The Importance of Crawlspace Ventilation

Posted by in Foundations, Mold Growth, Wood | March 27, 2017

The decay (and the foot step through the floor in the featured photo) seen here is characteristic of damage from prolonged exposure to excessive moisture. The severity of the decay indicates that the problem has been progressing over a period of many years. How does this happen?

Figure 1 Floor deck and joists under hole are severely decayed.

We have a number of blog posts that discuss or mention ventilation in an attic and rafter spaces, but don’t forget that space under the floor needs ventilation, too.

Just as excessive airborne moisture condenses on a chilled roof deck or on chilled roof rafters or joists and promote decay, excessive airborne moisture can also condense on a chilled floor deck or chilled floor joists. Repeated and prolonged wetting can often lead to decay and/or the growth of wood damaging fungus.

And just like an attic, industry experience has found that the condensation of excessive airborne moisture in a crawlspace is commonly prevented by ventilating the crawlspace. It also helps to reduce the source of the moisture, usually the evaporation of moisture from the floor, especially when the floor is exposed earth.

Industry standards recommend that approximately one square inch of free vent area be provided for each square foot of crawlspace area. This may be reduced to one square inch for every ten square feet of crawlspace area, if the earth floor is adequately covered with a vapor barrier (commonly a thick plastic sheet covered with a ballast to hold it in place and protect it from being punctured).

How can good ventilation be accomplished in a crawlspace without causing the freezing of pipes in the winter or causing reduced efficiency for heating or cooling when the heat/ac ducts are in the crawlspace? That is an issue for another discussion at another time.

However, we note here that excessive moisture can also be directed into the crawlspace through the foundation walls by poor handling of surface or roof runoff. Grade must pitch away from the crawlspace walls to direct surface runoff away. And the discharge from downspouts must be directed away from the crawlspace walls.

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