A freshly immigrated Norwegian carpenter built our two-story house in 1931. It has a walk-up attic that can be aired out in the summer with a pair of windows in the gable ends. What the house doesn’t have—like many others in the neighborhood—is any apparent ventilation in the roof.
Houses like ours are no longer permitted by code. Builders usually install soffit and ridge vents on new houses, not only because building codes require ventilation but also because we’ve been told that vents carry away moisture and heat, lower the risk of mold and rot, and make the house more energy-efficient.
The current ventilation standard for residential roofs emerged in the 1940s, roughly 15 years after our house was constructed. The International Residential Code (IRC) provides a formula for calculating exactly how much ventilation a roof should have. It is described as the ratio of “net free ventilating area” to the area of the vented space (net free vent area is the unimpeded area of a vent that allows air to flow through it). That ratio should be no less than 1/300, and it carries a couple of caveats:
There are many types of roof vents on the market: louvered vents for gable ends, soffit vents, continuous ridge vents, mid-roof vents designed for buildings that don’t have soffits, vents for hip roofs, vents that look like sheet metal mushrooms, and electrically powered vents that can move as much air as the biggest kitchen range hoods on the market.
What brought on that fundamental shift in thinking about attic ventilation? Lots, says William Rose, senior research architect at the Applied Research Institute, University of Illinois Urbana-Champaign, and author of Water in Buildings: An Architect’s Guide to Moisture and Mold. As Rose explained in a 1995 article, a number of…