foundation or closed?

booboo60October 5, 2009

First, I apologize for not knowing the correct terminology but....our house has vents all around the house at ground level. We have a crawlspace so I assume this is what is vented. They are in multiples of 2 with screens and little plastic flaps which open and close. Should these vents always be open for say, the summer and then closed in the winter? Right now, I am noticing some are open and some are closed :(


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My present dwelling has those. I open them in warm, dry weather and close them when it's cold as well as when tropical storms are expected. The general idea is ventilate the crawlspace, theoretically preventing molds and mildews. FWTW, I've had no problems with mold or mildew, but I do know of folks who have an ongoing problem; however, I don't know if they made any effort to adjust the vents.

Don't forget to thoroughly insulate underneath the flooring: the temp of a crawlspace differs little from the outside air, and freezing pipes are quite likely in cold climates.

    Bookmark   October 5, 2009 at 3:10PM
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twotogo, the ones we have are regulated by temperature. They stay open for summer and as the weather will turn colder, they will close to keep the cold air from blowing under the house. We live in a raised woodframe. I'm down South so we very rarely get freezing temps, but we still have all our pipes insulated. In the summer months they remain open to allow for air circulation because down here we have 90% humidity. As the weather cools the humidity drops so then it's OK if they close.

    Bookmark   October 5, 2009 at 11:49PM
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twotogo - the simple answer is open in the summer, closed in the winter. The more complex answer is that it probably depends where you live. Here in NC, probably the best practice is to close them all the time, cover the holes with cut foamboard, plastic on the ground (really sealed - not just the usual covering with out tape and with gaps), and run a dehumidifier during the summer.

The reality is that the vents don't circulate enough air in the summer to lower the humidity under the house. The vents are a throwback to truly ventilated crawlspace - like a house on stilts. The winter definitely sees some benefit in closing vents.

In my last house, the HVAC vents leaked enough that the crawlspace was somewhat conditioned. In that case, sealing is definitely the way to go.

I'm just glad that I have a basement now so I never have to deal with those stupid vents....

    Bookmark   October 6, 2009 at 5:00AM
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In my last house, the HVAC vents leaked enough that the crawlspace was somewhat conditioned. In that case, sealing is definitely the way to go.

Same thing here in our current house.

If I was building a new house with a crawlspace instead of a basement, I would go the sealed and conditioned route. Just make sure the building inspector is on board with it.

    Bookmark   October 6, 2009 at 8:44AM
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Thanks all!! I kind of figured that would be the logical answer. We are just now heading for freezing temps at night and if the wind blows too seems like closed vents would be in order. One more question....does closing these vents interfere at all with the HVAC or air circulation in the house?

    Bookmark   October 6, 2009 at 10:59AM
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No, closing the vents should not impact either one, unless you have a massive leak somewhere.

    Bookmark   October 6, 2009 at 11:08AM
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Foundation vents are required to ventilate underfloor spaces especially if the floor of that space is uncovered soil.

The vents are there to allow water vapor from the soil to escape to the atmosphere.

But this does not always work, and seasonal differences can actually create moisture problems in crawlspaces if the vents are left opened.

For example, it may seem counter-intuitive, but foundation vents should generally be left closed in summer and open in winter.

The reason is that if left open in summer, the warmer more humid exterior air that is drawn into the crawlspace will condense more readily on cooler crawslpace surfaces and cause more moisture and mold realted problems than if left closed.

In winter, vents should be opened to allow moister interior air from the crawlspace to vent to the outside.

That said, a better approach is to eliminate crawlspace vents altogether, but this requires:

1. Insulating the crawlspace walls
2. Sealing the floor of the crawslpace with a vapor barrier
3. Conditioning the crawlspace by either heating it, cooling, dehumidifying it or all three.

Conditioning the space can cut overall energy costs to heat and cool your home and is the best method for dealing with crawlspaces.

    Bookmark   October 8, 2009 at 7:34AM
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