Ventilation of an attic or crawl space is very important to allow warm, moist air to escape to the outdoors.
If the insulation in your attic has a vapor barrier (which should be toward the floor and not on top of the insulation where it may trap moisture), you should have at least one square foot of "free vent opening" (measurement of the opening not including the area taken up by screen or grillwork) for each 150 square feet of floor area. The net "free vent area" should be specified on the vent itself, or the information should be available from the vendor. It is preferable that the vents be located such that one-half of the vents are low and one-half are high.
Don’t try to substitute a vapor barrier for ventilation. According to Department of Housing and Urban Development standards, if you have a vapor barrier you should still have one square foot of "free vent opening" for every 300 square feet of attic floor opening.
Turbine vents and attic ventilating fans are also useful for ventilation.
Note: Take care to install enough attic ventilation to meet the needs of your whole-house fan. Requirements should be listed on the instruction sheet or available from the manufacturer or vendor of the fan.
Wintertime Attic Ventilation
If the house is insulated, attic ventilation should not be covered in the winter. If warm, moist air in the attic condenses on the roof decking, it may melt and drip on attic insulation, causing the insulation to be ineffective. Warm air in the attic can also cause other roof problems.
Crawl spaces should be vented to the outdoors in the summer. If the vents are located near each corner, the vents will permit good air movement through the crawl space. The total of all the vent areas where there is no vapor barrier as a ground cover should be at least one square foot for each 150 square feet of the floor area. Where such a vapor barrier is used, the vent area may be reduced to 1/1,500 of the floor area.
Crawl space vents should be closed and sealed in the winter. Exception: When vents are used for combustion air-to-gas appliances located in the crawl space.
Note: If your foundation walls are insulated, the vents should have insulation placed over them in winter.