Search the forums

Loading
KDESIGN's picture

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?

no (post #207279, reply #1 of 39)

no

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Request for Explanation: (post #207279, reply #2 of 39)

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 (post #207279, reply #3 of 39)

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.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Calvin, Thanks for your (post #207279, reply #4 of 39)

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........ (post #207279, reply #5 of 39)

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.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Its always special when (post #207279, reply #12 of 39)

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" (post #207279, reply #13 of 39)

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 (post #207279, reply #15 of 39)

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.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The "warm" surface of the (post #207279, reply #6 of 39)

The "warm" surface of the material will be no colder than the average of the two chamber temperatures.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Note that no one is claiming (post #207279, reply #7 of 39)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

So then if the warm side of (post #207279, reply #8 of 39)

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 (post #207279, reply #9 of 39)

Come back to this in 30 days.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


But in a homogeneous (post #207279, reply #10 of 39)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The temperature of the (post #207279, reply #11 of 39)

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 (post #207279, reply #14 of 39)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

? (post #207279, reply #16 of 39)

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 (post #207279, reply #17 of 39)

The rate of diffusion determines (given the boundary conditions) the dewpoint at a given point in the material.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

You may find the following (post #207279, reply #18 of 39)

You may find the following article helpful: "Confusion About Diffusion" Insight-049, May 2011 at buildingscience.com

Thanks graphien.  (post #207279, reply #19 of 39)

Thanks graphien.  Coincidentally, I just read that report earlier this morning to refresh my memory of it.  Lstiburek says that vapor will condense in fiberglass, but it just won’t hang around there.  He says it moves on to a place of interest, which happens to the sheathing layer, and then it condenses there. 

Right, condensation collects (post #207279, reply #20 of 39)

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 (post #207279, reply #21 of 39)

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 (post #207279, reply #22 of 39)

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?)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Yes, I can see fog coming (post #207279, reply #23 of 39)

Yes, I can see fog coming into play. That is what Lstiburek was taling about when he said the vapor would condense in firberglass, but it would not hang around there.  It moves on to a relatively soild surface. 

 

To a water molecule, (post #207279, reply #24 of 39)

To a water molecule, fiberglass is "relatively solid".


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

That is what I would think. (post #207279, reply #25 of 39)

That is what I would think.

Water will tend to condense (post #207279, reply #26 of 39)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

If the two lines don’t (post #207279, reply #27 of 39)

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 (post #207279, reply #29 of 39)

I have no idea what his stated conditions are, or, for that matter, whether his calculations make any sense.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Water vapor! (post #207279, reply #28 of 39)

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 (post #207279, reply #30 of 39)

That isn't the worst analogy I've ever heard, but it's right up there.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville