My question is about crawlspace vents.  Should they be closed during the winter months or what is the general rule of thumb concerning vents?  Do they have any bearing on the warmth of the home?-  Dan, Georgia 

            That is a difficult question since the science on this is quite complicated.  The consensus among moisture/building scientists tends to be that crawlspace vents do little good if you have a wet site and they increase the problem of crawlspace humidity in coastal states like your own particularly during summer months. A good layer of polyethylene plastic appears to be more effective than crawlspace vents. 

            The premise of crawlspace vents is that they will provide sufficient air exchange under a house to significantly reduce humidity levels under the house and protect floor joists and other timbers from becoming mold infested.  How well a crawlspace is ventilated depends on a number of factors.  These include the number, size and placement of crawlspace vents. 

            In theory good crawlspace ventilation will result if there are sufficient vents of the proper size and location.  In practice that is difficult to achieve.  Air flow through a space is enhanced when there is a cross flow from one vent to another.  Such cross flow cannot occur if vents do not occur on one or more walls.  In most crawlspaces there are no vents on the garage side. 

            Air flow into crawlspaces and into houses depends on temperature and wind-driven pressure differences.  If pressure differences are low, little or no air exchange occurs.  Such is the case on calm days and nights at moderate temperatures. 

            Highest air exchange and thus ventilation occurs on cold, windy days.  It is on such days that most homeowners will have vents closed to reduce the discomfort of cold floors and to reduce energy costs. 

            In my experience houses built on crawlspaces tend to have more mold problems than those built on slabs.  Our clay soils in Indiana require good drainage around building structures such as houses.  Contractors who build using crawlspaces seem to believe that crawlspace vents are a substitute for adequate site drainage.  They are unfortunately not.

 February 18,2003