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Climate Zone 5: concrete slab remodel insulation strategy

Mid Century Modern Boulder | Posted in Energy Efficiency and Durability on

What would you recommend for insulation strategy for a remodel that has an existing uninsulated concrete slab for a walk out basement?

Client Desires:

– maximum ceiling height (current cement to joist pre finished is 93-inch)

– air tight well insulated house (r-60 roof and r-50 walls)

– engineered hardwood floor in walk out basement

The Walk out basement will be wrapped with 15-mil Stego vapor barrier prior to finishing.

The current thought is (from top down):

– 1/2″ Engineered Hardwood

– 1/2″ advantech subfloor

– 1″ Neopor GPS – graphite polystyrene rigid foam

– 15mil Stego vapor barrier

– existing uninsulated concrete


Thanks for your insight and recommendations!

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Replies

  1. Jon R | | #1

    This is worth reading (especially the fully adhered slab vapor barrier part):

    https://buildingscience.com/documents/insights/bsi082-walking-the-plank

    Note that relative humidity under plastic will be sufficient to grow mold independent of interior dew points.

  2. Expert Member
    Dana Dorsett | | #2

    Do I correctly assume this is in high-dry Colorado 5B, and not swampy New England 5A?

    In zone 5B the subsoil temps are sufficiently north of the summertime dew point averages that summertime floor mold/condensation is usually not an issue. That makes the primary reasons for insulting it energy use & comfort. While the subsoils might be 50F, dry well drained soils also have a real R-value.

    What is the slab temperature well away from the walk-out end when the room air has been at a stable 68F for days/weeks?

    If the slab temp is north of 60F it could be fine to use a very moisture tolerant wood-grain ceramic tile (if the wood look is what they're after) rather than insulating it, but beef up /correct any deficiencies of the slab edge insulation on the walk-out side.

    If tile is a no-go, there are floating click-lock engineered hardwood flooring products that can be installed on concrete with only a (6-mil polyethylene minimum, Stego would be fine) vapor barrier between the concrete & flooring.

    If the slab temp is always cooler than 55F even at the deepest part of the basement, insulation will make a difference in comfort. But if every half-inch matters, the difference between R2.5 and R5 foam under the subfloor will be fairly meaningless. If a floating engineered wood is used, you might even use half inch MDF or asphalted (bottom side only) fiberboard sheathing for the subfloor, since it's not really structural, being fully supported by a concrete slab, and adds another R0.5 to the whole-assembly performance.

    How are the below grade and above grade walls of the walk-out basement being insulated?

  3. Tyler Keniston | | #3

    Is there a plan to attach the 1/2 advantech subfloor to the concrete through the foam?
    Is the Neopor just for maximum R value per inch?

  4. Mid Century Modern Boulder | | #4

    @DanaDorsett Yes, the house is in Colorado and high-dry 5B. I'll have to measure the slab. It'd 80-90-degrees right now so its a comfortable cooler temp now, just don't know if its cold.
    - Grade is about 1-ft from the ceiling at the highest, and drops quickly to near walkout height 2/3rd the way to the walk out.
    - the wall are going to be insulated with 4-inch of EPS, then 2x4 framing, then dense packed cellulose in exterior wall cavities.

    @TylerKeniston - the Neopor GPS was just an idea of maximum R per inch attempting to take the edge off. didn't have a plan of attaching the advantech to concrete, maybe could just skip it and have a floating floor on the foam?

    @JohnR that article looks good! I'll have to spend some time reading it.

    Much appreciate everyone's insight and recommendations!

  5. Mid Century Modern Boulder | | #5

    @JohnR - I think in a nice way you were telling me Figure 7 from that article... "don't be dumb and use a plastic sheet." but instead an epoxy fluid applied vapor barrier.
    - do you have an example recommendation?

    Prosoco make R-Guard but not sure if it can be used on cement?

  6. Tyler Keniston | | #6

    You may already be aware of these, but two more articles:
    https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab
    https://www.jlconline.com/how-to/foundations/insulating-over-a-structural-slab_o

    They mention use of tapcon fasteners, or two layers of sub-floor with staggered seams. Perhaps one t&g is fine?

    The Lstiburek article Jon posted is interesting. It seems to contrast the recommendations often made here, which do include the poly.
    I too would be interested in the specific product recommendation. My gut is telling me applying such product is more labor intensive and costly than a sheet good, so the question is: is it really necessary or all that much better than poly/sheet goods?

    1. Jon R | | #7

      I only have more questions. If you are going to use poly, is a mildewcide useful?

      1. Tyler Keniston | | #8

        The concern being the film space between the concrete and the poly growing molds or that moisture in that space could escape, causing molds? There isn't really food in the former is there? The later is the concern Joe appears to have, and this is based on his fear an air-tight install won't be achieved with plastic sheeting goods.

        Feels like a case of: If you detail and execute it carefully, it works, but if there isn't a guarantee of that happening, don't try it. Unless I'm misunderstanding the science of the issue.

        It might be a case for at least using a more robust plastic sheeting like 15mil vs 6mil and maybe a case for not fastening through the sheet. I guess saying all that does make it seem like his concern is a good one.

        1. Mid Century Modern Boulder | | #9

          I'm still curious about what epoxy fluid applied vapor barrier you guys would recommend?

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