Low Slope roof calculation errors

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Michael Hurd
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Low Slope roof calculation errors

Post by Michael Hurd » Tue Sep 06, 2016 11:44 am -1100

I have come across the situation on multiple occasions where I will run low-slope roofing simulations to determine whether or not a vapor retarder is required in the assembly. far to frequently the results indicate what appears to be unacceptable results whether a vapor retarder is included in the assembly or not. On one hand I'll get condensation between the insulation and the concrete deck, on the other, an ASHRAE 160 violation at the cover board directly beneath the roof membrane. The choice you seemingly are left with is which assembly isn't quite as bad as the other. I'm beginning to wonder if there is something different with WUFI as to how it models roofs versus wall assemblies. Or perhaps there is some key bit of information or a setting for roofs that I am missing or have forgotten from the classes. Have other users encountered this?

One specific question, If you have a camera location that shows condensation, but there is seemingly no real moisture build up in the material, is condensation really occurring? Is there perhaps so little moisture in the material that condensation is occurring but it really can't harm anything?
Respectfully,

Michael Hurd

Manfred Kehrer
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Re: Low Slope roof calculation errors

Post by Manfred Kehrer » Tue Sep 13, 2016 3:03 am -1100

Hello Michael,
long time not heard from you. As you see I am now with my own company.

I don't see any reason why WUFI should deal with roofs differently than wall assemblies, all based on physics and it is a matter of what to feed in. Also ASHRAE 160 evaluation criteria did change. We voted on that at the last ASHRAE meeting. We got rid of the running average evaluation and now have a criteria based on mold growth index according to the Vitaanen model.

To your specific question: I think you need to get rid of the "condensation thinking". small amounts of condensation happens already at about 80%RH and is called capillary condensation. I would recommend 3 approaches:
1) Model growth evaluation, based on temp and RH
2) wooden decay, based on water content in M.%
3) high moisture accumulation which leads to run off water.

Manfred
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI® :) .... It is easy and complex.

Michael Hurd
WUFI User
WUFI User
Posts: 96
Joined: Thu Oct 19, 2006 5:27 am -1100

Re: Low Slope roof calculation errors

Post by Michael Hurd » Tue Sep 13, 2016 3:15 am -1100

Thanks Manfred, I'll take that approach.

Any idea why WUFI 5.3 and WUFI 4 would provide different results for the exact same roof assembly? Consistently the outputs in WUFI 4 for the same assembly create better results than 5.3.

Is the ASHRAE change published yet? I heard that it was coming when I attended my last WUFI class.
Respectfully,

Michael Hurd

BEHROOA
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Re: Low Slope roof calculation errors

Post by BEHROOA » Wed Jun 05, 2019 6:03 am -1100

Hello Manfred,

I am having the exact same issue as Michael and want to take one of your suggestions to get a more accurate model. Can you please elaborate on what you mean when you say to perform:

"1) Model growth evaluation, based on temp and RH
2) wooden decay, based on water content in M.%
3) high moisture accumulation which leads to run off water."

Thanks,
Autusa

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