©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
During the winter months, wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is usually cold. Cold sheathing is risky, since it tends to accumulate moisture during the winter. Unless the sheathing can dry out during the summer months, damp sheathing can rot.
Cold sheathing can get wet from two directions. It can get wet from the exterior, due to leaks through defective flashing or a poorly detailed water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB). It can also get wet on the interior, due to a phenomenon traditionally called “condensation,” but more accurately called sorption. (As building scientist William Rose likes to say, “Capillary materials do not exhibit condensation at the dew point.”)
Most wood-framed walls are somewhat leaky. Interior air can leak into wall cavities through cracks around electrical boxes and cracks between the drywall and the wall’s bottom plate. When the warm air reaches the cold wall sheathing, one of two things usually happens: frost can form on the sheathing, or, at temperatures above freezing, the sheathing (which is hygroscopic or “sorptive”) can gain moisture from the air. (The source of the moisture taken on by sorption can be either interior or exterior moisture; for further details on moisture sources, see Bill Rose's posted comment below.)
Most cold-climate homes have wall sheathing that gains moisture every winter. Usually, however, the wall sheathing doesn’t rot, because:
Building components can survive occasional wetting, as long as the drying potential exceeds the wetting. If, on an annual basis, the wall dries more than it gets wet, it will probably be okay.
Probably — but not necessarily. In many areas of the U.S., OSB-sheathed walls have failed at an alarming rate. A combination of factors — poorly installed WRBs, air leaks through drywall, and the use of claddings (like stucco) that dry very slowly — have caused the OSB on thousands of homes to turn to oatmeal.
Even if the builder gets all the details right, there are still a few reasons to worry about new OSB-sheathed walls, especially if the wall is unusually thick. These days, builders are experimenting with thicker and thicker walls. In some parts of the U.S. and Canada, an increasing percentage of new homes have double 2x4 walls that are 12 inches thick — a design that makes the OSB sheathing colder than ever.
At least in theory, there are two reasons that thick walls are riskier than thin walls:
“By doubling the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the wall, we get half the energy available from the interior to drive the evaporation from wet wood,” says building scientist John Straube, a principal at the Building Science Corporation.
What factors make walls riskier?
Air leaks are bad. Leaks that allow interior air to enter a wall cavity are obviously risky — because these leaks allow moisture to piggyback on the exfiltrating air. So it’s important to create an airtight wall.
Air-permeable insulation is risky. Fiberglass batts do little to slow air movement. Switching from fiberglass batts to dense-packed cellulose raises the risk slightly (by keeping the sheathing colder); but more importantly on balance, it lowers the risk by reducing the chance of air movement through the insulation.
The thicker the wall, the colder the sheathing. The thicker the insulation on the interior side of the OSB, the colder the OSB. If you build a double 2x4 wall with a total thickness of 12 inches, you’ve made your sheathing colder than it used to be.
Sheathing temperature matters. Colder sheathing is at greater risk than warmer sheathing. (To quote Bill Rose again: “Cold, wet. Warm, dry.”) The more insulation you have on the exterior side of your sheathing — and the less insulation you have on the interior side of your sheathing — the warmer (and therefore dryer) your sheathing will stay. So an easy way to reduce the risk that your OSB sheathing will accumulate moisture is to install exterior rigid foam on top of the sheathing. That keeps the sheathing warm. However, the foam also reduces the ability of the sheathing to dry to the exterior, so it’s important to be sure that the foam is thick enough. Thick foam is better than thin foam (see "Calculating the Minimum Thickness of Rigid Foam Sheathing." )
Rainscreen gaps are good. If there’s a reason to believe that your sheathing is getting damp every winter, you want to be sure that it can dry quickly during the summer. One way to encourage faster drying: include a ventilated rainscreen gap between the sheathing and the siding. That reduces the risk of rot.
Avoid OSB. Finally, OSB is more susceptible to rot than plywood. So if you’re worried about the durability of your sheathing, choose plywood (or diagonal board sheathing) over OSB. One other possible (permeable) sheathing choice is structural fiberboard sheathing, which is available from International Bildrite  and Georgia-Pacific .
Get your flashing details right. Although it should go without saying, remember to install proper flashing at all wall penetrations, windows, and doors, and remember to integrate your wall flashing with the WRB. Upper courses should be properly lapped over lower courses.
After I mulled the issues raised in this blog, I sent an e-mail to John Straube, asking him about the riskiness of using OSB on a very thick double-stud wall, and about whether my advice is on target. Straube’s answer follows.
“We don’t know the full significance of this question, but the basic physics of wood and humidity tells us that OSB sheathing on a thick wall is risky, and experience has shown us that it is risky,” Straube wrote. “By risky, I mean riskier than historical practice.
“Anything we can do to reduce risk is therefore good. A major improvement is adding a ventilated gap over the sheathing: this allows for much better control of rain moisture (biggest concern) and encourages drying of the sheathing (particularly useful for air leakage condensation moisture). Switching to plywood adds more safety by further increasing the ability of interior moisture to dry outward.
“Your advice is good. I would differ by letting people know that it is more important to ventilate than switching from OSB to plywood — but both together are powerful allies.
“The presence of cellulose (rather than fiberglass batts) in the cavity is very helpful as it reduces any air leaks, stops convection loops, and add moisture storage and mold resistance (via borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments.).
“So, when you add the three components: ventilated cladding, plywood, and dense-packed cellulose, you have reduced risk tremendously. Are you back to the same safety that we had in the past? I don’t know. I don’t think so — but you’re likely really close.
“All of this assumes you have done a good job on the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. (tighter is better of course) and that rain control is managed (window subsill flashing, etc).”
Last week’s blog: “A Conversation With Wolfgang Feist.”