Moisture source not considered

[Benitoo]

Hello,

I’m a new worker on the humidity subject in walls, especially in rehabilitation, and I have an issue with the internal humidity source (internal convection).
Here is the situation: I am trying to estimate the risk of condensation in a wall surrounded with an internal insulation made of glass wool and an external insulation made of EPS. Specifically, to determine the minimum thickness of the EPS to avoid any condensation problems.
To represent the air infiltration, I have added an internal moisture source in the innermost side (5mm) of the masonry wall (first cold wall).
When I run the simulation, I don’t have any infiltration or moisture source (0 in the status of the last calculation)

I thought this was due to the low hygroscopicity of the material, so I added an air blade but the result hasn’t changed.
Furthermore, wherever I place this air blade in my composition, the results are the same.

I saw topics with a similar problem : viewtopic.php?f=13&t=436&sid=670af60dd1 ... 7561e753df
viewtopic.php?f=13&t=895&sid=670af60dd1 ... 7561e753df

Does anyone have any idea about this phenomenon ? Am I missing something ?

Thank you for your response ! Regards,

Benoît

[Thomas]

Hi Benoît,

did you make sure that either the stack height or the overpressure parameter of the air infiltration model is non-zero? If both are zero, the moisture source will be zero, too.

Kind regards,
Thomas

[Benitoo]

Hello Thomas,

Thanks for your answer.

Here are some precisions on my input :
- IBP air infiltration model
- q50 = 5 m3/(m2.h) (Classe C)
- free water saturation
- stack height = 5m
- overpressure parameter = 0 Pa

Could I have placed my source at a wrong place ?

Regards,

Benoît

[Thomas]

Hi Benoît,

maybe it is too warm at the location of the source? An air infiltration source does not automatically release a certain amount of moisture, it only releases moisture if the temperature at the location of the source is below the dew point temperature of the indoor air. (The model assumes that indoor air creeps through cracks or defects into the wall assembly and sheds some condensate if its temperature drops below its dew point.)

If the source location never drops below the indoor air dew point, or if the indoor air is so dry that it has a very low dew point, the air infiltration moisture source may never be activated.

You could test this by playing around with the outdoor or indoor temperatures (note the option to apply a positive or negative temperature shift to the outdoor temperature, using the "Details" button in the climate dialog), or with the indoor humidity.

Kind regards,
Thomas

[Benitoo]

Dear Thomas,

Thanks again for your quick reply.

I think that this misunderstanding of the air infiltration model explains my the results. However, I am still wondering how to represent this risk.

If I understand your explanation correctly, if the position of the moisture source on the model is at a point where the temperature is higher than the dew point of the indoor temperature, no condensation is considered and the air can flow out of the wall without affecting it.

This point where the condensation occurs changes at each time step. In my case, the risky points are at the interface between the internal insulation and the masonry wall and between the EIFS and the masonry wall.

I cannot have two sources because if the temperature in the wall before the first risk point is below the dew point, the amount of moisture is doubled, right ?

If I suppose that the condensation point moves within the thickness of the wall. The other option, which seems logical to me, is to create a moisture source in the entire masonry wall layer (option "whole layer").
I assume its operation as follows : The release of moisture is applied at a precise point depending on the temperature of the wall, which moves at each time step.
The other possible operation of this function would be to release moisture at any point on the wall where the temperature is below the dew point. In this case, my idea won’t work and I’m running out of ideas to consider the air infiltration in my wall.

Regards,

Benoît

[Thomas]

Hi Benoît,

I will check with the programmers about how the air infiltration source function has been implemented exactly, regarding the temperature evaluation and the moisture release within the source region.

Kind regards,
Thomas

[Thomas]

Hi Benoît,

the details of the air infiltration source are as follows:

For each time step, WUFI determines the lowest temperature occurring in the region covered by the source.

If this temperature is higher than the dew point temperature of the indoor air, nothing happens. If it is lower, condensation is supposed to occur, with the rate of condensation being determined by the formula given in the help file.

The condensed moisture is released uniformly within the source region.

It is possible to place two or more infiltration sources within the assembly; these work independently of each other.

.
Please keep in mind that the intention is not to simulate what the precise effect of given leaks would be (a one-dimensional program can not simulate a three-dimensional, possibly very complicated leakage path). The intention is to provide a plausible mechanism and amount of additional moisture which serves to "challenge" the drying potential of the assembly.

For application examples, please refer to the "Guideline for using the Air Infiltration Source in WUFI" in the download section of our website.

Kind regards,
Thomas