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I know hydraulic cement is designed to block the transmission of water, but does it also block water vapor to any appreciable degree that it is considered a vapor barrier at a certain depth/thickness (anywhere from "skim coat a foundation slab with it" to "cost not being a consideration, pour the full depth of the foundation slab with it")?

If not, is there another type of masonry formulation that can achieve a vapor barrier classification?

  • Normally a vapor barrier is installed before a concrete slab is poured. I'm curious what is the exact problem you're trying to solve here. – SX welcomes ageist gossip Sep 10 '17 at 19:00
  • @Fizz just wondering if one can make a concrete foundation water-proof and a suitable vapor barrier without installing another product which either can get cut or create a poor seal around penetrations (e.g. Plastic vapor barrier and holes for plumbing pipes) or otherwise have long-term adhesion problems (e.g. Waterproofing paint products). It seems like the concrete mixture itself could be formulated to address both issues (though, probably in a less cost effective manner if possible, thus making it less common). – statueuphemism Sep 10 '17 at 19:39
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No, hydraulic cement is not considered a vapor barrier but it can reduce moisture. I have had really good experiances with epoxy coating cement flooring. The first time I used epoxy I did not expect it to work As a moisture barrier as well as it did. The basement dehumidifier in the winter would remove 1/2 to 1 gallon a day in winter. The next year after coating the floor it was less than 1 quart per week. One important thing is to use a coating that can handle moisture if your floor has problems. Tape some plastic to the floor at least a square foot or 3 square foot for a larger test area. After 24 hours if there is water on the plastic or the cement looks wet you will need to use a brand that can handle the moisture or try again in the summer when it may be dry.

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Concrete is permeable to water and vapor.

It is possible to make it waterproof with additives, this is common when making concrete water tanks for example.

You should consult with the company that will be manufacturing and pouring the concrete, they will tell you all about pricing etc. Most likely a putting a plastic membrane under the slab before pouring it will be the cheaper option.

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