Standing Seam Metal Roofing

[Eric Penney]

I am modeling a roof structure which has a 24 galvanized standing seam metal roofing (seams at 18" on center) with a LEED Certified Kynar whtie finish (SRI 78). I cannot locate this material within the database am looking for a material database or something with equivalent properties - any suggestions?

In my training seminar it was suggested to treat a structural steel deck as paint with .7 permeance but this seems odd for an exterior material.

[Christian Bludau]

Hello Eric,
we have done some measurements about that a few years ago:
The equivalent values (related to the area) for the standing seams are between 25 to 75m sd-value (thats the equivalent air layer thickness), thats in perm 0,13 to 0,04. If the seams are sealed by eg a rubber tape the equivalent sd-value is about 300m, what is about 0,01 perm.
Regards,
Christian

[Eric Penney]

Thanks Christian.

Your response is helpful but a little over my head. How can I enter this into my model? Is there a material I should use? We are using an unsealed snap-lock standing seam so the permeability will be on the high end of the spectrum.

[Christian Bludau]

Hi Eric,
its easy to enter in WUFI. Set the value as a permanence at the exterior surface in the "Surface Transfer Coefficients" section. Doing that do not insert the metal roofing as layer in the assembly again. And make sure you switch off the rain (adhering fraction of rain = 0), as the rain is not affected by the permanence at the exterior side.
Christian

[Eric Penney]

That's what I needed. Thanks Christian.

[Andreas Hauser]

I have a question concerning equivalent sd-values for the metal roofing made by "KalZip" http://www.kalzip.com
25 to 75 meters sd-value is quite a range.
Did you test this roof system, too? What is the equivalent sd-value?

[Christian Bludau]

Dear Andreas,
this values are due to the different heights and tightness of the seams.
For the calculation you should use bracketing to see if there is an influence. The whole range is quite tight. So if your construction works with both values, its probably ok, if a special value is needed in this range for not reaching critical values, your construction probably have a to less drying potential and you should think about it again.
Christian