Preventing Attic Condensation
An attic that cannot breathe properly has problems. During hot weather, temperatures may reach 150°F, placing a lot of demand on your cooling system. And during cold spells, the temperature difference between the attic and the heated spaces below causes moisture condensation. This can lower the R-value of insulation, rot framing and sheathing, and shorten the life of your roof. The solution to both problems is to provide adequate airflow through inlets at the eaves (or soffits), then up and out the roof via vents near or at the ridge.
How much ventilation your attic needs depends on whether the insulation includes a vapor barrier. If it does, there should be 1 square foot of venting for every 300 square feet of attic floor space. If there is no vapor barrier, double this figure. Vents do no good if they are clogged. Make sure that insulation does not lap over a soffit vent; you may have to section off the area with wood blocking. Also check that gable and ridge vents are free of caked-on dust.
Caution! Working in an Attic - If the floor of an attic is unfinished so that insulation is visible, one misstep could put your foot through the ceiling of the room below, causing damage that is a pain to fix. Maneuver strips of plywood, as wide as possible, into the attic. Either screw them to the joists permanently or move them around as you work.
Soffit and gable vents. In an unfinished attic, you might get by with gable louver vents at both ends. The larger and higher the vents, the more effective they will be. A gable fan, controlled by a heat-sensitive switch, can keep your attic cool, reducing energy costs. Better yet, add eaves vents as well.
Soffit and roof vents. Finished attics need both eaves vents and roof louvers. Inlets and outlets should be about equal in area. A ridge vent (see below) may run continuously along a ridge. If the roof is insulated, special channels may be needed to allow air to flow from the eaves up to the ridge vent.
Choosing vents. There are ventilation components for almost any roof. Breathers are easy to install, but you need many of them. Soffit strip vents provide greater airflow. Up top, you can choose gable or roof-mounted louvers (shown above). A ridge vent is an unobtrusive answer, but a professional roofer must install it. Add design interest with a cupola. Penthouse louvers work well on big, flat roofs. Many of these projects are big jobs best left to a contractor.