anobiid

Crawl Space Ventilation

The purpose of crawl space ventilation is to keep the moisture/humidity level in the space at as low a level as possible.  This is important because wood destroying organisms thrive in moist environments.  Anobiid beetles (most common and most destructive wood eating insect in Western Washington) for instance, thrive in a wood moisture level as low as 13%.  An unventilated, or inadequately ventilated crawl space is apt to have high moisture levels.  Most newer homes are provided with adequate ventilation through building codes.  Homeowners often defeat this ventilation by installing foam block inserts, thinking they are protecting water pipes.  The proper way to protect water pipes is with pipe insulation.  Closing off foundation vents does not allow moisture to leave.  Older homes often have inadequate ventilation. Standard building practices require one square foot of vent area for every 150 square feet of crawl space.  Vents should be evenly distributed and within a few feet of corners to promote air circulation. One example of inadequate ventilation is the louvered vent cover shown below.

This type of vent actually has only half the vent surface area that it appears to have.  This restricts air flow.  A better solution would be to replace the louvered vent screen with 1/4 inch mesh galvanized hardware cloth.  This would give full ventilation surface area.  I always advise my clients to do this.  To sum up; keep vents open.  The only time, in this county, to block vents is when we are having a serious northeaster with the temperature and wind chill down around O degrees fahrenheit, and only for the duration of the northeaster. 

As always, thanks for reading.

David Helm, Bellingham home inspector

http://www.helmhomeinspections.com