Most homeowners and builders believe that attics should be vented. If you walk down to your local lumberyard and lean on the counter, the employees and nearby customers will offer a variety of opinions about why attics need to be vented. Unfortunately, it’s highly unlikely that the statements you hear will be true.
Here are the four most common reasons people suggest to explain the practice of venting attics:
- To reduce the chance of moisture build-up in the attic or condensation on the underside of the roof sheathing.
- To make roofing shingles last longer.
- To lower cooling bills during the summer.
- To reduce the chance of ice dams.
Although attic ventilation is sometimes able to contribute in a very small way to addressing the problems on this list, there are much better solutions to all four problems than ventilation.
What does the code require?
If you plan to install insulation on your attic floor, then most building codes require that the attic be vented. (For example, see Section R806.1 of the 2015 International Residential Code.)
The standard code formula requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the lower half of the attic (generally at the soffit) and half near or at the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.
Manufacturers of soffit vents and ridge vents usually specify the net free vent area of their products on product packaging or in specifications available online. (Researchers have shown that the net free vent areas reported by manufacturers are exaggerated, but that is a topic for another article.)
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