Cold sheathing question
user-1105327
| Posted in General Questions on
Martin, I read your article on cold sheathing. I’m still not clear on one point: how do you know if the sheathing is wet due to the sheathing being cold and taking on moisture or the sheathing being wet due to vapour making its way from the interior of the house?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Erik,
In general terms, we know about vapor transport in walls due to the work of researchers who have used three avenues to investigate your question: laboratory testing; field testing; and hygrothermal modeling (the use of computer software, which is verified and calibrated by lab tests and field tests).
When it comes to an investigation at any particular house with wet sheathing, the only way to determine the source of the moisture is by investigating the house. It's extraordinarily easy for a home inspector to be wrong on this issue, and it really takes years of experience and study to understand enough to determine where the moisture came from with any particular failure.
There are a few general guidelines, however:
- The most common source of water in walls is rain.
- The first suspects should always be flashing details and WRB details.
For more information on this issue, see All About Wall Rot.
Finally, when it comes to moisture accumulation in sheathing that is covered by exterior rigid foam of an insufficient thickness to keep the sheathing above the dew point in winter: researchers make assumptions about the interior relative humidity levels in the home, based on measurements made in thousands of homes, and use an energy modeling program to determine whether any particular foam thickness is safe or risky in a variety of climates.
I think cold sheathing is the Sasquatch of building science - often discussed, occasionally sighted, rarely captured.
The cold sheathing problem has raised many questions in my mind as well. I know there are many variables involved, but for simplification and a starting point, let me pose the following setup leading to a question:
To simplify the explanation, I would set aside the issue of rain entering the walls due to bad flashing. Certainly that can be a problem, but one does have the option of building with enough perfection that rain does not enter. Furthermore, while rain can add moisture to a wall, it does not seem like it is fundamentally related to cold sheathing because it does not involve condensing.
So, in setting aside the issue of rain, that leaves air leaks and/or diffusion creating the possibility of outward vapor drive condensing on the cold sheathing. However, building without air leaks is also an achievable and essential requirement. And it is said that diffusion is not significant enough to matter unless there is an unusually concentrated source of humidity inside of the living space.
Therefore, considering a house that has no exterior insulation, and thus has cold sheathing: Assuming that it has no rain leaks, has no significant diffusion, and has no air leaks; how can there be a problem where cold sheathing acquires moisture?
If it is cold enough to be a condensing surface and the moisture has no where to release to. Typically an air gap between the sheathing/WRB and the siding (rain screen, Homeslicker, etc) would be enough to allow that to happen, but do we know that's always true?
Atmospheric, if nowhere else. Condensing on the outside isn't much better than the inside of it's trapped agains the siding and can't evaporate or drain.
I understand your point that it would be a problem if atmospheric vapor were to condense on the outside of the sheathing and form water. And I agree that the atmosphere contains vapor. But what circumstances would cause atmospheric vapor to condense on the outside of the sheathing?
Good question. As I suggested in my first post, I'm a cold-sheathing skeptic myself. There could certainly be vapor drive inward through some (most?) claddings under the right circumstances. And there will always be some vapor drive, however slight, from inside. There is a whole "night time radiation" idea as well. I would love to see more photos of these problems that aren't traceable to some other problem.
If the sheathing is cold enough to be a condensing surface, why should that matter if there is no vapor available to condense on the sheathing? There won't be any moisture to release if there is no vapor to condense.
Dan,
You may recall that we had a couple long thread discussions on this issue, but several people connecting dots at the same time gets a little confusing. In those discussions, one prominent issue was the fact that cold hygroscopic materials can hold more moisture than warm hygroscopic materials. And therefore, due to the seasonal temperature drop, the siding and sheathing would become thirstier than they are during the summer.
The discussion evolved to discover that some regions had relative humidity levels that are higher in the winter than in the summer, despite the general assumption that the opposite is true in all regions. So this led to the premise of sheathing and siding taking on more moisture during the winter from the atmosphere because those materials are thirstier, and because more moisture is available.
Therefore, if this is the “cold sheathing problem,” it is not a problem of condensation. Instead, it would be like putting a wick into a bucket of water and running it into the wall cavity so it would transfer water by osmosis into the wall cavity and evaporate the wicked water into the warmer air inside of the cavity. The wick would be the cold sheathing. I assume that this model of moisture transfer is what Erik Olofsson is referring to when he says: “how do you know if the sheathing is wet due to the sheathing being cold and taking on moisture…?”
But other discussions seemed to define the cold sheathing problem as one due only to outward vapor drive condensing on the inside of the cold sheathing, which I assume Erik Olofsson was referring to in the second half of his question when he says: “…or the sheathing being wet due to vapour making its way from the interior of the house?”
Ron, nicely summarized.
The issue that I struggle with is a high r assembly where there is a good air and vapor control on the warm side of the assembly and the cold side is structural sheathing and therefore not permeable. In that case the cavity really can't dry well in either direction. In the marine climes of the Pacific Northwest, (where the Sasquatch is supposed to reside) the lack of permeability at the exterior sheathing inhibits high humidity in the cavity to balance out with exterior conditions. If it can't dry quickly through high diffusion potential at the sheathing, moisture can accumulate and then condense. A solution to this is to maintain a 10x1 ratio of permeability as you move from inward (1) to outward (10). 20 to 1 is even better.
Hence keeping Dans Sasquatch (cold sheathing) at bay with high perm sheathing so that the cavity can equalize with exterior humidity levels as quickly as possible. When its dry outside, the moisture, in liquid or vapor form, gets "sucked out" to satisfy the "imbalance".
Ron,
In Comment #3, you wrote, "Assuming that it has no rain leaks, has no significant diffusion, and has no air leaks; how can there be a problem where cold sheathing acquires moisture?"
Ah, if only such a building existed! One without rain leaks, without vapor diffusion, and without air leaks! Unfortunately, no one has yet built such a building.
Dan,
I know that you are a cold sheathing skeptic. I share your skepticism, and I await more data and evidence of failures.
In the meantime, I think that it is wise and conservative to choose plywood, fiberboard, or board sheathing when building a double-stud wall -- and to include a ventilated raincreen gap. These measures amount to cheap insurance, and provide many benefits. I don't want to be a guinea pig skeptic who builds homes that fail.
Martin wrote: "I think that it is wise and conservative to choose plywood, fiberboard, or board sheathing when building a double-stud wall"
translation: No ZIP for you
In my post #9, I described two separate principles that might be described as the “cold sheathing problem” as follows:
1) Cold sheathing adsorption/absorption principle fed by atmospheric moisture.
2) Cold sheathing condensing principle fed by living space moisture.
Which of these two principles is responsible for the cold sheathing problem?
"translation: No ZIP for you"
How does ZIP not fall somewhere under "...plywood, fiberboard, or board sheathing...", given that it's just painted OSB with taped seams?
Oops ... I see ... I guess ZIP can be considered as "Fiberboard"
My question for Martin :
I may have wrongly assumed you were not recommending OSB and ZIP
What type of commonly used sheathing do you NOT recommend for Double Stud Walls?
Dana,
I think that OSB is more vulnerable to moisture than plywood or boards. Fiberboard many not be durable, but at least it is vapor-permeable.
Hi Ron
I am "bumping" your question at #14
inquiring minds want to know
Ron and John,
Professor William Rose answered the question in a comment posted on my blog on this topic (How Risky Is Cold OSB Wall Sheathing?). The answer appeared in Comment #4 on that page.
Rose wrote, "Exterior sheathing gets wet by virtue of getting cold. ... Your discussion of ‘condensation’ leaves a lingering impression that wetness in cold sheathing materials comes from the inside, that is, from the high-vapor-pressure side (Northern bias throughout). Actually it comes from both sides, and predominantly from the side it’s closest to—the outside. There are ways to tweak both steady-state and transient analysis to answer the question of where the water comes from. We’ll have to leave that as an exercise. The advice you give here is good. Of course."
Martin: Yes, OSB is more susceptible if you screw it up, but it's about 1 perm (about as vapor-permeable as CDX ) when dry, but doesn't increase in permeance with moisture content to NEARLY the degree CDX does. Both plywood & OSB behave something like a "smart" vapor retarder, becoming more permeable when damp, but the exponent of the exponential increase in permeance with moisture content is bigger with plywood. When near saturation CDX runs about 25-30 perms to OSBs 8-10 perms, but that's still more permeable than #15 roofing felt, comparable to some lower-perm housewraps.
OSB is not a particularly risky material to use, as long as you have reasonable air-tightness on the interior (or good air-retardency in the cavity fill), and use any sort of rainscreen gap. Even where those are ignored most instances of real problems are due to missing or improperly lapped flashing, not air leaks or lack of rainscreen. It's not as if building with plywood is going to save the day on those types of construction errors- but it will look a bit uglier and might be discovered sooner on the OSB version. A bit less forgiving than plywood or plank perhaps, but it's still possible to build reasonably resilient structures using OSB.
Fiberboard is at least an an order of magnitude more vapor permeable than either OSB or CDX, which can create moisture issues on the interior layers of air conditioned buildings during hot humid weather. All stackups need to be reviewed for local climate appropriateness, and a super-high perm sheathing is important to keep an eye on.
"Even where those are ignored most instances of real problems are due to missing or improperly lapped flashing, not air leaks or lack of rainscreen. "
Dana, that's my contention as well. I also suspect that many of the remaining examples would be caused by a crappy insulation job (meaning voids).
In reviewing the comment by Bill Rose referenced by Martin in post #20, I see the explanation that the cold sheathing problem is due to the cold sheathing adsorption/absorption principle fed by atmospheric moisture from the outside; and also fed from the living space moisture from the inside; to the extent that there is a defective air barrier or no air barrier, and/or if there is an unusually high outward diffusion vapor drive.
But if those outward drive factors exist, the cold sheathing would also condense vapor which would lead to wet sheathing. So in that case, the wet sheathing would be a result of adsorption from the outside, and a combination of condensation and adsorption from the inside.
However, if there is an adequate air barrier and no high level of outward diffusion, the cold sheathing problem would occur solely because the hygroscopic materials would adsorb/absorb moisture from the atmosphere outside simply due to the seasonal temperature drop. This would occur even with an un-insulated, and unheated shed.
As I understand it, this cold material adsorption/absorption from the atmosphere is just a one-time moisture content adjustment for the seasonal average temperature drop. It is not a continuous generator of moisture that can accumulate like the continuous vapor drive being fed from the interior. So I have a hard time seeing an effect where the sheathing becomes saturated merely from this slight seasonal moisture change.
However, if unrestricted, outward vapor drive is added to the cold sheathing problem, then it certainly can saturate not only the sheathing, but much of the insulation as well. But it seems to me that this would be a construction defect or design flaw to allow this outward vapor drive, and thus not a fundamental component of the cold sheathing problem.
Much of the focus of the cold sheathing problem is on the deterioration of the sheathing from wetness. Is that the only negative effect of the cold sheathing problem?
Also, what does rain entry have to do with the cold sheathing problem? Certainly rain can get into a wall and wet the sheathing, but where does the “cold” of the cold sheathing problem enter into the action of wetting by rain?