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Question DetailsAsked on 12/8/2013

Should I keep my crawl space vents open or closed in the winter and summer?

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3 Answers


If water table is not extremely high

I would poly the crawl space and glue the joints and plastic to the wall. Insulate the walls of the crawl space with extruded poly (blue, yellow ) at least 1" or 2" and leave vent closed forever.

If that is not okie dokie, leave vent open in summer and closed in winter and would insulate the floor above the crawl space with fibergblass batts.

Think 6 mill black poly is way to go, and would do it regardless for moisture control.

Jim Casper Old Energy Conservation Guy

Answered 5 years ago by jccasper


Do not vent and reduce the moisture/ vapor infiltration into the crawl space- floor and walls if possible. For example, allowing warm, moist air into the crawl space the air will condense on the floor/walls adding moisture which will be drawn up to the living area through framing and floor penetrations. Then air seal any floor penetrations and sill/rim joist. If you do not air seal the fiberglass batting insulation is worthless.

Answered 5 years ago by hosey


Your crawl space MUST be ventilated - if it is not heated space, then you HAVE to (both to prevent moisture buildup and by code) vent it to the outside air. Putting down vapor barriers and board insulation and all can certainly reduce the amount of moisture coming in, but your foundation walls are still damp and release moisture to the crawlspace, and no vapor barrier is totally watertight, so you STILL NEED VENTILATION. IF you vapor barrier and insulate like Jim says, that will certainly warm the area and reduce moisture, but you still need ventilation, though it could be a humidistat and thermostat-controlled fan driven system rather than natural lventilation if cold floors are a problem.

Also, unless dehumidified, do NOT put any absorbent insulation on the underside of your floor - it WILL condense moisture. It is possible to insualte there, but has to be done right, because the tendency is to condense moisture in the insulation and on the underside of the floor sheathing in the summer (which causes rot), and on the bottom of the insualtion and joists in the cold winter, so typical insualtion and vapor barrier rules do not apply there.

Unless you are prepared to turn it into artifically ventilated and heater space, you need to keep your vents open year around, because the moisture source is there year around - the moist ground. I have seen dozens of homes with serious stubwall or subfloor damage due to people putting down vapor barrier (and even full concrete layers) on the exposed ground and then blocking off their vent openings. I had a house like this once - 4 inch concrete layer in the crawlspace just under the living room (about 14x20 feet) - a dehumidifier removed over a gallon of water a day from that area summer and winter till I turned it into a humidistat controlled fan-ventilated space the second summer.

Answered 5 years ago by LCD

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