Fear of cold plywood sheathing in a cold climate
415irwin
| Posted in Energy Efficiency and Durability on
Martin,
A while back I asked about the wall system I am planning (drywall, taped smart membrain, 2×3 framing, 12″ cellulose, 2×6 structural framing, taped plywood sheating, tyvek, strapping, siding). In your opinion it should perform well and with an adequate rain screen the plywood sheating would be fine. Given the mantra “sheating should not be cold”, I am wondering what your reasoning is? Is it that with an adeqauate rainscreen the sheating will stay dry and therefore mold free? And when you say rainscreen, I am thinking simple strapping, albeit thicker (1 1/2″??)
Thanks
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Replies
Jessie,
Here's the reasoning: cold sheathing will gain moisture during the winter. Although many builders mistakenly assume that the moisture buildup is due to interior moisture migrating through the wall assembly, that only happens if your wall has lots of air leaks. If your walls are relatively tight, the source of the moisture is the outdoor environment. Cold materials tend to be damp; hot materials tend to be dry.
Like all examples of wetting, the seasonal accumulation of moisture in wall sheathing isn't necessarily a problem. As long as the annual rate of drying exceeds the annual rate of wetting, the sheathing should be fine.
The main reason to install a ventilated rainscreen gap between the sheathing and the siding -- especially on a thick wall with cold sheathing -- is to increase the rate of drying. The air gap helps the sheathing to dry in the spring. Your drying rate now exceeds your wetting rate, so everything is fine.
Thanks. That explains it.
So when you say "ventilated rainscreen gap", I am assuming that strapping should do the trick. And if so how thick should it be?
Martin,
My understanding is that the warmer the air is, the more moisture it can hold. Therefore, wintertime outside air contains less moisture than summertime outside air. I assume that plywood will tend to equalize its moisture content with the moisture content of the air around it. Therefore, plywood in the colder winter air will contain less moisture than plywood in the warmer summer air. Is this not correct?
Ron,
Q. "Plywood in the colder winter air will contain less moisture than plywood in the warmer summer air. Is this not correct?"
A. No, that is not correct.
It's hard to improve on the words of the master, William Rose. I quote from his textbook, Water in Buildings:
"At the same vapor pressure, warm temperatures lead to low relative humidity, cold temperatures lead to high relative humidity. Cold materials tend to be wet, whereas warm materials tend to be dry. Larry Teesdale wrote quite forcefully that the wetness of materials is function of their temperature.
"Let us add a few common, almost trivial observations. Bread kept in the refrigerator gets soggy, bread pulled from the oven is dry. We use heat in the dryer to dry our clothes. ... At this point we may propose a general rule regarding the wetness of porous and hygroscopic materials: At the same vapor pressure, cold materials tend to be wet and warm materials tend to be dry."
Jessie,
Most builders use 1x4 strapping, which is 3/4 inch thick. But you can make the gap shallower if you want; it's possible to rip 1/2-inch plywood or even 1/4-inch lauan into furring strips if you want a shallower gap.
Shallower gaps work fine, although deeper (3/4 inch) gaps probably dry faster. If you want to maximize drying potential, be sure to have screened openings at the top and bottom of the gap.
Martin,
Regarding Mr. Teesdale’s proposed rule as follows:
“At the same vapor pressure, cold materials tend to be wet and warm materials tend to be dry."
I guess I can see hygroscopic materials that are warm absorbing less moisture than those that are cold if both are in air of the "same vapor pressure," as the rule stipulates.
But is the vapor pressure of warm summer air the same as the vapor pressure of cold winter air?
If not, then how does this rule apply to plywood in the summer/winter cycle?
Ron,
Vapor pressures depend on the temperature and relative humidity of the air — as temperature and RH go up, vapor pressure gets higher. However, that fact does not diminish William Rose's point -- nor does it alter the fact that hot bread from the oven is dry, and cold bread from the refrigerator is soggy.
Consider two rain-soaked pieces of plywood -- one at 33 degrees F, and one at 80 degrees F. The one at 33 degrees F will dry slowly -- in other words, it will stay damp -- because the surrounding air can't hold much water, and evaporation from the surface of the plywood is slow.
The plywood at 80 degrees F is surrounded by air that can absorb a lot of moisture, and the air is eager to do so. That's why the 80 degree plywood is soon dry.
80F plywood is only going to dry quickly if the dew point of the proximate air is well below 80F, eh? (Fortunately spring & summer dew points are much lower than that most of the time in the colder parts of the US. :-) )
The wintertime plywood model is more complicated though. because you have two bodies of air with differing vapor pressures. The air on the rainscreen side has a much lower vapor pressure than the conditioned space air, and the plywood itself is a vapor retarder. If the materials between the plywood and the conditioned space are highly permeable the plywood will adsorb significant moisture whenever it's below the dew point of the conditioned space air, and only release it slowly into the rainscreen cavity. There are limits to how fast it can move that moisture, but it's usually fast enough to deal with vapor diffusion through latex interior paints, but not necessarily fast enough if there is major air leakage from the interior.
In Jessie's stackup the cold edge of the foot of cellulose will take on the bulk of any moisture that diffuses through the MemBrain in winter, and release it when it warms in spring, and the plywood doesn't take on much at all. In addition to bumping the drying rate an order of magnitude, the gap also limits the amount of bulk moisture reaching it from wind-blown rain through the siding, and allows the siding to drain & dry quickly. Rainscreens should be required by code in most of the US, IMHO, just like they are up north of the border, since that change alone would alleviate the need for strong vapor retarders in all but the coldest US climate zones. Even in climate zone 5 building in a rainscreen would mean even 5 perm latex paint on the interior would be vapor retardent enough to protect the sheathing. In zones 6 & 7 you'd still need the MemBrain (or half-perm "vapor barrier" latex) on the interior.
Martin,
I am not sure what to make of this. But it is the first time that I have ever heard that building materials take on more moisture when they get cold, so I am curious. If this were true, and you had a wood floor in an unheated shed, for example, joints between the floor boards would open up in the summer and close in the winter. Is that what you would expect?
I can see a soaked piece of cloth drying out faster if you heat it. But we are talking about materials taking on moisture from the water vapor in the air. I am not sure of that is analogous to saturating material from a liquid source of water and then drying it to air that is much colder than the object being heated for drying. With building material outside of the envelope, the temperature of the material will match the air temperature.
Assuming as a given, air that has as much moisture available as it wants to absorb, do you agree that cold air contains less water vapor than warm air?
Ron,
Yes, warm air is capable of holding more moisture than cold air. That's why ventilating a house with outdoor air during the winter is a good way to lower the indoor RH.
And yes, hygroscopic building materials tend to be damp when they are cold, and dry when they are warm, just like clothes from the dryer or bread in your kitchen.
Building Science 101.
Martin,
But your rule from Rose and Teesdale about materials absorbing moisture stipulates that both the hot and cold environments containing those materials have the same amount of vapor pressure.
Whereas, in your real world application of summer/winter, the two environments will not be at the same vapor pressure.
And since when does bread get soggy in the refrigerator?
Ron,
Here is some bread storage advice: if you appreciate the crisp crust on a fresh baguette, you should never put your baguettes in your refrigerator.
Hi. I just want to nail down a few matters for this discussion.
Chapter 4 in the Wood Handbook, online from the Forest Products Laboratory, is the go-to resource on wood moisture content. 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.
Ron, you're right that VP will vary. The example of same vapor pressure was used, by me and by Martin, just for illustration. The rule--MC depends on RH--works at the whole range of VP.
Dana, I agree with your post but just for clarification: rainscreens are only required in the coastal zone of British Columbia. They haven't been mandated anywhere else in Canada.
Bill Rose,
Thanks very much for the additional information; much appreciated.
Thanks for your input Bill. But I am still uncertain about the terms of this matter. I have always assumed that wood takes on the moisture content of air around it. And I understand that the temperature of air plays a role in the moisture content of air.
But in this discussion, I do not understand whether we are talking about air changing its moisture content, and thus causing the wood to change its moisture content in response—OR—wood taking on or releasing moisture in a fixed moisture environment simply because the temperature of the wood changes.
So let me ask this very simple question to build a foundation to understanding what this discussion is about:
If you have a sample of wood in air with a fixed level of moisture content; and you vary the temperature of the wood sample only, does the sample take on more moisture as its temperature decreases?
Hi Ron, I think you are asking some good questions.
you wrote:"If you have a sample of wood in air with a fixed level of moisture content; and you vary the temperature of the wood sample only, does the sample take on more moisture as its temperature decreases?"
I think ...Yes
I think if you lower the temperature of the wood...
the air near the surface of the wood will get colder..
and the RH near the surface of the wood will go up.
When RH goes up...Moisture content in the wood goes up.
the attached snip-it is from:
http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/files/bsi-023_wood_is_good.pdf
John,
Thanks for that information, which gets to my question as to whether wood moisture absorption can depend on the temperature of the wood, or whether it depends on the air temperature, and its effect on the moisture-holding capability of the air. I conclude that it has nothing to do with the temperature of the wood per se, but is solely dependent on relative humidity of the air to which the wood is exposed.
Regarding the winter / summer cycle outdoors, for wood to take on more moisture outdoors in winter, it would require a higher relative humidity in winter than in the summer. Yet this is not consistently the case. Relative humidity, on one hand, goes up and down according to temperature (which is lower in winter), but on the other hand, relative humidity also depends on moisture availability for air to absorb. This varies with precipitation events and weather pressure conditions, so sometimes a cold day in winter can have a higher relative humidity than a warm day in some, and vice versa.
A cursory examination of annual weather charts shows no clear correlation between relative humidity and the winter / summer cycle. However, it does show a clear pattern of dewpoints being high in summer and low in winter. But dewpoints are beside the point of the issue here.
Therefore, I conclude that the ability of wood to absorb and release moisture:
1) Is not affected by the temperature of the wood (alone).
2) Is not affected by air temperature (alone).
3) Is affected by the amount of relative humidity, which is a result of air temperature and available moisture for the air to absorb.
Aside from the issue of wood absorbing moisture, these variables of temperature, relative humidity, dewpoint, and vapor pressure do raise some disquieting questions about vapor moving to and from buildings.
Ron, hmmm ... not sure that I agree with your conclusions
let me offer another example ...based on an "Experiment" on page 85 from Bill Rose's book:
[Water in Buildings: An Architect's Guide to Moisture and Mold]
http://www.amazon.com/Water-Buildings-Architects-Guide-Moisture/dp/0471468509
The experiment is not exactly the conditions you describe
However, for me it indicates that when wood (or bread) is heated it "releases" water...
and when wood is cooled it adsorbs/absorbs water
quoting the captions:
"Empty jar and jar with {small chunk of} wood are brought to equilibrium.
Lids are tightly closed"
................
"Upon heating, the relative humidity in the empty jar is lowered....
while the relative humidity in the jar with wood is slightly raised."
the illustration shows both jars starting at 50% RH
after heating .....
the "empty" jar shows 20% RH
the jar with wood shows 51% RH
To me this experiment shows that the Absolute Humidity of the air in the jar (with wood) is rising as the wood is heated.
To me it would follow that if the wood were cooled ...
it would adsorb/absorb water from the air and the Absolute Humidity of the air in the jar would go down.
John,
I draw the same conclusion from that experiment.
This would mean that a decrease in the temperature of the wood, and an increase of the relative humidity of the air surrounding it—or either factor alone—will cause an increase in the moisture content of wood.
HOWEVER: In the natural atmosphere, as the temperature of wood decreases, the relative humidity can either increase or decrease. So if the temperature of wood decreases while the relative humidity increases, the wood is being induced to expand and shrink simultaneously.
Therefore: At some times, the temperature of wood will decrease while the relative humidity increases, both causing the wood to expand; and other times, the temperature of wood will decrease, thus causing wood to expand while the relative humidity decreases, thus causing the wood to shrink.
During the summer / winter cycle indoors, the air temperature is approximately constant, while the relatively humidity increases in the summer and decreases in the winter. Therefore, indoor wood expands in the summer and shrinks in the winter.
During the summer / winter cycle outdoors, the air temperature increases in the summer and decreases in the winter, while the relatively humidity is approximately constant. Therefore, outdoor wood expands in the winter and shrinks in the summer.
Ron, I think you are saying
warm wood tends to be dry
and cold wood tends to be wet.
John,
Just to clarify, I meant to say above that I draw the same conclusion that you do from that experiment. Then I listed what would follow from that conclusion. However, this conclusion does surprise me because it differs from what I previously believed.
Regarding your comment:
“Ron, I think you are saying
warm wood tends to be dry
and cold wood tends to be wet.”
The experiment does show that wood loses moisture when heated, and gains moisture when cooled. That is the direct effect of temperature alone on wood. That is the part that I have never known before. I had always believed that the gain and loss of atmospheric moisture was solely dependent on the rise and fall of relative humidity. Therefore, my basic question at the outset here was whether we were talking about temperature of the wood alone directly influencing its absorption of moisture—or—whether we were talking about temperature indirectly influencing the process by causing a change in relative humidity.
The experiment shows that the former applies. And, of course, the latter also applies. So, there are two completely independent causes for wood to gain and lose moisture. Often times the two causes will occur in tandem with one reinforcing the other to cause a moisture change in wood. However, the two causes can also work against each other.
Fundamentally, a drop in temperature increases the relative humidity of air. In that sense, the two causes would both contribute to cause wood to gain moisture. However, a drop in temperature can also be accompanied by a change in air mass introducing dryer air from a different location. So, even though the temperature drop is increasing the relative humidity of the air, that air is being exchanged for dryer air from a different location. So, in that case, the relative humidity may be rising as the temperature falls. I assume that those two causes working against each other would tend to cancel each other out, leaving the wood moisture content unchanged.