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Question DetailsAsked on 5/21/2014

How to get mold and mildew smell from under home concrete slab.

When heating and air conditioning unit is running can smell mold and mildew. House is on concrete slab and water was noticed underneath old unit in hole where old unit was before new unit was replaced. Water was gone before new unit replaced, but that is when the smell started.

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1 Answer


Pour water down there with a substantial amount (several cups per gallon of warm water) of Borax in it - pure Borax powder, not Boraxo soap. Found in laundry aisle near powdered soaps. Comes in powder form, and will kill most odors and mildew - both in-situ and used (per instructions) when washing walls, clothing, garbage disposals, drains, etc. Really pretty versatile stuff - is a component of most heavy duty detergents, and pretty harmless as well. IF that does not work, then I would spray the heck out of the hole and immediately surrounding concrete slab with Lysol, and cover the hole with saran wrap to keep the vapor in there for a day or so.

If the hole is still there, unless you would be burying the A/C footings or pipes or wiring that should not be buried, you could get a sack of concrete mix for under $10 and mix it up and fill the hole after you treat the smell.

Answered 6 years ago by LCD

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