Perm database values

Everything concerning the adequate application of WUFI in the USA and Canada depending on local conditions e.g. architecture, building codes, standardization, laws et cetera.
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robinasu
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Perm database values

Post by robinasu » Tue Apr 19, 2016 2:04 pm -1100

Hello, great program-

Can you tell me why the Spun Bonded Poly (SBP) entry in the North American database has a perm-in value of 0.39? Based on typical manufacturer data this value is much greater than 5 perms. Tyvek Housewrap is about 50.

The same goes for the 0.1, 1, 5, and 10 perm entries.

Fiberglass, gypsum, fiber cement, and other perm values look reasonable.

What am I missing?

Thanks!

PS, can I edit the database to enter my own perm values?

Manfred Kehrer
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Re: Perm database values

Post by Manfred Kehrer » Tue Apr 19, 2016 8:45 pm -1100

Hi Robinasu,
you miss the difference between perm and perm*in.
0.392 perm*in at 0.008 perm results in 49 perm which is about what you thought.

If you have a hard time to understand it, it is similar to a thermal conductivity with units R and R per inch :wink:
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI® :) .... It is easy and complex.

robinasu
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Re: Perm database values

Post by robinasu » Wed Apr 20, 2016 7:13 am -1100

Ah, ok, thanks. Got it. In the database it says I need to use 1mm for the SBP options. Is this the case, or should I use 0.008 inches for the material thickness?

Manfred Kehrer
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Re: Perm database values

Post by Manfred Kehrer » Wed Apr 20, 2016 7:21 am -1100

The 1mm rule has been set up as a rough rule as too short thickness may result in too less calculation elements within the membrane. As long as your SPB has 3 elements you should be fine.
As I am now working with my own company Smart Solutions in the US I may find the time to take an extensive look at the database an correct those minor things for the next update.
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI® :) .... It is easy and complex.

robinasu
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Joined: Tue Apr 19, 2016 1:53 pm -1100

Re: Perm database values

Post by robinasu » Thu Apr 21, 2016 11:45 am -1100

Ah, ok, I have about 6 segments when I click on the layer, so that seems good to go. Thanks.

I've been coming up to speed pretty well, minus the perm-inch mix up. The tool has been pretty interesting. So far, I've been doing sensitivity testing on my condo wall assembly. I found some condensation there and I'm trying to figure out if the moisture is residual left behind from about 5 years ago, or if I have some air infiltration, etc. The stud cavities seemed pretty well sealed, though. There is no roof or flashing issue and the moisture I found has been condensing on the top plate of the stud cavities all along the length of the wall, which is about 80ft. So far, I've talked to about 4 consultants in the SF Bay Area and they all have different stories, so I decided to get educated before going down the hourly rate rabbit hole.

My assembly is: 10 perm exterior paint (verified), exterior fiber-cement board, SBP, plywood, woven plastic wrap (modeled with 10 perms), exterior gypsum board, 2x6 studs with loose FG, interior drywall, interior paint at 10 perms (3 layers of latex).

The plywood was pretty moist before the top of the wall was vented. Everything seems to be on a drying trend now. I'm hoping for some moist weather to run some additional experiments.

It's been pretty amazing to run the sensitivity analysis. Indoor humidity is a knob with a large effect. The adhering fraction of rain is another. Any advice on the rain fraction? The wall in question has perfect westerly sun exposure and gets a lot of sun and wind. The foggy weather comes in from the west in SF, as well, so this wall gets moist windy weather followed by sunshine quite a bit. I also know the fiber-cement board and SBP wrap was added about 5 years ago. The plywood was most likely the original siding and I was told paint was flaking off and so the fiber board was installed to "nuke" the problem. I also found rusty interior drywall screws at the bottom and top of the wall. The bottom of the wall shoes no evidence of excess moisture, except for the rusty screws. The North wall with the same construction seems fine.

Theory 1: moisture saturated the wall and the tail end of the moisture has been acting as fluid in a heat pipe in the stud cavities for quite a while. The drywall near the ceiling was water damaged and removed. Amazingly, there was very little mold. The Theory is after the initial bulk drying, the residual moisture would concentrate, then quickly disperse fast enough to limit growth. When I first moved in 5 years ago I do remember it being very stuffy in the summer. It's much better now and I never thought much about it because I sleep in a loft and the heat accumulated up there. Also, the drywall on the wall has some joint cracks, so there was some kind of moisture event.

Theory 2: there was/is high humidity in the building. The ventilation is poor and is in the process of an upgrade.

Theory 3: some kind of air infiltration. I've looked into this and I find it hard to believe. The way the bottom plates and floor are framed it seems very unlikely this could cause such uniform moisture across the entire width of the wall. A couple stud cavities had holes for electrical in the top plate and were dry, with wet cavities adjacent.

Theory 4: a little of all the above.

This is getting long. If you are interested in a tricky problem, let me know!

Great tool!

Manfred Kehrer
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Re: Perm database values

Post by Manfred Kehrer » Thu Apr 21, 2016 9:04 pm -1100

You should use a 1% driving rain moisture intrusion behind the cladding as ASHRAE Standard 160 does recommend that.
I think your theory 2 can be proofed wrong as in this case you would excpect more issues on a north oriented wall.
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI® :) .... It is easy and complex.

robinasu
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Joined: Tue Apr 19, 2016 1:53 pm -1100

Re: Perm database values

Post by robinasu » Fri Apr 22, 2016 9:32 am -1100

Good point about theory 2.

I added the driving rain and an air change in the fiberglass, then maxed out the initial RH, and internal humidity model (EN15026). It seems possible that if the wall was soaked before the fiber-cement board was installed, water could be in the fiberglass cavity for multiple years. This is a pretty extreme case. Changing the air change rate in the fiberglass layer didn't seem to change too much and I don't see any mosture % runaway when I crank the air changes - seems like the wall may have been very wet to begin with. The original paint on the original siding was really flaking off in the grainy picture I saw, so this wouldn't surprise me. I modeled without the fiber-board and left side moisture barrier and the wall would have dried in half the time and would have been $45k less. Not sure why they didn't just paint the original siding.

Here is an accelerated 8 year model of the worst case: https://youtu.be/vD6izMD9dRg

The RH in the layer left of the fiberglass stays pretty high, but the moisture level is low. Not sure about that.

All evidence points to residual moisture. This also seems to be the most simple possibility that best matches the evidence (rusty screws, consistent moisture along the entire width of the wall, no other issues in other parts of the building). I added a couple moisture sources from air infiltration and the wall always dried out. Unless I'm doing something stupid, the only theory that seems plausible (based on simulation) is the wall was totally saturated before the fiber-board and tyvec wrap was added. If that was the case, it's amazing mold did not infest the wall. That's the only wrinkle to this theory. I'm going to run some real world wall drying experiments to get another data point.

I'm thinking the WUFI model is most applicable at the point half way between the floor and ceiling. I would also think residual rising vapor would be trapped by the top plates and find it difficult to exit the wall once it cooled and re-distributed down into the the fiberglass and stud cavities. Upon reheating it would rise and encounter the top plates again, and repeat.

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