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WUFI ORNL Help: Permeability + other stupid novice questions

Anyone in this community skilled at milking out the limits of the WUFI ORNL version? I'm interested in modeling my service cavity style walls and finding it challenging.

First the obvious question: why not ask on wufi forums? Well, they require you to register on the site using a serial number and won't accept my WUFI-issued ORNL valid serial number. So I guess there are sites with more technical difficulties than GBA ;)

First, a basic question I think I have figured out but that might help future readers:
Could you confirm that permeability on WUFI is in perms per inch? For example, if you select Spun Bonded Polyolefin Membrane (the closest thing to Tyvek?) you will see a default thickness of 0.00787 inches and 0.392 permeability in [perm in]. If you divide the listed permeability by the thickness, you'll get a perm of ~50. Reasonable appropriate for Tyvek. Similarly, the Roxul Plus (the closest thing to Roxul ComfortBatt Plus in the ORNL version) a 5.5 inches batt has a perm-in of 117 or a perm of 21. I can't find a published perm number for Roxul but that seems reasonable.

However, things get a little fuzzier. My wall has a service cavity which is backed by a 15/32" plywood rather than a fancier smart vapor retarder (a mistake?). WUFI-ORNL lists the perm-in as 0.1195. Based on Joe's article, I would have expected dry perm of 0.5 instead of 0.1195/0.5 = ~0.24. Based on this article,, I'd expect a perm of 1.0. I also don't see a way to model the 'smart' vapor performance of plywood (perms rising upwards of 30x during high moisture times).

I've also wanted to model how the system might perform if I replaced the plywood (ugh but now is the time if I'm going to) with a smart vapor retarder like IntelloPlus. However, I can't see how to model such a variable permeability in the ORNL version. Has anyone found a material that works reasonably well as a substitute. It doesn't seem like I can create a new material in the ORNL version (I imagine 475 would help me with reasonable values).

Why the questions? So far the models show serious saturation with some silly results (1000% M-%).

Lest you think I just have the climate or software screwed up in general, I was able to model my older wall system (still present on much of the house) of stucco-1" XPS, 2" air gap, celotex, fb, pe, gypsum just fine. The model shows a bake to dryness over the limited two year period that the ORNL version allows.

Well, anyone have suggestions?

Asked by Keith H
Posted Fri, 01/31/2014 - 14:48


1 Answer

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We at can indeed help you along, with WUFI-ing assemblies. But using WUFI ORNL will give you a first idea if the wall is a really bad idea. It's functionality is limited to two years, so cannnot nodel long term behaviour and add new materials. However, you can see the "permeability (IP)/water vapor moisture diffusion resistance(SI), moisture dependent" on the right top of material screen (nr 4 on the list of hygrothermal functions). For a vapor variable material this will be a curve, not a straight line. To model ProClima INTELLO Plus in ORNL, you can use PA (nylon) as a reasonably close substitute material - it is pretty vapor variable, but in a less optimized way (open less at high humidity).

Regarding plywood - the perm/in is 0.119 - but at 60%RH it is about 1. So that is what you indeed describe - again look in the same chart in the material properties.

Not sure what is happening in your model. can have a look but would need more info (climate orientation, etc.) - please contact us via our website.

Answered by floris keverling buisman
Posted Sun, 02/02/2014 - 15:58
Edited Sun, 02/02/2014 - 15:59.

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