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I would be most appreciative for any of your experiences or preferences between Faswall versus typical ICFs.

user-3133848 | Posted in Green Products and Materials on

I’m planning a rural build in Southern Oregon –zone 4C– passive solar 2100 sq. ft. modular on conditioned crawlspace 2-3 feet high. The crawl which will act as a plenum will be insulated below the rat slab with EPS and it appears that ICFs and Faswall offer roughly the same R-value for through wall heat loss. I am trying to avoid EPS or XPS (unless encapsulated under the slab) due to their environmental negatives, possible out-gassing and fire risk. I am also concerned with potential insect issues with borers into the ICFs but will be parging the exterior surfaces of either product type. The Faswall product combines stabilized wood chips and concrete along with rock wool insulation into interlocking form blocks but appears to have more thermal bridging potential, along with being heavier and more expensive from source to my site . Any preferences or advice? Many thanks in advance for your thoughts.

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  1. user-3133848 | | #1

    Just for clarity: Either block system will form the perimeter stem walls only, to be filled with concrete and rebar. A central line of piers provides support for the interior load beams. Thanks.

  2. GBA Editor
    Martin Holladay | | #2

    I've never seen a report from an independent laboratory that shows the measured R-value of a Faswall wall. While the company claims R-21 for their walls, others have estimated the R-value as closer to R-11. The walls include concrete and thermal bridges, neither of which help, and the insulation inserts are not continuous.

    The R-value of an ICF wall can range from dismal to OK -- generally in the range of R-17 to R-23.

    I think it's nuts to consider using a crawl space as a plenum. This arrangement almost guarantees air leaks -- whether you are thinking about pressurizing or depressurizing the crawl space -- and these air leaks will guarantee high fan energy usage as well as energy losses due to infiltration or exfiltration. Using a crawl space for a plenum is probably a violation of your local building code. That approach also raises the chance of IAQ problems.

  3. user-3133848 | | #3

    Martin: Apparently I shouldn't have used the term "plenum". I am thinking that we would follow your option 2 approach and install "a supply register in the crawl space, the grille allows the air in the slightly pressurized crawl space to flow to the house above", not as the principle distribution for heated air. Our heating strategy is a mini-split and an ERV. Sorry for my error.
    The Faswall manufacturer emphasizes the benefit of thermal mass, but that would only provide real benefit with the addition of external insulation. Sounds like I might as well stay with ICFs. Thanks.

  4. GBA Editor
    Martin Holladay | | #4

    Yes, I agree with you that ICFs probably make more sense than Faswall blocks. There is less thermal bridging with ICFs.

  5. GBA Editor
    Martin Holladay | | #5

    One more comment on the "plenum" issue: you are correct that when a crawl space is pressurized by a forced-air register, and when the house has a grille that allows crawl space air to enter the house, the entire crawl space is, in effect, a type of plenum. I'm sorry that my answer didn't consider that possibility. While such a system is code-approved (and is clearly different from systems that use a crawl space rather than ducts to deliver pressurized air to heat and cool the upstairs rooms, which is what I was thinking of when I called the approach "nuts"), it does raise IAQ questions, and in my opinion is less preferable than a crawl space conditioning system that includes an exhaust fan mounted in the crawl space.

  6. Dana1 | | #6

    Using an encapsulated crawl space as the return has very low duct impedance, operating at a very low pressure difference, and would improve blower efficiency.

    It's not clear how that would be more of an air quality risk than dirty ducts, ducts that are more difficult to inspect or clean than an encapsulated crawl. In most homes the living space rooms &/or hallways are all part of the return path, and regularly contaminated with all sorts of things. Using the crawl space as the return keeps the crawlspace air from stagnating and accumulating concentrations of contaminants.

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