Crawl space ventilation Inadequate

b.  Crawl space ventilation Inadequate ?:
Ideally crawl space ventilation would be 1 sq. foot of ventilation, strategically located, for every 150 sq. feet of crawl space floor.  The need for ventilation lessens to 1 sq. foot of vent per 300 sq. feet of floor if there is a vapor barrier over the floor.

Few houses actually meet these standards.  This is, after all, a theoretical standard.  Older houses that have less crawl space ventilation and no negative effects should be considered safe to leave alone.  Those with moisture or rot conditions, however, should be improved.  Ideally, the vents should be placed strategically to allow cross ventilation to promote drying of the entire space.

Vents of adequate area, but located on only one wall, may prove to be inadequate (Fig. b1, Item A).  The actual airflow through a crawlspace is unpredictable. It seems to be mostly dependent upon wind, temperature differences, etc.

NOTE: The effective free opening size of vents with grilles or screening is one half the rough opening size.

Generally speaking, dry crawl spaces may have the vents closed during the wintertime to conserve energy but closing vents can prove catastrophic for moist or wet crawlspaces.  Wet or damp crawl spaces should never have vents closed.  Automatic vents are available at a relatively low cost from pest control companies but unfortunately can cause floor rot in houses with wet crawlspaces.

What to:  How to:
In all foundations, except stone or poured concrete, crawl space vents can usually be added easily by removing blocks or bricks.  Naturally, the vent grills are purchased first (make sure they are screened to keep bees and other large insects out) before the holes are cut and the vents set.  Try to place the vents symmetrically across the foundation walls from each other to promote cross ventilation.  In cold weather areas it is important to consider what might happen if vents are added.  Pipes can freeze and obviously the house can be much colder. Make sure the plumbing pipes are adequately wrapped and that the floor above is insulated once it has dried properly.

The average handyman can install crawl space vents adequately.  If cosmetic final appearance is critical, a professional mason should do the work.