Attic Cross Ventilation

Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.

Attic cross ventilation.

Proper insulation and air sealing also keeps attics cold in winter by blocking the entry of heat and moist air from below. Most building codes require a ratio of 1 150 ventilation space to attic floor space. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. The insulation will resist heat transfer into the house.

Intake vents located at the lowest part of the roof under the eaves allow cool. If the attic is insulated with fiberglass batts just pull back any that are blocking the flow of air. If there s blown in insulation like ours rake back the fluffy stuff with a 3 or 4 ft long 1 x 6 or use a garden rake or hoe. In other words the entire vent opening doesn t count as vented space.

That said air resistance and interference such as vent grates reduces the area of true ventilation. Ventilation of attic spaces is required by most building codes as well as by roofing material manufacturers and the national roofing contractors association nrca. Generally speaking you need a ratio of 1 300 where for every 300 square feet of ceiling space you need 1 square foot of attic ventilation.