Rain penetration

[WEI LI]

Dear experts,

For rain penetration, ASHRAE Standard 160 says the deposit site for the rainwater shall be the exterior surface of the water-resistive barrier.
In my target component, the outermost layer is the metal deck, and the layer next to it is a water-resistive barrier, followed by a PIR board insulation, gypsum board and another metal deck.
It is quite confusing where I should put the rain penetration source.
At the moment, I inserted a thin air layer (5mm) between the outermost metal deck and the water-resistive barrier and put the rain penetration source there.
Is that a rational choice?

Thanks in advance.

[Christian Bludau]

Dear Wei Li,

the rain source is thought to take in account not completely tight positions like window sills or similar. In your case I would assume, that the metal deck is rain tight and if it comes behind it for some reason it will run down the water-resistive barrier. So in my eyes no reason to use a rain source between this two layers as it nearly will effect the behavior of the rest of the assembly.

In the German standard DIN 4108-3 Annex D the same source like in ASHRAE 160 is recommended, but with the addition that you do not need to use the source if the first layer of the underlay construction is a water-resistive layer like a weathering barrier or some membrane.

Some more discussion about that in this topic:
viewtopic.php?f=32&t=1968&p=5808#p5817

Best regards
Christian

[saricioglu pelin]

Hello,
I want to ask about adding the moisture source. Now,I understand that the moisture source can be added if the wall section is near the window etc. So, if ı can add the moisture source under the 5mm of insulation layer (which is recommend in the guideline) is it right ? and also when i compare the 2015 and 2100 year analyses, can i add moisture source all of them ?
thank you.

[Christian Bludau]

Hello,
yes, the moisture source should be add at the position where the water can intrude the construction, the German standard recommends to spread it about 5mm. And yes, you can use that to compare analysis for the year 2015 and 2100 if both climate files contain rain.
Christian

[saricioglu pelin]

Thank you so much Christian. I do this.
Pelin.