Crawl space vents - open / closed ?

Hi, I have 1 year old ranch style house with a crawl space instead of a basement. The only thing down there is the furnace.
There are ventilation vents for this crawl space, and I wonder if these should be left open or closed. Should they be open part of the year, closed other times?
Thanks
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I have thermostatically controlled vents on my house for the crawlspace.
I did have one left open for combustion air for my furnace, but went with sealed combustion. Now they're all controlled.

closed
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Most people close them in the winter. If your winter temperatures don't get much below freezing, just leave them open.
Chopper wrote:

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They are their to reduce moisture. Moisture does not go away in the winter. They should be open all year to prevent moisture build up and help prevent mold and rot.
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Joseph E. Meehan

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This may surprise some: open in winter, closed in summer. Rationale: in winter, warm air and it's humidity will move from the crawl space out. In summer, closed vents will prevent warm air and it's humidity from moving into the cooler space.
Ed

Thanks,
Ed
===============================================Ed Siff Computing Consultant Library Systems CB# 3946 Academic Affairs Libraries UNC-Chapel Hill ed snipped-for-privacy@unc.edu 919.962.1288 =============================================== "Take me on a trip upon your magic swirlin' ship" -Dylan
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Vents in crawl spaces are supposed to allow air movement to remove the moisture that comes from the ground. Add the need for cumbustion air and the vents should stay at least partly open. You may have a particularly good vapor barrier in place. You may have particularly dry soil conditions. You may have particularly good insulation at floor level. You may have a very mild winter. I would suggest using the Building Science Corporation web site to look at your particular conditions. I'm not affiliated and think it sometimes overstates its opinon, but it is one place you can find researched information based on climate.
TB
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