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Community and Q&A

Faswall Block Construction and Human Health

mikeysp | Posted in General Questions on

In Zone 4a (Nashville) my friend is considering a Faswall house. He is not mechanical, so he is definitely going to hire a General Contractor to handle it.

A healthy home is his number one priority and number two is to build as small a budget as possible, BUT not at the expense of priority number one.

It seems to me that the trades will have a lot better time dealing with advanced framing with great details, executed well and using some materials that he is comfortable with such as mineral wool insulation and cellulose.

His primary weight toward the Faswall is the idea of a healthy assembly.

Faswall is made of 85% mineralized wood/15% cement and with a 3″ mineral wool insert, the total wall turns out to be R21 according to the manufacturer.

This area is a competitor for mold capital USA 🙂 and they lived in a trailer that had lots of mold and family members have some health issues, so they are fearful of ending up in the same bad situation.

I am not sure, but I think the thought of glue in OSB, or other manmade introductions into the construction causes a little fear of “unhealthy” and mold friendly.

I am trying to explain that it is good practices and assemblies that make a healthy house and even a Faswall assembled without proper drainage, subslab details, window flashing, ceailing air sealing, etc… will be as prone as any other house to problems.

Anyways, I am just looking for some wisdom to help a friend out.

PS. I included ICF in the equation, because he is also considering ICF.

Thanks for your wisdom. He is looking forward to hearing wise men.


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  1. Expert Member
    RICHARD EVANS | | #1

    Hi Mike,

    I tend to agree with you here. I admire the Faswall folks for their innovative design and they seem to be really passionate about their product.

    I have some initial thoughts on the Faswall as they pertain to your friends objectives:

    1.) Rockwool is not known for being a healthy insulation. Rockwool is still manufactured with Formaldehyde which some people aren't too happy about. If your friend has sensitivities, they may not like mineral wool.

    2.) At quick glance, the block seems to be at least 50% stone-ish (mineralized wood) 25% air and 25% insulation. Sorry, but I call BS on the R-21 claim. Using the dimensions provided, I plugged it into the Ekotrope wall R-value calculator as best I could. The result was around R-11, at best.

    Imagine building a stick-built wall with studs 10 inches on center and the studs are 2 inches thick. Now assume those studs are made of concrete (R-zero). No thank you!

    (I realize these blocks are made of "mineralized wood" not concrete but if they are claiming that they have thermal mass then I assume it performs similarly to stone or concrete and has a similar R-value.)

    3.) Batt insulation generally needs 6 sides to prevent convective loops and R-value reduction. The large void in the core will likely reduce the R-value further.

    4.) How do you air-seal these things? The website says that you stack them without using grout. If your friend uses drywall inside then the significant air leakage will undoubtedly result in mold somewhere- not to mention needlessly higher energy bills.

    A Healthy, Simple wall:

    Your friend can opt for a 2x4 wall filled with dense pack fiberglass. This is the healthiest cavity insulation of all. Add plywood instead of OSB as sheathing. Tape the seams with Zip tape to avoid air leaks. Then add 1.5" of foil-faced polyisocyanurate outside to keep the sheathing above dew point. Ventilate the house with balanced ventilation. Use a small-ish AC to dehumidify in the summer. Opt for low VOC paint and interior finishes. Include good drainage under and around the foundation (as you stated). Use a class I vapor retarder under the concrete slab.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      I think Rick has given y0u very good advice.

  2. user-2310254 | | #3


    I invested a lot of time investigating Faswall and aerated autoclaved concrete. I really like the mass aspect of both products but ultimately abandoned these strategies for a more conventional build (similar to what Mike recommended). One thing I found is that is is difficult to find trades who are skilled with these systems and interested in working on residential products (more an issue for Faswall than AAC). I would only go with one of these alternatives if your friend is considering a DIY approach and comfortable with masonry. In that case, the AAC would probably be more user friendly and easier to source than the Faswall, which has to be shipped from the west coast. Also keep in mind that silica dust exposure is a potential risk when working with AAC.

  3. Expert Member
    BILL WICHERS | | #4

    Man made vs natural products isn’t all there is to mold problems. Mold feeds on natural things like wood. Mold seems to like more heavily processed wood more (I like to think of this as “mold likes wood that someone else has already chewed for it” :-). OSB is known to be more mold prone than framing lumber, for example, but none are immune.

    Many synthetic products are basically mold proof since they offer no food source for the mold (urethane, rigid foam, spray foam, concrete, steel, etc).

    Regarding Rick’s comment about mineral wool and formaldehyde, the manufacturing process is supposed to burn all that off and leave a pretty safe final product. I’m not aware of mineral wood being considered any better or worse than Fiberglass in that regard. owens Corning does make their “puresafety” brand of fiberglass that is specifically manufactured to be safe for sensitive people. I’ve used it before and it’s a good product in terms of installation and handling, but I have no way to test the allergy sensitivity part of it.

    If you use foil-faced rigid foam, you can encapsulate any other materials you deem less healthy. The foil facer is a heavy aluminum foil, so no food for mold, and no products with any possibility of offgassing since it’s just a sheet of metal. If you tape the seems, you can use this material as a safety barrier to help to isolate you from any other materials in the wall assembly. You need to be careful about the ability of the wall to dry depending on how you do do this though.


    1. andy_ | | #6

      Even though this is an old thread...I still have to take issue with a blanket statement that OSB is inherently riskier for mold than plywood. Depending on the product I've actually had the opposite experience building in the true mold capital of the US, the rain forests of the PNW.
      Zip sheathing left out in the winter rains for a month showed a little edge swelling in a few places, but no mold. Plywood for the roof deck delivered in spring and kept covered developed extensive mold even after the roof went on.

      1. Expert Member
        PETER G ENGLE PE | | #7

        Andy, I think you're right about the difficulty of blanket statements. With OSB, one product is not at all like another. It seems that the ZIP products are manufactured and treated differently/better than run-of-the-mill OSB. I'd include Advantech as another that seems more durable than others. You're right that plywood can get moldy when left out to get wet. Sawn lumber can certainly get surface mold. In my experience, there's also a difference between surface mold and structural decay. Sawn lumber often comes with surface mold already on it, but decays much more slowly than plywood, and plywood decays more slowly than run-of-the-mill OSB. I haven't seen enough building failures with Advantech/ZIP to really say how they do, although the lack of observed failures may in itself be significant. As in everything BS related, it depends.

  4. dianacherry | | #5

    Hey mikeysp,
    I was wondering if your friend ended up using Faswall in his Nashville build? We live in Nashville as well and I am seriously considering this product as well for the low VOC, less chance of mold, and thermal mass benefits. Please let me know if you happen to be able to connect me with him or builders he used so I can figure out the best way to proceed. Thank you!!!!


  5. Desert_Sasquatch | | #8


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