Image Credit: Bruce Sullivan
Image Credit: Bruce Sullivan Relative humidity inside the wall varied by 4 to 12 points over the course of any given day. The peak generally occurred at the coldest time of night.
More Guest Blogs
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 results
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.
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
16 Comments
90% RH in Bend OR !
Good thing its cold.
A dry climate
Before GBA published this article, I let Kohta Ueno, a researcher at Building Science Corporation, read it. He provided several interesting comments. (I hope he posts his reactions here.)
I won't quote all of his email to me, but I will quote some if it. Ueno wrote:
"I’m good with this method of RH measurement at the sheathing-insulation interface—i.e., where condensation is going to happen in wintertime. The HOBO remote probes are small enough that they’re not disturbing the insulation significantly. This stands in contrast to the OmniSense probes—a plastic “box” that pushes insulation out of the way. The fact that his RHs never get above 91% means to me that things are incredibly safe… not even really worth measuring MCs, unless you were curious.
"The conclusions make sense (and are not too earth-shattering)—double stud wall, Climate Zone 5 (5B, right? So super-dry), and 1 perm paint inside = safe."
I wonder what the south facing wall is like
Assuming he has solar exposure, the implication is that the south facing wall may not be experiencing even high % RH. I'd be very curious about the effect of aspect.
Response to Keith H
Keith,
Researchers have monitored the moisture content of sheathing in dozens of homes, in many climate zones. Here in the northern hemisphere (in the U.S. and Canada), north-facing sheathing is almost always more damp than south-facing sheathing, for fairly obvious reasons.
That's why people who are worried about damp sheathing always check the north-facing sheathing -- ideally, toward the bottom of the wall.
Adsorb vs condensation, and the vapor permeance of OSB.
"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."
It's pretty safe to say that even when the moisture content of the OSB is well above that necessary for it to rot away quickly, there will NEVER be "...liquid water on the sheathing...".
The reason it never hits 100% RH at the sheathing/fiberglass interface is because the moisture in the cavity's air is being taken up by the OSB as adsorb, not liquid. Only when the moisture content of the OSB is ultra-high (north of 50% moisture content by weight, way above rot-risk levels) would it be possible for liquid water to form on the surface.
Also " The OSB has a perm rating of about 0.7, so it qualifies as a vapor retarder." carries some misconceptions. The vapor permeance of OSB isn't a single number, it is variable with both moisture content and the RH of the proximate air. When the proximate air continuously dwells at an RH of 91% the OSB's vapor permeance rises to more than 10 perms, at which point it's not even a Class-III vapor retarder. See Figure 1:
http://www.norbord.com/na/cms/wp-content/uploads/Moisture%20Vapor%20and%20Perms%20J450.pdf
Of course the RH on the interior side of the OSB may be somewhat higher than on the exterior side during those coldest-hours periods, but the coldest outdoor temperatures are often determined by the outdoor dew points, and the RH on the exterior during those coldest-hours is also going to be high, if not quite 91%. Either way you can count on it being WAY above 0.7 perms whenever the RH at the OSB/fiberglass is measuring 91% RH (or even 50% RH.) The mean January temperature in Bend is about 30F, and the mean January low is about 24F. (see: https://weatherspark.com/y/1215/Average-Weather-in-Bend-Oregon-United-States-Year-Round ) When the outdoor dew point is 10F (pretty dry) and the outdoor air & sheathing is 24F, the air on the exterior of the siding is still about 50% RH, which means the OSB will be over 2 perms even if the air on the cavity side were only 50% RH (rather than >75% as indicated in the monitored data.) The actual vapor permeance is unknown, and will vary a bit with moisture content as well, but it's still over 2 perms, most of the time in winter, and WAY over 2 perms some of the time.
It is this characteristic that allows buildings in zone 5 to get away without anything more vapor retardent than standard latex paint (about 5 perms) on the interior side for a vapor retarder, as long as the siding is back ventilated. With vented siding the number of cold hours over a winter are few enough that the moisture passes through the OSB to be diluted by the outdoor air, and the moisture content stays within relatively safe limits. With foot thick double studwalls the number of seasonal moisture accumulation hours increases a bit compared to conventional framing, but the low-perm paint gives it a huge safety margin.
ERV balanced negative.
Should we be balancing all HRV and ERV units slightly negative in cold climates? What effect will this have on stack effect?
Response to Randy Williams
Randy,
Most homeowners overestimate the effect of a ventilation system on infiltration and exfiltration rates. Ventilation systems move small volumes of air -- generally in the range of 60 cfm to 100 cfm. Air exchange rates in that range are easily overwhelmed by wind effects and the stack effect.
The best advice is to follow manufacturers' recommendations for commissioning an ERV or HRV. These systems are designed for balanced operation.
If you are worried about indoor air entering your wall cavities, the best approach is to pay attention to air sealing measures.
91% humidity not a concern?
I am a bit perplexed here... Isn't 91% a dangerously high moisture level? I know the test wan't scientific, but aren't these figures dramatically higher than the BSC tests a few years ago?
I hope I am just confusing the differences between MC and RH...
Response to Rick Evans
Rick,
You are probably confusing moisture content (MC) and relative humidity (RH). The sheathing in Bruce Sullivan's wall is not at risk for rot.
Some relevant quotes:
A paper by Mark Willians (“Developing Innovative Drainage and Drying Solutions for the Building Enclosure”): "Most water-induced deterioration of wood-based products and mold formation requires wood moisture content above 20% (Morris, 1998). Readings from 20% to 28% are typically considered moderate and indicate that damage may occur if moisture levels are sustained. Readings of 30% and above indicate that wood-based components are saturated and damage is likely if sustained (Morris and Winandy, 2002)."
Bill Rose: "Wood moisture content depends strongly on RH of the surrounding air, somewhat regardless of the temperature or vapor pressure at which that RH is achieved. In one project I'm working on, the January outdoor RH is 84%, the January indoor RH is 10% and the June RH, indoors and out, is around 60%. That sort of pegs where the wood moisture content will be, or tend to be. Vapor pressure is often the same indoors and out, except with strong moisture sources or dehumidification."
http://www.germology.com/moisture_survey.htm : "Wood moisture content between 0% and approximately 28% is dependent upon the relative humidity of the surrounding air. As the air’s humidity increases, so does the moisture content of wood exposed to air. Wood exposed to 90% ambient relative humidity will reach a Wood Moisture Content (WMC) of about 20%."
Thank you Martin
The information you provided is extremely helpful. Thank you!
Measuring Moisture in a double-stud wall
The better way to construct a double stud wall is to place foam in the middle, not on the outside.
Example: ( stud face to stud face) begin at the outside of the outside wall. 2x6 outside wall filled with
your choice of fiber insulation - then 2 inches of polyiso on inside face of outside wall - then 1/2" gap filled with blown in cellulose - then 2x4 inside wall also filled with blown in cellulose: Total wall thickness 11 1/2 inches -
Total R rating approximately 33 to 39 depending on fiber chosen and outside temperature (note this is the real effective R
rating considering 20 to 25 % wood in walls). Treat the inside wall as another inside non-structural wall
to be constructed after fiber and polyiso are installed, this makes construction easier - the polyiso acts as a moisture barrier (both directions) and the insulated outside wall protects the foam against both physical damage and R rating deterioration due to temperature drop. Need less R rating - less foam - no gap and no cellulose - need more - add foam expand gap, No more moisture problems!
Safe and Simple wall systems
Thanks to Bruce Sullivan for collecting more evidence that this wall assembly is safe. This research is important because the IECC codes that require exterior foam insulation are adding unnecessary cost to these wall systems. We've studied this at GBA many times and my favorite wall of this type was discussed thoroughly in 2014: https://www.greenbuildingadvisor.com/blogs/dept/musings/dense-packed-cellulose-and-wrong-side-vapor-barrier
As an infill developer also using this type of wall, here are my recommendations (that don't have any significant cost penalties.)
1. Dense packed cellulose is safer than fiberglass (see Dana's comments on adsorption above) and as a bonus, it blocks air infiltration better.
2. Use real plywood sheathing instead of OSB because it is more rot-resistant than OSB, and OSB actually leaks a lot of air in blower door testing. https://www.greenbuildingadvisor.com/blogs/dept/musings/osb-airtight
3. A liquid-applied weather resistant barrier (WRB) on the outside of the sheathing is also the vapor and air barrier. That's a real cost-saving 3fer. These coatings are self-healing and less hassle to apply than the expensive building tapes (which are still required at windows and other penetrations.
4. I really don't see much risk in the 5b climate zone.
Further Clarification on the Risks of 91% RH
Just some further clarification on the risks of a peak value of 91% RH in winter. After doing a bunch of instrumentation and monitoring of building enclosure assemblies (wow... back to 2003 at this point), a peak RH of 91% in winter is not risky at all.
First off--other work has shown that when you have condensation--i.e., liquid water on the surface, that's when mold growth really takes off. I believe that's Susan Doll's ScD thesis. This observation definitely matches up to what we've seen when we've opened up monitored assemblies, and looked at damage in forensic investigations.
Next, temperature plays a huge role in whether you have problems--91% RH when the sheathing is a frozen hunk of wood is not risky. In the words of Mark Bomberg (mentor to many important building scientists), "... we worry about wet walls in the winter, but they rot in the summer." (for proper effect, must be stated with a thick Eastern European accent).
We can quantify this quite nicely--some of the current work on mold growth is the Viitanen mold index--for instance, it is adopted as the new moisture failure in the latest addendum to ASHRAE Standard 160. I've attached plots below of a "flatline"
91% RH condition, at 4 C (39 F), 10 C (50 F), and 20 C (68 F). The various color lines show the effect of substrate/material sensitivity. Below 4 C, there's no growth. At 4 C, you'd need to use a microscope to see any mold growth, on the "most sensitive" material. At 20 C, you've got some nasty fuzzy sheathing.
Just a quibble with part of the article:
I agreed with the "cold sheathing" explanation before I ran BSC's double stud wall project. In that job, I compared a 2x6 wall and 12” double stud wall side by side, and looked at sheathing temperature. I tried to look at differences in sheathing temperatures between the two walls. Takeaway: huh… you can barely make out a temperature difference—maybe a degree F? It is more about the heat flux/heat flow (per the first half of the sentence) than the sheathing being substantially colder than “normal” construction (see 4.6 Sheathing Temperature Measurements in report below).
BA-1501: Monitoring Double-Stud Wall Moisture Conditions in the Northeast
https://buildingscience.com/documents/bareports/ba-1501-monitor-double-stud-moisture-conditions-northeast/view
Re: Safe and Simple wall systems
First off--I agree with all of the numbered points here--cellulose is safer than fiberglass (moisture storage and borate preservatives); plywood is safer than OSB, liquid-applied membranes are a nice safety factor, and this is a low-risk wall in Zone 5B.
But one point I'd like to push back on:
It is true that the prescriptive tables in the IECC (e.g., TABLE R402.1.1 INSULATION AND FENESTRATION REQUIREMENTS BY COMPONENT) show cavity + continuous exterior insulation as a requirement. However, the next table is the U-factor equivalents (TABLE R402.1.3 EQUIVALENT U-FACTORS). It should be reasonable to calculate the whole-wall/opaque U factor of a double stud assembly, showing that it meets target U values. Martin did a nice job of calculating thermal bridging through studs and overall U-factor here https://www.greenbuildingadvisor.com/articles/dept/musings/installing-closed-cell-spray-foam-between-studs-waste. This could just be applied to the multiple layers of a double stud wall (inner wall, continuous between, outer wall).
Convincing your local building official that math works, and that your math is right... yeah, that's a different problem... can't help you with that one.
Reply to Khota Ueno
Khota, Thank you for your expert contributions here!
100% not needed for condensation
See below for "This means we can get condensation on porous surfaces at relative humidities less than 100 percent."
The good news is that you won't get a lot of moisture accumulation below 100 percent.
See building-science-insights/bsi-099-its-all-relative
Log in or create an account to post a comment.
Sign up Log in