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Q.I have read conflicting reports about the use of housewrap. Does the building code actually require the use of housewrap under siding?

A.There are hundreds, if not thousands, of individual code interpretations in this country. A community adopts a code or some version of a model code and enforces it locally. Until recently there were three model codes that were used to develop local codes. However, in 2002, the three code-writing bodies, BOCA, ICBO, and SBCCI, consolidated all operations under one organization called the International Code Council. The ICC's International Residential Code for One- and Two-Family Dwellings (IRC) is catching hold and has currently been adopted by Washington, D.C. and 18 states as a statewide code. This is nice, but if you live in one of the other 32 states, your local code may still use and enforce some other code, like the 1999 BOCA National Building Code, for example. So you'll need to check your local code for a definitive answer.

From a technical standpoint, you should absolutely use felt or housewrap. But from a legal standpoint, you probably don't have to in most cases. The 2003 IRC deals with this in section R703.2, "Weather Resistant Sheathing Paper." It states that asphalt-saturated felt or other approved weather-resistant material shall be applied over sheathing of all exterior walls as required by Table R703.4. In general, you must use wrap behind masonry, wood shakes and shingles, hardboard, and fiber cement siding. You do not have to use it behind aluminum, steel, vinyl, wood, and panel products. But stayed tuned: The 2004 Supplement to the IRC will change this section, and jurisdictions that adopt the 2004 IRC will thus require weather-resistant sheathing paper under all types of siding.

Keep in mind, too, that the 2003 International Energy Conservation Code (Section 502.1.4.2) requires that you control air leakage through exterior penetrations, and one of the allowed ways to do this is by using a "moisture vapor-permeable housewrap." While this is not a strict requirement to use housewrap, it gives you another reason to do so.

Paul Fisette is director of Building Materials and Wood Technology at the University of Massachusetts at Amherst and a contributing editor to The Journal of Light Construction