Ventilation in crawl space

I live in northern Westchester County, N.Y. My house is built above a crawl space with a dirt floor. Last year I had expanding foam insulation installed between the floor joists above the dirt floor. The height between the dirt floor and the bottom of the floor joists varies from a high of about five feet to a low of about six inches (the house is built on a slope).
I have heard conflisting theories about ventilating a crawl space. One theory says keep vents open in winter and closed in summer. That seems counter-intuitive to me.
At one end of the house I have a window opening to the crawl space measuring about three feet by three feet. In spring, summer and fall I place a screen in the opening. During winter a place a solid door in the opening.
At the other end of the house there is a small window (27 x 17). In spring summer and fall it is covered by a screen. In winter by a solid board.
Here's my question.
Should I cover the two aforementioned windows with the screen, or solid covering, in the spring, summer, and fall? What about winter?
Is it necessary to cover the dirt floor with polyurethane?
Thanks.
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Dejola wrote:

The reason for ventilating is to reduce moisture build up. Since different areas will need different amounts of ventilation, you are going to hear different responses and they all are likely right, at least where they started from.
Personally I would want ventilation. I would never want to seal it up even if it were in a dry area. You are not going to gain much insulation from sealing it up, if you have insulation above the craw space. Trapping moisture there will mean rot and mold. Of course I live in an area that is a little moist.
As for the dirt floor, I would say the same there. Different local conditions would give you different answers. I see no reason not to cover it however. You got to win there.
--
Joseph Meehan

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wrote:

This all boils down to mold/mildew issues which grows best with warm temperatures and moisture. I think what you have been doing makes the most sense, especially for your location having cold winters. The exception would be if there is excessive moisture (water puddles), then you'd want to vent all year round. I do not believe a plastic ground cover is necessary.
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Dejola wrote:

If the crawl space ground stays ,moist the I say cover with poly, If it stays dry there is no need. But do ventilate in warm months and close off for winter for sure. unless you live in tropics. Jack
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Many thanks, Joseph, Phisherman, Jack, and Jay, for your responses to my questions. This is a very confusing issue indeed. I'm happy to say that the dirt floor below my floor joists is cool and dry. No sign of water infiltration.
For now I will continue to vent from spring to fall and close things up for the winter months.
I am especially grateful, Jay, for the link to the various crawl space studies. Thanks.
Dejola wrote:

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dirt may feel dry but still allows moisture travel.
I would cover with poly........
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Yes you should seal the crawl space and yes you should put down a vapor barrier.
Check the studies done http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces /
The technical analyses as to why one should seal a crawl space and place in there a secure vapor barrier are explained at Advance Energy.
I did it for my own crawl spaces and it improved the air quality in my house tremendously.

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