temp/dew point quick graph

[eordner]

The temperature/dew point quick graph is new to WUFI 4.0. How do I interpret this data? When the temperature line and dew point temperature line overlap, there is a potential for condensation? If this is the case, it seems that in every model I run, to the exterior side of the vapor barrier, there is a likelyhood for condensation.

Thank you in advance for your help!

[Manfred Kehrer]

The temperature/dew point quick graph is new to WUFI 4.0. How do I interpret this data? When the temperature line and dew point temperature line overlap, there is a potential for condensation?

Yes, if the dew point is higher than the temperature you have a RH of 100%

If this is the case, it seems that in every model I run, to the exterior side of the vapor barrier, there is a likelyhood for condensation.

What kind of construction do you examine?

(P.S.If you use WUFI® 4.0 please change your profile "used WUFI® version"

[Michael Hurd]

To follow up on your previous comments on the temp/humidity quick graph - when I select the humidity function in this quick graph and you have a condition where the temp and humidity lines cross does this indicate that condensation is occuring? When I switch to the dew point function of the same material and the lines don't cross, you had previously indicated that there will be no condensation - I guess I'm not fully understanding if while in the humidity function, if temp and humidity lines crossing indicates anything bad is happening in the material or if this is simply a normal occurrence

[Manfred Kehrer]

To follow up on your previous comments on the temp/humidity quick graph - when I select the humidity function in this quick graph and you have a condition where the temp and humidity lines cross does this indicate that condensation is occuring?

Yes, Temp, RH and dewpoint have exactly the following behavior:

1) Temperature below Dewpoint => RH = 100% and condensation
2) Temperature = Dewpoint => RH=100%
3) Temperature above Dewpoint => RH lower than 100%

Number 3) does not necessarily mean that everything is OK.
E.g. 90% RH in a wooden material can also lead to problems.

[Michael Hurd]

Manfred, forgive my lack of understanding on this issue, but if I am understanding your response it would seem that when the temp/humidity lines cross you would have a condition where condensation occurs - but when you flip the quick graph over to the dewpoint version the temp/dewpoint lines don't cross which would seem to indicate that condensation is not occuring. Put another way, do both versions of the graph indicate the occurence of condensation or is the dewpoint version the only one that truly shows whether this is happening? Thanks for you patience on this one by the way.

[Manfred Kehrer]

I think the Temp/dewpoint graph is a little better to estimate global condensation, because you see how much the temperature falls below the dewpoint.

To estimate capillary condensation (which occurs at about 80% depending on the material) or mould growth of wooden material the RH graph seem better to me.

[Michael Hurd]

Thanks Manfred!

[Michael Hurd]

Another follow up question for the wonderful WUFI team - I have run many different WUFI simulations over the past months and have never seen a condition on a surface where the dewpoint line and the humidity lines actually cross in the Temp/Humidity graph - if I understand the previous posts about this subject it leads me to believe none of my simulations would experience condensation on the surfaces I am looking at in these assemblies. So, I ran some far fetched simulations and all I ever got was the dewpoint and temp lines coming very close together, not actually crossing - my question is; if the dewpoint line and temp line are only separated by a degree or two is condensation still likely to occur or do the lines actually have to cross for condensation to occur?

[Thomas]

Dear Mr. Hurd,

in porous, hygroscopic materials such as virtually all of those in WUFI's material database, "condensation" is a somewhat fuzzy concept.

In air layers or very light-weight non-hygroscopic materials which consist mostly of air, condensation is a clear-cut event: if the relative humidity of the air is below 100%, you only have water vapor in the air and no liquid water. At the moment when 100% RH are reached, condensation begins and the material contains some amount of liquid water. The water may damage the material or degrade its thermal insulation properties, so condensation is to be avoided.

In hygroscopic materials, however, you always have a certain amount of liquid water. Up to about 70% RH or so, you have a growing amount of water molecules adsorbed at the pore walls (not exactly liquid, but certainly more liquid than the water vapor in the pore air). Above about 70% RH you have a growing amount of liquid water condensing, first in the smallest pores, then in the larger pores.

The amount of liquid water in the material is given by its moisture storage function. This function describes how the (liquid) water content depends on the RH. In all hygroscopic materials, there is a gradual increase of liquid water content until "free saturation" is reached at 100% RH. So you have some liquid water even far below 100%, and if your material is susceptible to moisture damage (mould growth, rot, frost damage...), you may have some damage already at say 80% RH, well below what one might call "condensation". In these cases, condensation is not a useful criterion for assessing the damage risk. Moisture content should be used instead.


And in hygroscopic materials, "condensation" (i.e. an RH of 100%) occurs very rarely anyway. In air layers, the RH depends on both the temperature and
the water vapor concentration. When the temperature drops, massive condensation may occur. Because of the low sorption capacity of the air (a few grams of water per cubic meter at most), a small change in water content may result in a large change in RH, too.

The moisture storage function of a hygroscopic material, on the other hand, is not temperature-dependent (in reality, there may be a slight dependence, but this is ignored in WUFI), so a drop in temperature will not cause condensation. Most materials also have a quite considerable sorption capacity, so that any modest change in water content does not affect the RH very much.

Even if the boundary conditions are such that moisture accumulates in the construction and RH would eventually reach 100%, an RH of 100% would necessarily imply that the water content reaches free saturation (as dictated by the moisture storage function), and all that water must first be transported towards the point of condensation. The RH will therefore rise only slowly, and the boundary conditions may have changed before RH=100% is reached. Any changes in moisture content are thus strongly damped by the material's sorption capacity. So if you observe an increase of the moisture content, you cannot know whether the current boundary conditions, if acting long enough, would eventually lead to an RH of 100% (condensation) or, say, only 80% (no condensation). Here again, what matters is the change in moisture content, not the question whether condensation conditions prevail or not.
(Another mechanism that dampens moisture accumulation is liquid transport which tries to dispel the moisture.)


Can there be bona fide condensation in the construction at all? Yes, it can. If the water content of a hygroscopic material has reached free saturation, the pore spaces are not yet completely filled with water; some air pockets usually remain unfilled. If such a material, filled to free saturation, is exposed to condensation conditions, water vapor will condense and fill the air pockets. The water content may then rise up to maximum saturation where the pore spaces are completely filled.

So we reserve the term 'condensation' for those situations where the moisture content of the material has exceeded free saturation and continues to increase. Since no liquid transport occurs any more at these high moisture contents, further accumulation of liquid water can only be caused by vapor transport and subsequent condensation.

But this is a rare condition, and those moisture contents are far beyond acceptable anyway. So what you should look for when assessing a construction is the behavior of the moisture content, not the possible occurrence of condensation conditions.


See also items 17 and 18 of the WUFI FAQ.

And it should be noted that currently WUFI treats air layers in the same way it treats hygroscopic materials, so one should not expect to see frequent condensation events there, either.

Kind regards,
Thomas

[Michael Hurd]

I think I understand your response, What I am after is whether or not condensation is occuring on the surface of my sheathing, I've already satisified myself that the moisture levels within the sheathing are acceptable but don't want water dripping down the back of my sheathing onto the steel studs and rusting them away - it would seem as if the temp/dewpoint graph would tell me this - does it?

Also, in some of our buildings we will stop the gyp board just above the ceiling so air in the building could travel fairly freely through the fiberglass batt insulation and get to the back of the sheathing without having to go through a material by diffusion - in this scenario it would seem that WUFI in fact would be giving a true indication as to whether or not condensation will occur on the back of the sheathing - am I correct?

I also have modeled many buildings with precast concrete wall panels, in colder climates and am always interested in whether or not condensation is occuring on the back of these panels, it won't hurt the panel obviously but could hurt other interior finishes - does WUFI affectively tell me if condensation is occuring on the surface of the panel or do I need to look at some other form of vapor drive/dewpoint analysis to determine this?