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What is the best moisture barrier to use on a basement ceiling?

Deaven | Posted in Green Products and Materials on

We own a 1903 cottage in a mountainous, wet (zone 6) area of Central Pennsylvania and have a major moisture problem. Half the basement has a dug out red clay floor and other half has a concrete basement. The cottage is surrounded by trees and doesn’t get a lot of direct sun. Our cottage is like a sponge… a constant battle with mold and mildew! We are looking into a home dehumidifier system, redirecting the rain fall and finishing off the red clay flooring. ( we have heavy duty plastic down now – helps)

Does anyone have any good recommendation for a spray sealant for the basement ceiling that would create an effective moisture barrier?

Thank you in advance for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Does anyone have any good recommendation for a spray sealant for the basement ceiling that would create an effective moisture barrier?"

    A. No such product exists. That would be an ineffective approach, even if the product existed. I also strongly suggest that you don't install a dehumidifier.

    The correct approach in your case is to address the moisture issues in your basement and crawl space. Here are the steps you need to take:

    1. Install roof gutters connected to conductor pipes. The conductor pipes should lead away from the foundation, to daylight or a distant dry well.

    2. Make sure that the grade around your foundation slopes away from the foundation in all directions. If there is an uphill side, make sure that the grading includes a swale that directs rain away from your foundation.

    3. If possible, cut down some of the trees that are shading your house.

    4. If the above measures fail to address your problems, you may need to hire a backhoe or excavator to excavate the exterior of your foundation down to the footings, so that you can install drainage pipes near your footings. These pipes should lead to daylight or a distant dry well. While the foundation is exposed, you can install a waterproofing system that includes dimple mat.

    5. In your dirt-floored crawl space, you should consider removing some of the clay and replacing it with crushed stone. Then install 6 mil (or heavier) polyethylene on top of the crushed stone, followed by a concrete slab.

    Good luck!

  2. Deaven | | #2

    Hi. Thank you both for your advice! Elizabeth

  3. Expert Member
    Dana Dorsett | | #3

    What he said!

    The slab doesn't have to be a fully structural 4" slab- a 2 "rat slab will do. It's primary function is to protect the poly vapor barrier.

    In a zone-6 climate it's also going to be worth putting 3" of rigid EPS insulation (~R12) under the slab when it goes in, and insulating the foundation walls once the bulk water issues are under control. At the slab edges stub up the foam against the foundation and come up ~6" with it, floating the slab rather than having it in full contact with the foundation. This provides both a thermal break and a mechanically compliant buffer to limit expansion/contraction stresses on the slab & foundation. If you plan to finish the floor it would be important to place the poly vapor barrier between the foam and concrete, otherwise it doesn't matter- between the foam & gravel would also be fine.

    EPS (the stuff with the macroscopic visible bead structures, like packing material and cheap coolers) is the preferred insulating material for this application. Polyisocyanurate is much more susceptible to moisture, and could load up with water losing effectiveness over time. XPS (pink/blue/green board) would do the job but has a hefty green house gas footprint due to the blowing agents used in it's manufacture, and loses ~15% of it's rated R-value over the first 50 years as the climate damaging blowing agents escape. EPS is blown with comparatively innocuous pentane, most of which has already outgassed by the time it hit's the distributor's warehouse, and is rated at it's fully-depleted R-value, which is stable over time. It's usually cheaper than XPS too, at ~9-10 cents per square foot per R. (eg: R12 would run ~$1.20 per square foot.)

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