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Plaster on Interior of Concrete Foundation Walls - Mold Risk?

Plaster on Interior of Concrete Foundation Walls - Mold Risk?


Old 11-13-17, 02:09 PM
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Plaster on Interior of Concrete Foundation Walls - Mold Risk?

Hi DIYers, I would like to apply a layer of plaster directly to my concrete foundation walls (with some dash in between for adhesion) to give my basement a more finished look. Is this a bad idea in terms of mold forming on the plaster - is moisture likely to find its way through the wall, into the plaster & then form mold? Any other thoughts or feedback on my plan? Please see attached pic for details.

-The Happy Hobbit

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Old 11-13-17, 03:29 PM
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You have no insulation, no vapor barrier, so water vapor would just pass through.

Without insulation it will make for a cool basement, temp wise!
Old 11-13-17, 03:42 PM
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Hi Marq1, thanks for your reply. Isn't the waterproofing coat and membrane outside the concrete wall much more effective than a vapor barrier, or do I need a vapor barrier in addition to waterproof coating? Sorry I'm new to this!
Old 11-15-17, 12:20 PM
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I STRONGLY URGE you to use Portland cement plaster. Gypsum plaster will likely lose bond between the dash coat and the gypsum. Or if you use gypsum plaster as the dash coat it will lose bond between the concrete and the dash coat. Here is what I think happens. eventually moisture migrates and condenses in the wall somewhere. When that happens it dissolves a microscopic layer of the gypsum plaster and that dissolved layer is the bonded layer so all that holds the plaster is itself. Portland cement plaster on the other hand does not dissolve in the lamina and is monolithic with the concrete foundation.

Anything other than a dash coat of Portland cement mixture is a bond breaker unless you use a liquid bonding agent which is still somewhat likely to fail, though it is recommended for applications like yours it is a poor substitute for a coarse dash coat.

You specifically ask about mold. For some reason I can't explain because both gypsum and Portland cement plasters are inorganic, mold seems to happen more on gypsum than Portland. Some extra lime in the Portland cement might have some anti fungal properties, I don't know.

Make sure there is nothing that will inhibit the bond between your dash coat and the concrete. Anything like residual form release or dust or dirt or efflorescence can inhibit the bond. And speaking of efflorescence if you have any it will probably come through the new plaster so neutralize it with a strong vinegar solution or very mile muriatic acid solution and rinse well before the dash coat. An d solve the water wicking problem causing the efflorescence before you proceed.
Old 11-16-17, 06:21 AM
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What's your location? Climate will affect your answer.
If the wall surface temperature is below the dew point, interior moisture vapor will condense on the surface.
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