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What is
your opinion on venting a crawl space with a moisture problem? I know
that you need to control moisture, but all the current research points
to closing the vents, and encapsulating the crawl space. This practice
is backed by many building scientists. I cannot find any documented
research that concludes that foundation vents will “dry” the space.
Mike,
Connecticut
Crawlspace moisture is a common
problem in residential buildings as many, if not most, builders
apparently believe that the installation of crawlspace vents is an
effective (and less expensive) alternative to implementing adequate
drainage measures around newly constructed buildings.
I have conducted many residential
mold investigations over the years. As I look back crawlspace houses
are disproportionately represented in those investigations. The use of
crawlspace substructures varies from one part of the country to
another, and from builder to builder. Crawlspaces come into and out of
fashion.
In our area the crawlspace/slab house
ratio is approximately 1:1. Nevertheless in my mold investigations the
proportion of crawlspace to slab houses is about 2:1. Because of heavy
clay soils and relatively high water tables basements are relatively
uncommon here in the central Midwest. As a consequence I do not
recommend the use of crawlspace substructures. They appear to be at a
higher risk (than other substructures) to moisture/mold problems.
I have no doubt that crawlspaces can
be used in some regions without experiencing moisture/mold problems.
These would be in areas where the soil is relatively sandy. Such is
the case in coastal areas. In such circumstances most of the moisture
that crawlspaces experience in the summer months is high humidity air
that comes in through crawlspace vents. Under such conditions it
appears that crawlspace vents contribute to moisture problems and
closing/sealing crawlspace vents would seem to be appropriate. Some
building scientists recommend not using any crawlspace vents at all in
such areas. Others recommend a sealed crawlspace with conditioned air
passing through it just like any other space in the house.
Unfortunately building scientists in
conducting research are limited by cost to studies on a few houses in
a small geographical region. Therefore it is quite risky to make
generalized recommendations on ventless crawlspaces .What may work on
one type of soil in one geographical area may not work on another soil
or in another geographical area.
In typical applications crawlspace
vents are located on only three of the four crawlspace walls. This
reflects the fact that the fourth abuts the slab of the garage.
Achieving good cross-ventilation requires proper placement of vents
relative to outdoor air flows. If the flow is from the west with the
garage on the west crawlspace ventilation is seriously compromised.
Under calm weather conditions air
flows into and out of crawlspaces as a result of temperature-related
pressure differences. Such pressure differences are near zero when
outdoor temperatures and crawlspace temperatures are the same. Such
conditions occur at certain times of the day in Northern U.S. regions
in the Spring, Summer and Fall months and on a number of days in the
Spring and Fall. Similar occurrences may occur in Spring, Fall and
Winter in Southern states.
I have been in houses where the
crawlspace was subdivided into sections, with some sections with vents
and others without vents. Mold infestation was most severe (with
structural failure of floor joists) in those sections without
crawlspace vents. This suggests that at least in those houses
crawlspace vents were beneficial
It is hard to generalize on whether
it is better to enclose a crawlspace and not use vents or to use
crawlspace vents. It will vary from one soil type to another and one
climatic/geographic region to another. If one does not have to build
on a crawlspace don’t; if one chooses to build on a crawlspace then
insure that adequate site drainage is provided.
August 14, 2003
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