Vent and Code Requirements | |
Before the mid-1970s, few people thought about establishing precise requirements for attic ventilation. Homes weren't built as air tight as they are today. If a home had any attic ventilation at all, it usually consisted of some undereave vents. In some warmer areas of the country, one or more louvers might supplement those vents (the purpose being, as already mentioned, "to catch the breeze"). In especially warm regions, an attic fan might be installed (even though there might not be sufficient intake venting to assure proper functioning). Even if homeowners or contractors had wanted to calculate specific requirements for temperature or moisture reduction, how would they have gone about it? At the time they had little research-based information to guide them. The Federal Housing Administration tried to close that information gap with minimum property standards for buildings with one or two living units. Since then, other standards have been developed. An example of current minimum requirements for ventilation comes from the 1994 Uniform Building Code (ICBO). Section 1505.3 of the Code specifies: Where determined necessary by the building official due to atmospheric or climatic conditions, enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow. The net-free ventilating area shall not be less than I/] 50 of the area of the space ventilated. EXCEPTIONS: 1) The area may be 1/300 of the area of the space ventilated provided 50 percent of the required ventilation area is provided by ventilators located in the upper portion of the space to be ventilated at least three feet above eave or cornice vents with the balance of the required ventilation pro- vided by eave or cornice vents. 2) The area may be 1/300 of the space ventilated provided a vapor retarder having a trans- mission rate not exceeding I perm is installed on the warm side of the attic insulation. Let's assume people read that recommendation and follow it exactly. Is adequate attic ventilation now assured to all who desire it? Far from it. The intent of that recommendation, after all, is to establish minimum standards. Take, for example, the net-free area requirement established in the exception - the 1/300 ratio. That amounts to less than 1/2" of vent area for each square foot of attic floor area, barely enough to create a minimal flow of air. In addition, that standard assumes a proper balance of high and low venting. Unfortunately, it's probably safer to assume that assumption rarely holds true. As a result, H.S. Hinrichs, a noted ventilation expert, concluded that "The minimum-free ventilation area presently specified by F.H.A. (.48 wq. in. per sq. ft. of ceiling area) is inadequate to provide proper attic ventilation. So if you want to do more than meet minimum requirements - in other words, if you want to install an effective, year-round ventilation system -you have to turn to the work being done at leading private and university research labs. Responding to the need for greater energy conservation, these groups have taken up the challenge of determining comprehensive requirements for attic ventilation. Their findings provide an easy, five-step process for creating an effective and efficient ventilation system. Source: Air Vent Inc. A Certainteed Company Reprinted with permission |