Your recent response to a question concerning crawlspace mold (Soundings, January/February 2007) addressed almost all areas with the exception of this one: oversizing air conditioning units. Doing so creates many more problems than it solves. In the summer, it is both cold and damp (very high relative humidity) inside the house, and the unconditioned space (be it attic or crawlspace) has a very high moisture content, often with areas of mold. The mold problem can be exacerbated when the HVAC draws outside air through the house to feed the air-handling requirements but the return air from the inside is insufficient to do so (primarily caused by improper sizing of vents, particularly return-air vents).
I still do not understand why codes have not been changed to allow for conditioned crawlspaces.
Drainage vs. Drying
I liked Mike Guertin's article on sidewall shingles ("Best-Practice Wall Shingles," May/June 2007). I have to dissent, though: He says that if you're using furring strips to create the air space behind sidewall shingles, "back kerfs are essential for allowing drainage and airflow between the horizontal strips." The expense of these kerfs has dissuaded him from using furring strips.
In my experience, his concerns are unfounded. We've been using horizontal 1x3 strapping to batten out wood cladding — not only shingles, but vertical tongue-and-groove and shiplap siding as well — for nearly 20 years now. We've never bothered to back-kerf and, having monitored a range of such projects (including my own home) during those two decades, I can say with great confidence that lack of back-kerfing has never caused a problem.
The building science behind the idea of the air space behind wood siding still seems rather fuzzy even to the top researchers. I've been hearing about the air space for 20 years now at a range of conferences and symposia, and the rationale for it keeps shifting. At first it was all about air pressure equalization; from time to time about moisture and temperature consistency on both faces of the siding; now, apparently, it's all about drainage.
The fact is, if you have so much water getting behind your cladding that you really need significant "drainage," then you've probably got some serious issues that even the best-designed building envelope could have problems handling. A good exterior wall assembly, for the most part, needs to deal with only moderate amounts of water getting where it doesn't belong, and in my experience it deals with that water by giving it a chance to evaporate before the water does any real damage. It turns out water can take a pretty long time to evaporate before it does any damage, if I'm properly interpreting moisture meter readings I've taken over time on a range of projects we've done.
What seems more important than quick drying is even drying. A piece of wood that does not experience wide moisture variations within itself seems to hold a coat of paint much longer than if there's significant variation from one point to another. My experience indicates that the mere fact of a sizable air space behind your wood siding seems to be enough to ensure even drying — whether or not there's significant drainage allowed for (there's always going to be some drainage, planned or not) and whether or not it's vented. This is important because placing an unwarranted emphasis on drainage or on venting will add costs incommensurate with the benefit. I would be keenly interested (and, I have to admit, surprised) if anyone knows of a situation where wood siding failed because it was installed on an air space that was unvented and "undrained."
Another big factor in how long exterior wood siding lasts, not specifically mentioned by Mike, is whether the siding is sealed on all sides prior to installation. We have found that wood siding primed on all sides and installed over an air space can hold a coat of paint for at least 15 years — and still counting, with no sign of losing adhesion. This has been true not only of shingles but of clapboards and tongue-and-groove boards. This is important in the context of the article because siding that holds a coat of paint longer will also keep water out of your walls longer.
Thanks very much for a great magazine.