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Modified Edgewater Double Stud Wall

Tim L | Posted in Green Building Techniques on

Marine Climate 4C

I have been working on a double stud wall design for a Pretty Good House. It is a variation the Edgewater design with inspiration from various other sources. I have redlined the air barrier.

It is close to complete, still working on some flashing details and material specifics. But perhaps good enough at this stage to share my progress.

Would appreciate any feedback or comment.


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  1. Charlie Sullivan | | #1

    Looks a lot better than pretty good!

    The part that might be interesting to debate is the fiberglass insulated floor above the encapsulated crawl with no insulation on the crawlspace floor. What is the ground temperature in your area, and what are your plans for conditioning or dehumidifying the crawlspace?

  2. Expert Member
    Malcolm Taylor | | #2


    As Charlie said, it's a better that Pretty Good assembly. Two layers of sheathing won't be cheap, but yield a very build-able and resilient wall.

    I share Charlie's questions about the crawlspace insulation too. The value of an ICF foundation is lost when the crawlspace isn't conditioned and the floor insulated instead.

  3. Expert Member
    Dana Dorsett | | #3

    The wall performance of a double-studwall with 12" of dense packed fiberglass is more than what would be necessary to hit PassiveHouse performance in a zone 4C climate. (Walls of similar construction & thickness have made it even in zone 6A type climates). Seriously, if taking the performance of the house to THAT level of thermal performance, buy the PassiveHouse design tools and tweak it for best bang/buck, or at the very least download and simulate with BeOpt. Even if you don't take the foundation insulation to PassiveHouse levels, you're probably better off going with a thinner studwall and spending the money on other aspects.

    The XPS under the foundation wall probably won't pass the engineering analysis, and even if it does it's pretty much a waste without continuous insulation on the crawlspace floor. At your above-grade wall R it would make more sense to add 4" of continuous EPS under a 2" rat-slab. If the foundation wall needs to be insulated from below, it goes under the footing, which distributes the load over more area (and the footing width can be adjusted to suit by the engineering analysis), with 2.5" of Type-II EPS on the sides & top of the footing to where it meets the ICF foam, and under the 2" Thermax.

  4. Tim L | | #4

    - Charlie/Malcolm, the 5-year soil ground temp high and low (@8" depth) are 38.2 and 71.3. My understanding is that the fiberglass insulated floor (between floor joists) are not strictly needed. It was part of a belt and suspender approach and I have removed it from the updated detail (unnecessary cost). The crawl space will be kept under negative pressure with an continuous extraction fan with an Atmox monitoring system. Make up air will be provided by air from the above sub floor structure.

    - Thanks Dana for the thorough reply. Good note, regarding the XPS under foundation wall, I have removed it accordingly. Alas the cost considerations prevent me from doing EPS and a rat slab for the crawlspace floor (wish I could though). In general I am working through a lot of good feedback from various sources and I am trying to find the sweet spot between expenditure and gains.This first cross section was likely more nice to have than must have.

    Will post an updated cross section soon, holding fast to the 60-40-20 insulation (attic-wall-foundation) but conceding on some other elements.



  5. Expert Member
    Dana Dorsett | | #5

    If cost is preventing you from insulate the rat slab, seriously run a BeOpt simulation. You can probably get more performance (better payback) out of a couple inches of EPS under the slab than the last 4" of dense packed fiberglass wall + sub-floor insulation above the crawl space.

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