Ventilation: A Roof Must Breath To Stay Healthy.
Just as a roof must keep water from entering a house, moisture buildup under the roof must be removed through proper ventilation. Improper ventilation results in warm, moist air generated in the house meeting with colder air in the attic, which causes condensation. Ventilation problems can be seen in a variety of ways, such as blistering and peeling paint on the gable end exteriors because of excessive heat and moisture. Buckling roof shingles are another sign of poor ventilation, as are ice dams in the Midwest and Northeast. Water stains on a ceiling may actually be a result of condensation moisture dripping from under the roof rather than a leak.
Ideally, air circulates under the roof by being drawn up through a continuous soffit vent, through the attic space, and then out the ridge through a continuous ridge vent. This is the smoothest and most efficient system. But few houses have continuous soffit and ridge vents. Most houses, however, do have some type of vent in the roof or an opening at gable ends. Houses with no rafter overhang, or a very short one, may not have soffits. Older houses may not have vents between the rafters leading into the attic space. Venting was not a major problem in older houses because they leaked air everywhere. But with the advent of better roofs and tighter house construction, moisture became a more significant issue.
Calculating the number and size of roof vents is not an exact science, but instead is influenced by the climate, the roof pitch, available locations for vents, and the house's orientation to prevailing winds. The Asphalt Roofing Manufacturers Association states:
"In most cases, a minimum 30sqm of attic floor area must be designed and properly installed to provide proper ventilation. Combination eave and ridge venting is generally recognized as a superior venting technique."
Vent screens should be cleaned regularly because accumulated dust and grime can significantly restrict air movement. Fans connected to thermostats can be placed at gable end openings to draw hot air out of the attic space when temperatures reach a preset level.
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