Rain penetration

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Rain penetration

Post by WEI LI » Tue Aug 09, 2022 4:42 pm -1100

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
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Re: Rain penetration

Post by Christian Bludau » Fri Aug 12, 2022 12:40 am -1100

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:

Best regards

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