One of the key principles of high-performance, zero-energy homes is reducing energy use to a minimum. Since space heating and cooling have traditionally been the biggest residential end uses of energy, there is considerable emphasis on building insulation and air sealing. In most climates, it’s less expensive to increase wall insulation than it is to install a ground-source heat pump or more solar panels. For this reason, the walls of zero energy homes in heating-dominated climates usually require something thicker than 2×6 framing.
This leads to the inevitable comparison of two high-R wall options. You could attach several inches of rigid insulation sheathing on the outside of the walls or build a thicker cavity using the double-stud approach. I compared those two approaches in a previous post called High-R Walls, Part 1: Wall Assembly. While exterior foam sheathing is widely accepted as a good way to prevent condensation in wall cavities, the moisture performance of the double-stud approach is rightly questioned because of the potential for condensation. Since this was such a big issue, I discussed it in a follow-up post called High-R Walls, Part 2: Moisture Content where I mentioned that I embedded a data logger in the wall cavity of my new home to test the moisture performance. Now it’s time to look at the results.
Here’s the setup. I used a HOBO UX100-023 Ext Temp/RH data logger to record the data from my own net-zero energy house. This data logger measures temperature and relative humidity (RH) using a probe on a 6-foot cable, allowing me to mount the probe inside a north-facing wall during construction. I placed the probe on the inside surface of the 5/8-inch OSB wall sheathing.
Theory vs. measured results
The house has two stud walls: a structural wall on the outside and a second interior wall where the drywall is attached. Both walls are framed with 2x4s with an overall thickness of 10 inches. Packed with blow-in-blanket fiberglass, the insulating value of the assembly is around R-40.
In theory, this is a recipe for condensation and all the problems that come with it. The OSB has a perm rating of about 0.7, so it qualifies as a vapor retarder. Thick insulation blocks heat flow from the inside making the sheathing very cold in winter. When humid indoor air seeps into the wall cavity, you would expect there to be a considerable amount of condensation. If condensation occurs it tends to collect on the sheathing, since it offers a large cold surface.
I live in Bend, Oregon, which sits in Climate Zone 5 with about 6,800 heating degree days. I collected data over two winters. It’s not the arctic, but we do have our share of sub-freezing weather. The second winter saw an extended cold spell with many single-digit days in a row and several nights dipping below 0°F. This would seem to be a reasonable test of condensation in double-stud wall construction.
The highest RH measured at the exterior sheathing was 91%. Since condensation occurs at 100% RH, we can say that there isn’t liquid water on the sheathing. The graph shows a stretch of time with the highest RH levels over the two-winter test period (see Image #2, below).
One interesting result is that the RH varies by 4 to 12 points over the course of any given day. The peak was generally at the coldest time of the night.
This particular double-stud wall with fiberglass insulation didn’t experience condensation. If you looked up the temperature and water vapor amounts on a psychrometric chart or did a WUFI computer model, you would expect to see condensation. Why did condensation not occur in this case? While my test can’t provide conclusive evidence, there are several beneficial factors that reduce the condensation potential.
Air sealing: The blower door test on this house showed 1.0 ach50. It’s not as tight as a passive house, but it’s tight. The primary air barrier is the exterior sheathing which is glued to the studs, plates, and subfloor. Drywall was glued to the interior face of the studs and plates (although I didn’t see this with my own eyes). I, myself, caulked the drywall to the subfloor and coated all the electrical boxes with a thick layer of duct mastic. Taken together, these measures allow almost zero air from the inside to migrate into the cavity.
Why is air leakage a moisture issue? Because moist air carries much more water vapor into building cavities through air leaks than does diffusion through the wall materials themselves.
Ventilation: Our house is equipped with a Lifebreath energy recovery ventilator (ERV). The humidity of air in the living space varies between 35% and 45%, a level low enough to protect the building. The ERV is balanced slightly negative. That means that slightly more air is exhausted than is supplied. The slight negative pressure means that most leakage through the building shell will be to the inside. This tends to prevent moist interior air from flowing into the structural cavities.
Vapor retarder paint: The vapor retarder in this wall is a coat of poly-vinyl acetate primer with a perm rating of just under 1. Normally any water vapor that manages to squeeze into the wall through air leakage, diffusion, or other means has the opportunity to diffuse back through the drywall to the inside.
Wood framing: For decay organisms to grow, wood must be at saturation, which is around 20% moisture content. So, wood framing, if installed dry, generally has a significant capacity to store moisture before becoming saturated. Framing will absorb moisture when RH is high in winter and release it when the RH drops during warmer, dryer months. This moisture storage capacity buffers the moisture level in the walls. This process is highly dependent on the local climate. Once fully cured, framing lumber around here has a moisture content of around 8%. (Moisture content of wood should not be confused with relative humidity of the air even though they are both expressed in percent.)
Local climate: We live in a high desert climate with annual precipitation of only 8-11 inches. The climate itself is very forgiving. If wetting does occur, it will quickly dry out.
The bottom line of this experiment was that under my specific conditions, double wall construction did not lead to a moisture problem. But given that this is backyard science with a sample of one, I won’t claim that it applies across the board. Nevertheless, I’m willing to speculate that the beneficial factors taken together allowed my wall cavity to avoid condensation conditions. And these same factors should be taken into consideration when designing wall assemblies in any climate.
This post originally appeared at the Zero Energy Project.
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