For the past eight years I’ve lived in the high desert of Northern Nevada, where I’ve grown more accustomed to the peculiar traits of a high-altitude arid environment. On any given day, the temperature may vary by some 40 degrees, however, as it is so dry, one scarcely seems to notice.
During the winter months, when this region receives most of its annual precipitation, comfort periodically gives way to distress whenever a wet Western storm front saturates the air with moisture. While the temperature may stay consistent, as the humidity rises so does the perceived chill factor. While a heavy jacket may not be warranted, when it’s 50 degrees F at 20-percent humidity, you won’t see anyone out in shirt sleeves once the humidity jumps by 50 points.
Two building research groups have recently deduced that extreme fluctuations in humidity are the culprit behind a spate of drywall cracking and crowning problems in wood-framed homes.
The two groups—one based in the Western United States, the other in Melbourne, Australia—independently reached similar conclusions in their study of drywall cracking and crowning problems in the arid regions of the Southeastern United States, Australia and New Zealand. More importantly, their research has identified specific remedies to prevent similar problems in new construction.
Dry goods
Readers may already be familiar with the U.S. group, comprised of wallboard and finishing product manufacturers, code officials, builders, drywall contractors and truss fabricators. Prior articles (W&C September 1997 and January 1999) discussed The Drywall Finishing Council’s sponsorship and participation in the group’s research efforts, which focused on drywall cracking problems in Southern California and Las Vegas. Unbeknownst to the U.S. collaboration, a research effort had also been launched in 1996 by Dr. Bob Milner at Monash University in Melbourne.Both research teams sought to identify the cause of an outbreak of drywall cracking and crowning problems in areas with typically dry climates. Although construction practices in Australia differ somewhat from U.S. practices, the nature of the cracking problems under study were strikingly similar.
In most instances, during the wet winter months, large cracks would appear at taped joints in the ceilings of newly constructed homes. However, come summer, those same joints would crown. Attempts to patch the cracks proved fruitless, however, as with each change of season the problems would reappear. While it was obvious that movement of framing members was responsible for the phenomenon, the specific cause of the movement was unknown. Those involved in the U.S. study had a number of theories as to the cause, but finding an answer required an extensive and expensive collaboration of efforts, including the installation of electronic monitoring equipment in two Las Vegas homes. The Australian research effort went one step further: constructing a 20-square-meter framed wall, ceiling and roof structure within a climate control chamber.
It's not the heat, it's the humidity
Both research groups had certain expectations as to what the testing would bear out, but they were caught off-guard by the results. While it was expected that temperature fluctuations—which cause nearly all building materials to expand and contract—were a likely contributor, temperature changes alone did not account for an appreciable amount of movement. Both wood framing and drywall have similar coefficients of expansion, meaning they expand and contract in response to changes in temperature at pretty much the same rate. However, the groups learned that when you add a significant change in humidity to environmental conditions, certain wood framing members are far more responsive by expanding or contracting to a much greater degree than gypsum drywall. As the moisture content of timber increase, the framing expands—in all dimensions—pulling apart any wallboard attached to it. When wood framing dries, it contracts, compressing sheets of wallboard together.Fortunately, the expansion or contraction does not occur every time the humidity shifts. Both groups noted that humidity levels must change by at least 50 percent and must be maintained for a prolonged period of time. The Australian research team were able to introduce cracks in their test assembly by changing the humidity of the climate chamber from 20 to 90 percent, maintaining the higher level for 18 days. Similarly, the U.S. group’s electronic monitoring of Las Vegas homes found that a change of 50 percent in relative humidity lasting for at least 30 days was needed to cause a significant change in the moisture content of framing members, resulting in dimensional movement.
Michael Jundt, a registered structural engineer in Fresno, Calif., worked closely with the U.S. group in its effort to pinpoint the cause of the drywall cracking. According to Jundt, the use of juvenile, rapidly grown timber in truss assemblies makes them much more responsive to extreme humidity fluctuations. Framing cut from the center of a tree is more reactive to humidity changes, as is framing cut from rapidly grown trees, as evidenced by wide growth rings.
“If you see two to two and one-half growth rings per inch, that’s bad,” said Jundt.
Further, Jundt noted that while the use of kiln-dried lumber provided a certain degree of relief, it is not enough to prevent the seasonal expansion and contraction that results in cracking.
“Kiln-dried does work better, but it does not guarantee that it won’t happen,” he said. “What it does do is get rid of some of the lumber that may be more reactive.”
Unfortunately, most drywall contractors don’t have much of a say in what framing is used in trusses. However, the U.S. group was able to identify effective countermeasures that contractor’s can employ to prevent future problems.
Pre-conditioning interiors for just 24 hours prior to installing board can minimize cracking by some 20 percent, according to Jundt. While temperature changes were not a major factor in the cracking problems studied by the research groups, if board is installed without being pre-conditioned to the indoor environment, problems can appear within 24 hours after turning on the heat (or A/C). Jundt also cautions that propane heaters should not be used for conditioning, as their exhaust dumps excessive amounts of moisture into interior environments.
However, the most effective way to prevent the seasonal cracking is to use resilient channel to buffer drywall panels from truss movement. Jundt reported that installing resilient channel provided the only effective means of remedying problems in troubled homes, and that—as a preventative measure—it’s a relatively cheap and practical way of keeping problems at bay.
