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I am going to build a test fixture to conduct an experiment in building science.  It will consist of a box divided into two chambers.  The air in one chamber will be 100 degrees F and 80 degree F dew point.  The air in the other chamber will be 65 degrees F and dew point of say 40 degrees F.  These air temperatures and dew points will be maintained during the time of the experiment which will last for 30 days.

The only material separating the two chambers will be a layer of ½” unfinished drywall.  The drywall will be mounted by its four edges, which will be sealed to the walls of the fixture box. 

What will be the effect, if any, on the drywall?  

One would have to calculate (post #207264, reply #1 of 33)

One would have to calculate the dewpoint and temperature profiles of the drywall, plus explore the aborption characteristics of gypsum to determine what will happen.

This site might help: http://www.vesma.com/tutorial/uvalue01/u...


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

I would use the blue drywall. (post #207264, reply #2 of 33)

I would use the blue drywall. (moisture resistant), just to avoid a potential problem.

Greg

I am asking what will happen (post #207264, reply #3 of 33)

I am asking what will happen over the time of this experiment.  Please predict the result based upon your knowledge of the science.   

The drywall on the 100f side (post #207264, reply #4 of 33)

The drywall on the 100f side will get below 80f and it will be wet.

Greg

According to this site:  (post #207264, reply #5 of 33)

According to this site:  http://dew-point.us/?rid=20121230021957H...

 


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

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unscientific answer (post #207264, reply #6 of 33)

nothing will happen to the sheetrock.

tho one could ask, what is it's moisture content at assembly. 

and what's the box made of and it's moisture content or ability to  allow moisture to pass through it.

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

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


The box for the test fixture (post #207264, reply #7 of 33)

The box for the test fixture is airtight and moisture impermeable.  But in any case, the conditions of dewpoint and temperature inside of the two chambers will be maintained by power equipment during the test.  So for instance, if vapor from the hot side were to pass through the drywall to the cool side, that passing vapor would not be allowed to accumulate in the cool side and raise its dewpoint.   In the same way, the hot side will not be able to raise the temperature of the cool side.   

The cold side of the drywall will match the cold side air temperature of 65 degrees F.  The warm side of the drywall will be slightly warmer than 65 because there is some R-value to the drywall.  So let’s just guess that the warm side of the drywall will be 68 degrees F.   That would mean that a 68 degree surface of the drywall is contacting air that has an 80 degree F dew point.   This continues for 30 days just to capture some performance over time.    

The cold side of the drywall (post #207264, reply #8 of 33)

The cold side of the drywall will match the cold side air temperature of 65 degrees F.  The warm side of the drywall will be slightly warmer than 65 because there is some R-value to the drywall. 

Are you saying that's the initial conditions, or that's what's going to happen?  Clearly, neither side will be exactly at the temperature of the adjacent ambient air, once steady-state is achieved.  Precisely what temps will be achieved on the DW surfaces will be dependent on airflow in the two chambers.


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 would be the fixed (post #207264, reply #9 of 33)

That would be the fixed conditions persisting throughout the test period.

But I understand your point that some convection and air film conditions will affect the heat transfer though the drywall.  But the main point is that the drywall will be in contact with air that has a dewpoint that is much higher than the coldest region of the drywall.  Obviously there will be heat transferring through the drywall from the warm to cold side.

Perhaps the whole test chamber needs to be insulated in order to minimize heat transfer to or from the outside, and thus minimize internal convection.  For the experiment, I want the air in each chamber to be a monolithic temperature and stationary. 

As I think about this, there probably needs to be air permeable insulation on the warm side of the drywall as there would be in the actual practice represented by this experiment.  That would slow down the heat transfer to the drywall, and keep the drywall temperature close to the 65 degree level throughout its thickness.    

You can't have the air at a (post #207264, reply #16 of 33)

You can't have the air at a "monolithic" temperature and still have one wall surface that is not at that temperature.

If you're going to put insulation against the drywall that entirely changes the equation.


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

I think you will find that if (post #207264, reply #10 of 33)

I think you will find that if this is still air, the drywall will end up somewhere between the high side and the low side and it is very likely that will be below the dew point on the high side. You are actually talking about a RH of about 40 and that is low for where I am at so I am not sure what you are trying to establish.

Greg

I am assuming that the (post #207264, reply #11 of 33)

I am assuming that the drywall will be no warmer than 70 degrees on either side or within the sheet thickness.  What happens when air of 80 degree dew point contacts drywall that is no warmer than 70 degrees? 

When air at a given dewpoint (post #207264, reply #17 of 33)

When air at a given dewpoint contacts a surface below that temp then condensation will occur -- that's the definition of dewpoint.  But keep in mind that there's a gradient of dewpoint in the air, in addition to the temperature gradient.  The less stirring of the air that occurs, the greater these gradients will be.


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

Well........ (post #207264, reply #12 of 33)

if this is a science experiment and you wanna know what's going to happen to the drywall.........

you'd best get a moisture reading of that board b/4 you stick it in the box.

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

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Yes the experiment will (post #207264, reply #13 of 33)

Yes the experiment will monitor moisture content of the drywall as well as the RH and temperature of the two chambers.  So what will happen?

I already answered. (post #207264, reply #14 of 33)

Nothing will happen to the drywall.

Will it's moisture content change?  probably.

to the point of destruction?  no.

 

What else do you want to know?

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

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


I am not concerned about (post #207264, reply #15 of 33)

I am not concerned about small changes in the drywall moisture content.  The experiment offers a lot of water vapor to the cold side.  If the cold side accepts it, it will make a lot of water.  So what I want to know is if that available water vapor will move from the warm chamber to the cold chamber through the drywall, and if it will, what happens to the water vapor as it passes from the warm side to the cold side. 

The water vapor will have an impetus to move from the warm side to the cold side because of the much lower vapor pressure in the cold side air. Heat will also naturally transfer in that direction. The only thing standing in the way is the drywall.

 

Here are some things I want to know:

 

Would vapor move through the drywall due to diffusion, and pass from the warm side to the cool side?

If it does, does it ever condense when it reaches drywall that is below the dewpoint of the warm side air?

If it does condense, where does it move to from there and in what form? 

If it does not condense, where does it move to?

Can the warm air diffuse its water vapor through the drywall without condensing even though the drywall is below the dewpoint temperature of the warm air?

Would vapor move through the (post #207264, reply #18 of 33)

Would vapor move through the drywall due to diffusion, and pass from the warm side to the cool side?

Yes.  I believe the movement will be from higher RH to lower, and the cold side is a slightly lower RH.

If it does, does it ever condense when it reaches drywall that is below the dewpoint of the warm side air?

Not based on the computations I tried.  But there are a lot of variables.  (Note:  See the graph.  When the temperature drops below the dewpoint in the two graph lines you get condensation.)

If it does condense, where does it move to from there and in what form? 

Moisture always moves from areas of higher RH to lower.  Condensation is 100% RH.  (However, the absorption ability of the gypsum upsets the equations somewhat.)

If it does not condense, where does it move to?

If it does not condense it either moves out or stays wher it is.  Depends on the "drive".

Can the warm air diffuse its water vapor through the drywall without condensing even though the drywall is below the dewpoint temperature of the warm air?

Yes.  Since the temperature and humidity both follow "profiles".


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

DanH, I understand what you (post #207264, reply #19 of 33)

DanH,

I understand what you are saying about air moving around in response to heating of the chambers, etc.  The objective of the test would be to smooth out those variables.  Maybe the interior temperature of the two chambers is controlled by heating all surfaces. 

If insulation is used on the hot side of the drywall to similate wall insulation, then yes, I can see that the air on that side would not be monolithic in temperature or RH.  But on the exterior side of that insulation, the air starts out at 100 degrees F and an 80 degree dew point.

I understand your point about the lines crossing, but in this case, they lines would definitely cross.  I see that they are not crossing in the graph you posted, but with this experiment, I expect all of the drywall mass to be well below the dew point temperature.  Assuming that is the case, would the hot moist air condense moisture onto the surface of the drywall? 

The drywall is one of your (post #207264, reply #20 of 33)

The drywall is one of your "surfaces".  I don't see how you can heat that.

If you add insulation that would change the graph, of course.  But absent such insulation the graph is as accurate as anything one can come up with.


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 drywall would not be (post #207264, reply #21 of 33)

The drywall would not be heated or cooled except for its exposure to the air chambers. 

I don't understand where the graph came from.  But I think I see what you are saying.  If there were insulation on the hot side of the drywall, the air temperature would fall off as it approached the cooler drywall.  As that air temperature drops, it will have to condense vapor as soon as it drops to 80 degrees.  Yet I am told that it will not condense on fiberglass insulation even if the air at that point is below the dewpoint. 

I don't see any way to conclude that the two lines will not cross.  You have the temperature line starting at 100, and then passing through the insulation, while dropping to 65 once it reaches at the drywall. 

You have the dewpoint line starting out at 80, and dropping as it approaches the drywall.  Why would it not be possible for those lines to cross? 

Gypsum board is a mediocre (post #207264, reply #22 of 33)

Gypsum board is a mediocre insulator but also passes humidity fairly easily.  With a homogenous cross-section, the two graphs will tend to vary in sync.  In fact, gypsum products are probably very popular as building materials because of this attribute -- condensation is relatively unlikely in most configurations.

(Keep in mind that condensation does not occur when the air temp drops below 80 because the dewpoint is also dropping as you approach the wall.  It's not that condensation can't possibly occur, but rather it simply doesn't under stated conditions, close as I can determine.)

A piece of waterproof plastic with the same insulating ability would probably condense badly in your test conditions, since very little moisture would pass through.  And condensation definitely CAN occur in fiberglass insulation, if the temperature drops to the dewpoint at some point inside the fiberglass.

Ultimately, though, it's all in the physics and math.


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

Question (post #207264, reply #25 of 33)

DanH wrote:
(Keep in mind that condensation does not occur when the air temp drops below 80 because the dewpoint is also dropping as you approach the wall.  It's not that condensation can't possibly occur, but rather it simply doesn't under stated conditions, close as I can determine.)

A piece of waterproof plastic with the same insulating ability would probably condense badly in your test conditions, since very little moisture would pass through.  And condensation definitely CAN occur in fiberglass insulation, if the temperature drops to the dewpoint at some point inside the fiberglass.

If the dew point is dropping in approach to the drywall; and if therefore, the dewpoint temperature will always be lower than the drywall temperature; then why would moisture condense on a piece of waterproof plastic if it were substituted for the drywall? 

If the dew point is dropping (post #207264, reply #27 of 33)

If the dew point is dropping in approach to the drywall; and if therefore, the dewpoint temperature will always be lower than the drywall temperature; then why would moisture condense on a piece of waterproof plastic if it were substituted for the drywall?

Because moistrure isn't diffusing through the plastic.  (At least not at a measureable rate.)


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

I understand that the vapor (post #207264, reply #31 of 33)

I understand that the vapor cannot diffuse through the impermeable plastic, so I would expect it to just stay on the one side of the plastic.  I would not expect the vapor to convert to moisture on the plastic surface just because it cannot permeate the material.  

The only way it can convert to moisture is by condensing on that plastic surface, and the only way it can condense is for the plastic surface to be below the dewpoint temperature of the vapor.   Since the plastic temperature will be above the dewpoint, no wetting should occur.   

Yep, after looking at the (post #207264, reply #32 of 33)

Yep, after looking at the specific numbers, with your conditions you likely would not get condensation on the plastic.  The average of the two temps is 82.5, so the plastic surface could never drop below that on the warm side (unless, eg, airflow was different in the two chambers). 

In this situation the only way to get condensation is probably to have a non-homogeneous wall, with a good insulator/poor vapor barrier on the warm side, and a less-good insulator/better vapor barrier on the cold side.  Eg, the fiberglass/drywall combo.


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

for the most part, nothing will happen (post #207264, reply #23 of 33)

"The air in one chamber will be 100 degrees F and 80 degree F dew point.  The air in the other chamber will be 65 degrees F and dew point of say 40 degrees F.  These air temperatures and dew points will be maintained during the time of the experiment which will last for 30 days."

Since you are magically maintaining a steady state environment on each side of the box where the skin temperature of the drywall is exactly the same as the atmospheric temperature of the respective side of the box, and where the atmopheric and drywall skin temperatures are always higher than the dew point temperatures...no changes will occur either to the drywall surface or within the atmosphere on either side of the box.

You've described your experiment as having steady state atmospheric conditions and the drywall skin mimicking its respective atmosphere's condition. So how can there be any change within a steady state?

The only potential change will be what Calvin alluded to when he asked about the initial condition of the drywall. The drywall will eventually develop a temperature gradient through its core from 100 to 65 degrees, and a moisture vapor gradient based on the atmospheric vapor levels on each side of the box.

Regardless, if you're wondering if there will ever be any condensation, there won't be. Your imposed conditions dictate that the temperature curve will always be higher than the dew point curve.


There are 10 kinds of people in this world; those who understand binary and those who do not.


Clarification (post #207264, reply #24 of 33)

Mongo wrote:

"The air in one chamber will be 100 degrees F and 80 degree F dew point.  The air in the other chamber will be 65 degrees F and dew point of say 40 degrees F.  These air temperatures and dew points will be maintained during the time of the experiment which will last for 30 days."

Since you are magically maintaining a steady state environment on each side of the box where the skin temperature of the drywall is exactly the same as the atmospheric temperature of the respective side of the box, and where the atmopheric and drywall skin temperatures are always higher than the dew point temperatures...no changes will occur either to the drywall surface or within the atmosphere on either side of the box.

You've described your experiment as having steady state atmospheric conditions and the drywall skin mimicking its respective atmosphere's condition. So how can there be any change within a steady state?

The only potential change will be what Calvin alluded to when he asked about the initial condition of the drywall. The drywall will eventually develop a temperature gradient through its core from 100 to 65 degrees, and a moisture vapor gradient based on the atmospheric vapor levels on each side of the box.

Regardless, if you're wondering if there will ever be any condensation, there won't be. Your imposed conditions dictate that the temperature curve will always be higher than the dew point curve.

 

Well it is a steady state on each side of the drywall, but the change I would anticipate is the passage of water vapor through the drywall by diffusion from the higher vapor pressure on the warm side to the lower vapor pressure on the cold side.  And in the course of that diffusion, the water vapor would encounter a temperature below its dew point. 

I have added insulation to this test model on the warm side of the drywall.  That would mean that the warm side of the drywall would be below the 100 degree ambient outside of the insulation.  Perhaps then as the temperature drops approaching the warm side of the drywall, the dewpoint falls off and remains below the warm side temperature of the sheetrock.  So, in that case, the lines do not cross.  But what happened to all the moisture that had to condense out of the 100 degree air as it became pregressivley colder approaching the drywall? 

And in the course of that (post #207264, reply #26 of 33)

And in the course of that diffusion, the water vapor would encounter a temperature below its dew point.

Why?  The temperature is also "diffusing" at a similar rate.


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

I have added insulation to (post #207264, reply #28 of 33)

I have added insulation to this test model on the warm side of the drywall.  That would mean that the warm side of the drywall would be below the 100 degree ambient outside of the insulation.  Perhaps then as the temperature drops approaching the warm side of the drywall, the dewpoint falls off and remains below the warm side temperature of the sheetrock.  So, in that case, the lines do not cross.

Much the opposite.  Moisture diffuses quiite easily through unfaced fiberglass, so the dewpoint on the side nearest the drywall will be relatively high, while the temperature will be low, due to the insulating properties of the fiberglass.

But what happened to all the moisture that had to condense out of the 100 degree air as it became pregressivley colder approaching the drywall?

The decreasing dewpoint closer to the drywall (in the original model) isn't due to moisture "condensing out", but rather it's due to diffusion through the drywall.  If you add the fiberglass then condensation becomes more likely, and, if it occurs, it will occur in the fiberglass or on the surface of the drywall.  (Probably not inside the drywall, since the dewpoint gradient will be downward.)

And there is another factor that is probably simply adds to the confusion at this point:  Gypsum is hydroscopic.  This means that it can absorb more moisture than air at the same temperature, without becoming "saturated".  So once moisture is inside the drywall condensation is even less likely to 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