DO NOT USE FOIL FACED goods, but there are many asphalted paper or fiberglass facers out there that deliver 0.5-1 perms of drying capacity @ 1 layer only , which would be fine. The published specs are usually a somewhat squishY " < 1 perm", but that's fine. Just don't double them up. The 3" of cc foam is about 0.3-0.4 perms, unless you go with the higher-perm/lower-R water blown Icycene, which would run about 1.3 perms @ 3"/R15. And while the difference between R18-R20 and R15 may sound like a lot, with the thermal bridging of the framing at a even 20% framing fraction (lower than average) that's still a difference of only R1 in whole-wall R. Were it my house I'd do open cell foam in the cavities and spend the money on fatter exterior iso where you at least get the full-R out of it.

    If the 2lb foam is already in and you're concerned about the iso facers being too vapor retardent, 4" of unfaced EPS would still be running about 1-perm. And at the low-temperature extremes in zone 6 the 4" EPS would be performing at about R18, to the iso's R22 (at that temperature and thickness), since EPS gains R with lower average temp, and iso gives up some. At warmer temps the difference in performance between the two would be more pronounced.

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