Page 103 - 2018 National Home Improvement

2018 National Home Improvement Estimator, All Rights Reserved Page 83

If soil in the crawl space is covered with a vapor barrier, foundation vent area can be as little as 1/1500 of
the ground area (1 square foot for a 1,500 square foot crawl space). It’s usually much cheaper to cover
the soil in a crawl space with membrane than to add more foundation vents. Ground cover should lap at
least 2” at all joints (Figure 5-4). Lay brick or stones on top of the membrane to hold it down and prevent
curling.

Attic and Roof Ventilation
Insulation laid between ceiling joists helps hold heat inside the home. But above that insulation, air
should circulate freely. Excess moisture accumulation will damage insulation and other attic materials.
Good air circulation is especially important in colder climates and when the roof cover is less
permeable. With built-up roofing or asphalt shingles, the only way to get moisture out of an attic is with
vents.

Inadequately-vented attics can reach temperatures above 140 degrees F on a sunny day. Some of the
heat will work its way into the house, overburdening the air conditioning system. Properly-vented attics
will stay cooler and can save up to 30 percent on cooling costs. In addition, a cooler attic will keep
asphalt shingles from degrading prematurely in the heat.

Air circulation works best with soffit vents around the edge of the roof and outlet vents located high, near
the ridge. Warm air rises in the attic, exits at the peak, and draws fresh air through soffit vents at the
perimeter. Under those conditions, inlet vents should be 1/900 of attic floor area. Outlet vents should
have the same area. A home with only gable vents needs more free vent area. Air circulation through
gable vents depends on wind blowing the right direction. Provide one square foot of free vent area for
each 300’ of attic area.

Hip roofs don’t have gables and flat roofs don’t have peaks. That reduces your venting options. The
estimating section that follows includes power ventilators that can be installed on nearly any roof.

Flat roofs with no attic require some type of ventilation above the ceiling insulation. If this space is
divided by joists, each joist space should be ventilated. A continuous soffit vent strip is the best choice.
Drill through all headers that impede passage of air to the opposite eave.

A post and beam roof with a roof-plank ceiling has no attic and needs no ventilation. But the roof surface
requires insulation board coated with a walkable sprayed-on insulator such as urethane.