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Thermal mass lessened with overlay over slab?

ktot | Posted in Energy Efficiency and Durability on

I have a passive solar off-grid home that works extremely well with a 2 in. concrete slab that is finished with decorative concrete stain/paint. However, the slab and stain/sealer are completely defective and very likely have to be replaced, but one party is proposing a concrete overlay product be put atop the existing slab. The overlay product (ArtCrete Deck Coat, I think it is) consists of two parts–1) Portland cement, aggregate, and a stain, and 2) a modified acrylic latex resin. My belief is this is not good thermal mass (atop the concrete) as the resin component synthetic or at best partly plant material and only earth materials such as stone, clay, water, concrete, provide optimal thermal mass. That in fact the entire slab needs to be removed and the floor rebuilt from scratch to resolve the serious problems and still provide me with optimal thermal mass.

I’ve also been told there’s a table that shows even natural tile atop a slab causes a loss of thermal mass, but I have been unable to find that table. If anyone is aware of this or similar, please let me know.

Note also the slab has severe cracking–cracks at least 3/8 in wide in multiple locations.

Does anyone have experience or knowledge about this? Using or not using a concrete overlay product for the top layer of a floor for thermal mass, vs. a plain darkly stained concrete slab–whether there would be any loss of thermal efficiency.

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  1. GBA Editor
    Martin Holladay | | #1

    K Tot,
    It seems to me that if the entire slab needs to be removed, then you have the opportunity to replace it with any kind of slab you want -- including a concrete slab with dark stain.

    I'm not familiar with the concrete additive you refer to -- "modified acrylic latex resin" -- but I am guessing that the resin component of any concrete mix would be a tiny percentage of the mix, and is therefore unlikely to interfere with the concrete slab's performance as thermal mass.

  2. ktot | | #2

    Of course if the slab is removed I can redo it in any manner, but the contractors who did the work are likely to argue for overlay. As I do more and more research, besides the non-earth material component of the overlay product issue, some people believe (and one cited a table but cannot remember the source) putting two distinct layers (of anything) will degrade the thermal mass due to the need for the heat to transfer from (in this scenario) the overlay (if it's adequate thermal mass) to some sort of mesh covering the cracking and finally down to the slab which is the main thermal mass element.

  3. GBA Editor
    Martin Holladay | | #3

    K Tot,
    Do you have a link to the manufacturer of this overlay -- to a web page describing the product?

  4. GBA Editor
    Martin Holladay | | #5

    K Tot,
    I have looked at the link. I can't imagine that the small about of resin used as an additive will make much of a difference in how these slabs perform as thermal mass.

  5. jinmtvt | | #6

    Do you know why the slab cracked? what is the spawn of the slab ? is it on grade or on beams?
    who is reponsible for the slab ?

    Someone needs to loof at the 3/8" cracks ... 3/8" is pretty large and looks like a mechanical failure.
    Could you take a picture of the slab and describe its setup ?

    I surely wouldn't accept a cracked slab that has openned up by 3/8" as a floor slab in a house where the slab is decoreated and act as a finish

  6. ktot | | #7

    Jin Kazama, can you please elaborate on your last sentence? That's the kind of thing I need as much detail about--factual information--as possible. I agree it's unacceptable but the concrete contractors are trying to claim it's common and normal (one said 99% of their jobs have cracks at least 1/4 in. wide, the other said 50% do--for residential and commercial stained slabs, i.e., decorative finishes on the slabs.

    The cracks have been looked at by an expert PhD PE concrete engineer and the report is forthcoming.

    The cracks are due to way too much water added on-site during the pour, probably an improper mix, and the trucks sitting around too long. I don't know if the expert report will say the following or not, but my view is the improper finishing/curing processes and techniques, sealing the floor with a non-breathable sealer only nine days after the pour with inadequate testing before doing so (in fairly cool weather in the northern Rockies in April before the house had heat on, i.e., when it was still under construction), and a few other things also may have contributed to the problem.

    The floor is overbuilt with 11-7/8 in. joists. The engineering report on this has been done and it's a fact that the floor itself is fine. There has been zero settling of the house in two years--no drywall cracking, no sticking windows/doors, and so on that would be possible signs of settling (again, this is based on comments by an expert as well as observation by me and others).

    So can you please elaborate on what you mean by "looks like a mechanical failure"?

    The slab is elevated, not on grade. The concrete contractors poured the slab as well as did the staining/sealing. My builder of course built the floor prior to the concrete going in.

    I'm not positive what you mean by spawn of the slab. The house isn't rectangular but it's probably more square than elongated (it just has more than four sides) and is about 1300 sq. feet on the main level, give or take. So if spawn is the diagonal, it's around 51 feet, roughly.

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