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
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
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?
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
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.
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.
Many thanks, Joseph, Phisherman, Jack, and Jay, for your responses to
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
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
Yes you should seal the crawl space and yes you should put down a vapor
Check the studies done
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
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