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Measuring Moisture in a Double-Stud Wall

A data logger embedded inside a double-stud wall helps evaluate the risk of condensation

Posted on Oct 9 2017 by Bruce Sullivan

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 pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. or more solar panels. For this reason, the walls of zero energy homes in heating-dominated climates usually require something thicker than 2x6 framing.

This leads to the inevitable comparison of two high-R wall options. You could attach several inches of rigid insulation 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. 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 energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. 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 houseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., but it’s tight. The primary 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. 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(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.). The humidity of air in the living space varies between 35% and 45%, a level low enough to protect the building. The ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by 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-vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). 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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Image Credits:

  1. Bruce Sullivan

1.
Oct 9, 2017 9:47 AM ET

90% RH in Bend OR !
by Foster Lyons

Good thing its cold.


2.
Oct 9, 2017 10:03 AM ET

A dry climate
by Martin Holladay

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."


3.
Oct 9, 2017 2:55 PM ET

I wonder what the south facing wall is like
by Keith H

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.


4.
Oct 9, 2017 3:02 PM ET

Response to Keith H
by Martin Holladay

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.


5.
Oct 9, 2017 3:20 PM ET

Edited Oct 9, 2017 4:25 PM ET.

Adsorb vs condensation, and the vapor permeance of OSB.
by Dana Dorsett

"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%...

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-St... ) 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.


6.
Oct 10, 2017 7:12 AM ET

ERV balanced negative.
by Randy Williams

Should we be balancing all HRV and ERV units slightly negative in cold climates? What effect will this have on stack effect?


7.
Oct 10, 2017 7:19 AM ET

Edited Oct 10, 2017 7:49 AM ET.

Response to Randy Williams
by Martin Holladay

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.


8.
Oct 11, 2017 8:52 PM ET

91% humidity not a concern?
by Rick Evans

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...


9.
Oct 12, 2017 4:54 AM ET

Response to Rick Evans
by Martin Holladay

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."

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%."


10.
Oct 12, 2017 6:14 AM ET

Thank you Martin
by Rick Evans

The information you provided is extremely helpful. Thank you!


11.
Oct 14, 2017 9:02 PM ET

Measuring Moisture in a double-stud wall
by G-STAN

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!


12.
Oct 20, 2017 4:08 AM ET

Safe and Simple wall systems
by Kevin Dickson, MSME

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: http://www.greenbuildingadvisor.com/blogs/dept/musings/dense-packed-cell...

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. http://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.


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