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Community and Q&A

Sealing indoor concrete floors: always a good thing?

Kenneth Gartner | Posted in Green Building Techniques on

I have a new construction house in MA. Slab on grade, literally, a frost-protected shallow foundation that is wrapped underneath and up sides in 6 mil poly, isolated by at least 3 inches of EPS, atop and surrounded by crushed stone. It is 10 months old and bone dry — never any accumulated moisture under plastic that may sit on the surface. If any moisture does unexpectedly enter the under-surface of slab, then the passive radon vent should take care of it.

Since we will live with this slab surface for a few years (until wallet is replenished and some click-snap bamboo flooring is added) I wanted to reduce dust meanwhile. Low VOCs are a must, so I chose AFM Penetrating Waterstop sealer.

However, as I apply concrete sealer I realize that this might not be a Good Thing in my circumstance, so wanted to run by GBA for sanity check.

I suppose a primary reason for having a concrete sealer is to prevent moisture FROM the slab from entering the home. In this case, one would probably wait until the concrete aged enough to have low moisture release from its genesis before applying sealer. Not applicable in my case.

A second reason would be to prevent absorption of nasty substances from above (oils, dog urine, etc). Not so applicable to me.

A third reason might be to prevent dust and otherwise improve durability of the surface. Yes, for me here.

However I am much more concerned about bulk water from above (ie, spilled drink) seeping through a future click-snap wood floor and thus remain trapped by the sealed surface to foster fungal growth. This would seem to be an excellent reason to avoid sealing the concrete floor — if the concrete floor remained porous, the spilled water would be drawn into the concrete and not pose mold hazard.

In a similar vein, it seems to me that surface water from a bathtub area (oversplash) might find itself migrating (via gravity in un(der)sealed grout, cracked tiles, etc) below wooden laminate in an adjacent room and be similarly sequestered.

A secondary concern is that a sealed surface might be more often slippery when minor water is present (ie, tromping in water on footwear during rainy season). We have to be mindful of elder safety here.

So, in my logic, the perceived risks outweigh the benefits and so I plan to NOT seal the concrete in kitchen/living room area.

Looking forward to any pertinent GBA edification, thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Briefly, you are overthinking this.

    First of all, if you are living on this slab (and therefore walking on it, putting furniture on it, inviting dogs to step on it, vacuuming it, and sweeping it), your sealer won't be unaffected by friction and traffic. It's going be degraded over time. Any coating is going to wear off. So you have to stop imagining that this magic sealer is an impenetrable layer of plastic. It isn't.

    Second, concerning the "spilled drink" problem: Evaporation prevents significant problems in the vast majority of mishaps in this category. Don't worry. Of course, if you get a major flooding event due to a plumbing leak, all bets are off. But that's true whether or not you install sealer on your slab.

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