Using air flow to prevent condensation problems
A well-ventilated attic is especially important in keeping a house comfortable and free of moisture damage all year. When installing insulation between attic rafters, leave at least 1 inch of space between the insulation and the underside of the roof sheathing: or install preformed vent channels.
Ideally, a rafter cavity should be vented at the soffit and at the roof ridge, allowing air to enter at the roof’s base and to exit at its top.
If venting the soffit isn’t possible, install sufficient gable or roof vents to permit good cross-ventilation: consider adding an exhaust fan to supplement air circulation. In an attic equipped with vapor barriers and soffit vents, install at least 1 square foot of open ventilation area for every 300 square feet of floor area; if there are no vapor barriers or soffit vents, double the ventilation area.
Ventilation is also important in a crawl space to eliminate moisture rising from the soil. Install at least two-preferably four-vents high on opposite foundation walls, close to the corners. With a vapor barrier on the ground, provide 1 square foot of open ventilation area for every 1,500 square feet of ground area. Close a crawl-space vent when humidity is high; leave attic vents open all year.