I want to narrow this down so that the objective of the question is a simple as it possibly can be.
A sheet of drywall is subjected to a chill on one side that holds the temperature of that side to 65 degrees F. The other side of the drywall is being exposed to air with an 80 degree dewpoint.
Will vapor from the 80 degree dewpoint air condense on or inside of the drywall?
Replies
no
Request for Explanation:
How can vapor fail to condense when it encounters a material at a temperture below the dewpoint of the air containing the vapor?
easy peasy
it passes through.
As simple as I can make it.
I was outside splitting wood-temp 25.
I walked into the house-temp 70.
Glasses steamed up.
Clothes didn't.
Boots, don't know-they were wet from the snow outside.
Calvin,
Thanks for your answer. I understand your observation about the behavior of glasses and clothing when coming in from the cold. I have seen the same thing. And I also see the logic of your conclusion from the observation. But I have another explanation that flows from the same observation.
Here is an explanation that I see:
Your glasses have much more mass density than your clothing does. So clothes warm up to the interior temperature quicker than glasses do. I conclude that both glasses and clothes will condense vapor into moisture as long as they are at a temperature below the 80 degree dewpoint temperature of the air. But since the clothes warm up faster, they stop condensing water vapor before glasses do.
It is also true that the glasses are an impermeable mass, so any water that does condense on them will accumulate on the surface and be obvious. Whereas any water condensing in the clothing will disperse out into the volume of the material and become unnoticeable.
But I contend that as long as that clothing is below dew point temperature, it is condensing water vapor out of the air and depositing wet water in the clothing. But the quickly rising temperature of the clothing will end the water generating process before enough water can disperse into the clothing to make it feel wet.
The effect would be like spraying your clothing lightly with a spray bottle. Within seconds, there would be no perception of wet clothing.
And also, because the clothing is so permeable, the condensation will take place on so many surfaces that it will be almost thoroughly distributed and dispersed throughout the clothing upon origination of the moisture.
With the drywall sheet, I can see one scenario where vapor could pass completely through. Vapor would diffuse into the drywall, reach the dewpoint temperature, and condense into water. From there, it would move further through the drywall mass until it reached a zone near the other side where it would begin to evaporate. Then it would evaporate into the air of the cold side. If the water can evaporate as quickly as it is produced, it may never reach a state of saturation in the drywall great enough to cause decomposition of the drywall. So, you would have vapor passing through the drywall, but changing phase to water in the mid zone, and back to vapor upon exiting the drywall.
However, it is also possible that vapor input could overwhelm the evaporative capability of the opposite surface. If that happened, wetness saturation within the drywall might reach the level of causing decomposition.
So I want to set up an experiment to test these theories.
if this is the same experiment as the other thread........
then I think you blew it starting another one.
The 10,000 words in the other thread now might become lost and I doubt anyone would want to re-phrase them in this new thread.
But, I've been wrong a zillion times b/4 and pretty much conclude that this is the case once again.
Personally, the moisture is not going to overpower the drywall in your test.
Unless you keep introducing more and more and leave it no way to get out on the other side.
Its always special when someone who has all the answers poses a question not wanting the answer but really looking for an excuse to show how brilliant they are. After we throw all the lawyers into the sea, I hope we pick engineers next.
"But I contend that as long as that clothing is below dew point temperature, it is condensing water vapor out of the air and depositing wet water in the clothing."
I your most briiliant and exhaustive way, can you give us a thousand or so words on the salient properties of WET WATER vs DRY WATER?
Clarification of "wet water"
cussnu2 wrote:
Its always special when someone who has all the answers poses a question not wanting the answer but really looking for an excuse to show how brilliant they are. After we throw all the lawyers into the sea, I hope we pick engineers next.
"But I contend that as long as that clothing is below dew point temperature, it is condensing water vapor out of the air and depositing wet water in the clothing."
I your most briiliant and exhaustive way, can you give us a thousand or so words on the salient properties of WET WATER vs DRY WATER?
I’m not sure why you are being so contentious, but I will provide an explanation for my “wet water” comment. Since we are talking about both the liquid and gas phase of water, I think it pays to be as clear as possible which one I am referring to. The terms “vapor” and “moisture” are often used to refer to either the liquid or gas phase.
Furthermore, I have been told that no condensation is possible on drywall even if it is below the dewpoint temperature of the contacting air. Instead, I am told that a different kind of process takes place where the water moves though the drywall in the gas phase.
Calvin was alluding to this in his clothing analogy. So to address that in the clearest terms, I chose to use the term “wet water” to distinguish from water moving through clothing or drywall in the gas phase.
Furthermore, I have been told that no condensation is possible on drywall even if it is below the dewpoint temperature of the contacting air.
No one (to my memory) has said that. It's simply that condensation will not occur under the conditions stated.
(You need to remember that nature abhors discontinuities. Temperature and humidity will always follow a gradient when moving from warmer to colder, wetter to drier.)
The "warm" surface of the
The "warm" surface of the material will be no colder than the average of the two chamber temperatures.
Note that no one is claiming that you can't get condensation on drywall -- it certainly happens. But to force the SURFACE of the "warm side" below 80F when the warm side chamber is at 100F will require a cold side chamber temperature of well below 65F. If air circulation conditons are similar on both sides the center of the drywall will be at the average of the two temps, and the warm surface will be warmer than that average.
So then if the warm side of the drywall will not be below the dewpoint of the warm air, the only way condensation will ocurr is if vapor diffuses into the drywall and reaches the dewpoint temperature inside of the drywall mass at some point.
Also, since the warm side of the drywall will not be below the dewpoint of the warm air, if that drywall surface were covered with polyethylene, no condensation would occur on it.
KD
Come back to this in 30 days.
But in a homogeneous material, in "steady state", if the dewpoint is below ambient on both sides, it can't drop below ambient in the middle.
The temperature of the drywall is above the dewpoint of the warm air on the warm side. However, on the cold side of the drywall, and to some extent in the thickness of the drywall, the temperature is below the dewpoint of the warm air. So condensation should ocurr in those colder regions of the drywall as vapor diffuses into those regions.
The temperature of the drywall is above the dewpoint of the warm air on the warm side. However, on the cold side of the drywall, and to some extent in the thickness of the drywall, the temperature is below the dewpoint of the warm air. So condensation should ocurr in those colder regions of the drywall as vapor diffuses into those regions.
But, as I've shown, the rate of diffusion of the water vapor is such that condensation will not occur.
DanH wrote:But, as I've shown, the rate of diffusion of the water vapor is such that condensation will not occur.
I don't understand what you mean. How does the rate of diffusion affect whether or not condensation ocurrs?
The rate of diffusion determines (given the boundary conditions) the dewpoint at a given point in the material.
You may find the following article helpful: "Confusion About Diffusion" Insight-049, May 2011 at buildingscience.com
Right, condensation collects on surfaces and must dry--if it is to dry at all--from those surfaces. If it cannot dry, then other, usually undesirable, things happen.
graphein,
So are you saying that if it does not dry fast enough, wetting will take place and damages materials by swelling, disintegration, rot, etc.?
Right, condensation collects on surfaces and must dry--if it is to dry at all--from those surfaces.
Actually, condensation can collect in midair, and can "dry" from there. (Ever seen fog?)
To a water molecule, fiberglass is "relatively solid".
That is what I would think.
Water will tend to condense on a rough surface sooner than a smooth surface, but fiberglass fibers are probably sufficiently small to count as "rough".
Otherwise, the difference between fiberglass and drywall would be that drywall is absorbant while fiberglass is not -- this wouldn't affect condensation, but would cause water to "hang around" to a degree (whereas, beyond a point, moisture will run off of fiberglass).
Probably there are slight effects due to different surfaces that affect how "supersaturated" the air must be to condense. But these effects won't cause condensation ABOVE the dewpoint -- rather they control how much BELOW the dewpoint the air must get before condensation begins. I'm thinking in extreme cases the air can get maybe 3 degrees below the dewpoint without condensing, if there are no "nucleating" agents present.
On the other hand, condensation releases heat, so even slight condensation, in the absence of air movement or some added source of cooling, will raise air temperature and cause condensation to halt.
If the two lines don’t cross in my experiment, why do they cross in Mr. Lstiburek’s experiment?
I have no idea what his stated conditions are, or, for that matter, whether his calculations make any sense.
Water vapor!
Put simply - the water vapor molecules are very tiny and they can slip through many things, without as it were touching the sides.
Think of it in terms: A cardboard box of footballs. The air/whatever are the footballs, the large open spaces in between are where the water vapor moves without touching the air/footballs/whatever.
The water vapor is of such small size - that it slips through the surface molecules of many things.
That isn't the worst analogy I've ever heard, but it's right up there.
At the start of this thread #2, I removed the insulation for simplification, but the point of the experiment is to find out what happens with reverse vapor drive with maximum air conditioning and outdoor heat and humidity. Therefore the experiment needs fiberglass insulation on the warm side of the drywall.
Therefore the insulation side of the drywall will be colder than the highest dewpoint on the outside of the insulation. Some distance between the drywall and the midpoint of the insulation will also be below the highest dewpoint. So I would expect condensation to begin occurring at some point midway through the insulation, and then move on to the site of interest, which is the drywall, and then accumulate on that surface. It is exactly what happened in Mr. Lstiburek’s experiment. And he says that it is the typical response.
His experiment is to show that what is expected is not what happens. He shows that although the dew point temperature is reached just outside of the interior wall finish, the condensation ends up forming at the opposite side of the insulation and on the sheathing. The condensing action is somehow swept beyond the dewpoint boundary, and accumulates on the final barrier of the cavity.
KD
You mention the bldg science finding on their building............
but you also say (and I thought I read-been a while) that the vapor collected on the backside of the sheathing.
Your drywall cannot be considered sheathing.
Best pull the sheetrock and insert plywood or osb.
Calvin,
Yes, Lstiburek's experiment was done with a layer of sheathing rather than drywall. And that may mean his result cannot be compared to my experiment. His temperature spread is also larger. I will use drywall because that is to represent the interior finish of the living space.
I am curious about the basis for the idea that the lines of temperature and dewpoint do not cross. I have heard that before stated almost as a revelation where modern science turns tradional building science upside down. But so far, I have heard no clear explanation of why the lines won't cross. They crossed for Lstiburek. They cross when it rains.
You keep going around in circles. Your "experiment", as originally stated, and as best as one can calculate, will not result in condensation. This definitely DOES NOT mean that there is no circumstance in any building anywhere where condensation will not occur -- there are obviously too many existence proofs to the contrary.
A wall with some sort of wallboard, fiberglass insulation, and exterior sheathing is far different from the simple structure you described. And a structure (can't call it a "wall", quite) with open fiberglass insulation and wallboard/sheathing on only one side is different still.
But anyway, WOULD YOU PLEASE GET TO YOUR POINT???!!! What do you want to prove in all this?
Looking at the Lstiburek article (http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion/?searchterm=diffusion) I don't see anything that contradicts what we've said here.
When you put fiberglass insulation in a wall then strange things happen. Fiberglass allows a substantial amount of airflow and convection, and you cannot simply apply mathematical formulae to the overall structure without somehow accounting for that airflow.
Also, with regard to condensation, it does not occur instantly when the air temp drops below the dewpoint. Time and some sort of nucleating action is required before condensation occurs. In a static environment (with no airflow to speak of) this is of minor consequence, but, again, in a fiberglass-insulated wall there's enough convection for the super-cooled/hyper-saturated air to be drawn away before a large amount of condensation occurs.
And in real live temperature is not constant. Condensation may occur within a wall but then, when the wall warms a bit, the moisture will migrate outward. This cycle can repeat day after day.
I would expect the chilling of the drywall surface to chill air near it, and cause that air to fall as its density rises with the falling temperature. As the chilled air falls, it will be replaced by hotter, humid air. So I see a convective loop that would pump the hot, humid air down the face of the drywall and through the insulation along that boundary. So it would be pumping a flow of 80 degree dewpoint air through a region of temerature well below that dewpoint temperature.
I might clarify that I am not saying than any answer so far is right or wrong. I have my own theroy, but I cannot prove that it is right either. That is why I want to build this experiment, but I thought I would ask for people's thoughts about what will happen.
So for this science question, the consensus seems to be that either there will be condensation and wetting of the drywall; or there will not be. When I read the article called Confusion About Diffusion by Joe Lstiburek, page 1 seems applicable to the question I posed.
From the article, I conclude that the answer to my question is that condensation and wetting will occur in the context of my question. The only difference between my question and Lstiburek’s experiment is that he has a 70 degree temperature drop versus a 35 degree drop for my question; and he uses sheathing whereas I use drywall.
My question concerns reverse vapor drive against an air conditioned interior whereas Lstiburek’s experiment shows the effect of outward vapor drive. However, I assume that the effects will be the same. In my question, the interior drywall will be what Lstiburek calls the “condensing surface of interest.” If it is below the dew point, vapor will condense there.
Mr. Lstiburek’s point of the experiment is to show that moisture does not accumulate at the point of condensation, but rather, it moves on to accumulate on the first surface it encounters. However, he has stacked up enough qualifications and disqualifications in his footnote #2 to pretty much lead to any conclusion you want to explain the result of his experiment.
http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion/?searchterm=diffusion
One thing the article doesn't discuss is that the temperature/humidity profile in the fiberglass will not be linear, due to its highly porous nature. Rather, it will be closer to a exponential profile, with the temperature and humidity remaining near "room" conditions until maybe halfway through the stuff. This will likely place the condensation point much nearer the outer surface.
And, as I stated earlier, several effects tend to delay condensation until the air is "super-cooled", moving the point of maximal condensation toward the colder surface.
Finally, temperature variations of only a degree or so will re-evaporate condensation near the "boundary", and some of that moisture will then migrate further toward the cold surface. Even with no temperature variation this process occurs at a low level.
Nothing particularly mysterious about any of it, and none of it invalidates the basic computations. In general the computations determine the earliest that condensation will occur.