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Handling slab moisture under hardwood floors

chadwik66 | Posted in General Questions on

Our 1960s colonial was built in a low lying area that gets very wet 4-5 months of the year. It has a slab on grade foundation with no signs of a vapor barrier having been installed at construction. The result is incredibly high humidity rates – often in the 70-80% range throughout the house. If left unattended, moisture can build up in areas like the brick entry way with noticeable dampness.

What options should we investigate to better handle the moisture? Seal the underlying slab? Install a vapor barrier? Some other option?

Additional details:
-Border of climate zones 5A-6A in Southern NH, but humidity is notably higher than expected due to lot location, proximity to water, high water table, etc
-We monitor humidity and take precautions as needed (open windows on low humidity days, run dehumidifiers and mini split AC on high humidity days, etc)
-The majority of the house is slab on grade with no crawl space and very little room (1-3 few inches?) between wood floors and slab.

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  1. Expert Member
    Zephyr7 | | #1

    I’d check air sealing too — a leak home in a humid area will constantly be bringing in new humid air. That’s probably part of your problem too.

    For the slab, you could seal it with an epoxy or other vapor barrier sealer. There is no practical way to install an under slab vapor barrier after construction, unfortunately. I would NOT use a vapor barrier on the floor that is not adhered (like a sheet of polyethylene) due to the possibility for moisture to pool under it and cause other kinds of problems.


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