Rain penetration in vented cavity- overlapping sources error

[Jay Keazer]

I am simulating a wall with brick exterior, air space, vapor barrier, exterior sheathing, steel stud with batt insulation and interior gyp board. I am attempting to follow the ASHRAE Std. 160 rain penetration requirement which states that 1% of the water reaching the exterior surface should be deposited on the water-resistive barrier or other layer if there is no barrier. The way I implemented this was to create a moisture source in the air layer at the last few elements on the right, closest to the vapor barrier. Because this is causing the RH in the air layer to be at or near 100% all of the time, I wanted to try adding an air change source to allow some drying out between rain events. When I add an air change source to the air layer and try to run, i get an error stating there are overlapping sources. My question is this: If two different sources cannot be specified within one material, what is the best way to simulate rain penetration in a ventilated cavity?

Thanks,
Jay

[Manfred Kehrer]

Dear Jay,

the ASHRAE/ANSI 160 says: "exterior side of the water-resistive barrier".

We figured out that this cause numerical problems sometimes. Therefore normally we deposit the rain penetration on the the water resistive barrier or on the OSB board (if exists).

You can also split your air layer into 2 layers. One for the rain penetration and one for the air exchange, but be aware of numerical problems.

[Jay Keazer]

Manfred,

Thanks for your response. Just to be clear, you are recommending that the moisture source be located in the actual membrane material rather than the air layer, correct? Does it matter if this is one or several elements?

Thanks,
Jay

[Manfred Kehrer]

Just to be clear, you are recommending that the moisture source be located in the actual membrane material rather than the air layer, correct?

Yes

Does it matter if this is one or several elements?

No.
Just use multiple elements if you run in numerical problems

[eordner]

When inputting a moisture source as a fraction of driving rain, for 1%, do you input "1" or "0.01". I'm a bit confused what the units are b/c it's a fraction of driving rain, which would be 0.01, but the text box reads "Fraction [%]," which leads me to believe I should input "1". Thanks in advance for your help!

[Manfred Kehrer]

To have 1% of the driving rain, just type in "1".