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Why your crawlspace smells and how to fix

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1The short version

Nobody goes down there for fun, which is exactly why the smell has years to build before anyone notices. Built instead of a full basement when a basement would be impractical or too expensive, a crawlspace gives access to plumbing, wiring and HVAC without excavation, provides a place to add insulation, and with adequate sealing can help with radon mitigation. It also, less usefully, provides a protective buffer between damp ground and the wooden parts of the house directly above it.

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2The smell, explained

A musty smell, mold on surfaces, water stains, and damp cardboard boxes are all listed together in home-inspection moisture guidance as signs of excess indoor moisture, and a crawlspace is one of the most common places that moisture originates before it works its way up into the house above. The smell is rarely mysterious once you're down there with a light; it's almost always standing moisture with nowhere to go.

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3What it takes, and what goes wrong

A standard crawlspace specification calls for insulating the floor joists and band joists with unfaced fiberglass insulation, covering the crawlspace floor with at least 6-mil plastic sheeting lapped 6 inches and turned up 6 inches on the foundation walls, and making sure ductwork and water lines down there are also insulated, with proper ventilation maintained throughout. The common mistake is insulating without addressing the ground moisture first, which just traps the same damp air the insulation was supposed to help with.

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4The first hour

A head torch, not a handheld flashlight, is the right tool, since the job needs both hands free for crawling and lifting the edge of the vapor barrier to check what's underneath it. The first pass is just looking: standing water, a barrier that's torn or never sealed at the walls, and anything living down there that wasn't invited.

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