Moisture Control in Double Stud Wall
Hi Folks: I’m in the final design and modeling stage for a passive solar net-zero energy home and have a question on moisture control.
Wall system is traditional stucco over building paper and OSB sheathing (vapor semi-impermeable between 0.1 and 1 perm), offset double-stud 2×4 walls 24”OC framing with 5” gap, cavity filled with Spider blown fiberglass, then 5/8” gypsum covered with either latex paint or American Clay system (46 perm). Inside cavity seams at the sheathing will be sealed with closed-cell spray foam. Whole-wall R-value of approx R-47.
Location is Colorado in Climate Zone 5, 6353 heating degree days, and avg low for Dec, Jan Feb is 18 degrees F.
My concern is vapor and moisture control in the stud cavity. Using Joseph Lstiburek’s equations, the inside face of the structural sheathing could drop to 19 deg F in the winter, well below the 37 degree dew point (assuming an interior rel humidity of 30%), and lead to condensation in the cavity.
Q1: If I flash the whole inside surface of the structural sheathing with 1” spray foam, do I all but eliminate the risk of condensation in the wall?
Q2: I am trying to minimize (not eliminate) use of spray polyurethane foam until the off-gassing concerns are fleshed out, so what alternatives do I have? With the vapor semi-impermeable outer surface (stucco+OSB), if I keep the interior wall permeance at greater than 1 perm, does that allow for sufficient drying to the inside in the event of cavity moisture?
Thanks.
Jim Riggins
Colorado Springs
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Replies
Jim,
Stucco over building paper over OSB is very risky; there are many failures of this combination of materials. The colder the OSB, the riskier the wall becomes -- so your proposed wall needs to be reconsidered.
The first thing I would do is eliminate the OSB, which is the least durable sheathing option. Rigid foam sheathing is the logical product to use under stucco. Provide wall bracing with diagonal steel T-profile strapping.
In you insist on OSB, put at least 2 or 3 inches of rigid foam on the outside of the OSB, to help keep it warm.
If you really hate foam, then use plywood sheathing -- and be sure you have a good ventilated air gap between your plywood and your stucco, so the plywood can dry out.
Jim
I’m in a hot-humid climate but in the last 20 years I’ve built dozens of stucco homes. I’ve seen many builders here use OSB, but I have only used plywood. I’ve recently used the new Zip Wall product by Huber, which is OSB coated with a protective barrier. You might take a look at that system.
I believe 95% of the water intrusion problems with stucco are flashing related. Code requires 2 WRB behind stucco, grade D paper is recommended, but house wrap like Tyvek can count as one of the 2 required WRB’s. I typically use 3.
99.9% of homes here do not use foam sheathing, although I have used foam and I’m a proponent, based on Lstiburek's recommendation. I’ve only used ¾” foam, you have an awkward detail if you build on a slab and have 3-4” of sheathing/stucco/foam hanging of a slab. And you do have a weep screed Code requirement so that detail needs to be addressed. I would like to see someone show a weep screed detail with sheathing (½”), foam (2”), stucco (7/8”).
So our detail is plywood sheathing, 2-3 layers of grade D paper/Tyvek, then 3 coat stucco applied (with lathe) directly over the wrb. Martin mentioned a ventilated gap, but I’ve never seen that detail used. I’ve been told that as the scratch and brown coat dries on stucco, that it “pulls away” from the wall enough to create tiny capillary channels for water to drain down to the weep screeds.
In 20 years I’ve never had a major or even minor warranty issue with stucco. If applied correctly it is an amazingly good product, and I would not think you would have moisture or vapor in the wall cavity.
Another way to control moisture in stucco (stucco will crack) is to use elastomeric paint as a coating on the stucco. Elastomeric paint is an amazing in that it is flexible and stretchable and will fill cracks that develop. You can see several stucco homes I’ve built painted with elastomeric paint at this site:
http://picasaweb.google.com/edwards.allan1/NewHomesBuiltByAllan#slideshow/5344992258038124946
Allan
Actually this is a better link to see stucco homes I've built:
http://picasaweb.google.com/edwards.allan1/NewHomesBuiltByAllan#
Allan,
Colorado is not Houston.
You can not draw conclusions from what may have "worked" in the past in a Hot Climate.
John
I understand Colorado isn’t Houston, that’s why I mentioned I was in a hot humid climate. My question to you (or anyone), if stucco is applied properly, good flashing detail, proper WRB’s, coated with elastomeric paint, do we really need to worry about moisture in the wall cavity?
Jim,
I and many others have designed many homes in northern NM and Southern CO, CZ5, with no problem with the wall assembly as you describe, that’s the way we normally do too. NM as well in CS, CO is dry and no humidity. However, the devil is in the details, as in water management details.
Many years a go, Dr. Lstiburek, BSC, Mark LaLiberte and many others taught us that in NM the BEST WRB is 2-layers of 30# building paper, period. I do like to use 2 layers of 60 min. Jumbotex. Yes, Plywood is better than OSB; yes, rigid foam outside the OSB is better than not, but you have 12” of insulation in your wall, so I doubt at the OSB layer you would get condensation. As you said, your only problem there is water getting inside the wall, and with your system and good execusion, your method has a better way to dry in-n-out.
(Forgive them Yankees, for they not know better... ;-)))) poor fellers....)
Martin, Allan,
Thank you very much for your replys.
Martin- I'll definitely look at foil-faced poly-iso under the stucco. I need to check on local code requirements (this county uses 2003 IRC and IECC), but believe I'll still be required to have structural shear panels at the corners for wind loading in our high wind area. I don't know if local code considers the added strength of the double-wall, and would not require shear panels. If shear panels are required, it sounds like I need to go with plywood, not OSB under the stucco. Or even better, a thinner layer of rigid foam over the plywood to be flush with rest of the foam sheathing, making sure all seams are taped. Does that sound reasonable?
Thanks again for your quick and comprehensive response (as always).
Jim
Coating 3-coat stucco with elastomeric paint can be dangerous as it becomes a moisture membrane and it would not allow the stucco to dry out.
I do not agree with Martin on the foam sheathing since you need to attach the stucco mesh closer than 24" o.c.
Using American Clay or your walls is great, as it absorbs some moisture when the indoor RH is higher for a short time and it releases it when it gets dry again. Best product to coat a wall, that's why, is so common in the SW to use clays and plasters on the walls.
Armando: thanks for the 2 replies. In fact, I did get this wall technique, stucco-2 layer building paper-OSB from Dr Lstiburek's book "Builder's Guide to Cold Climates." He does state that either plywood or OSB can be used and I think the arguments above in favor of plywood are valid, even in my mostly dry climate.
The discussion so far above has been focused on moisture external to the sheathing. But if I could go back to my original question and ask you if, in your experience, I am creating a condensation risk on the inside face of the sheathing by going with this highly insulated 12+" thick double wall?
Thanks
Jim
Jim,
Why are you using stucco? It is a good exterior finish but it does tie you up a bit when it comes to wall options. My perferred double wall in a cold, dry climate is warm side poly, insulation of your choice in the double wall (blown cellulose being the greenest) 3/4" fiberboard sheathing, housewrap. a rainscreen, then lap siding.
If you are opposed to a poly air barrier, drop it in favor of ADA. You do not need foam sheating in my opinion with a double wall. If you go with stucco make sure you have the right underlayment and a vented space.
Jim,
The two layer building paper has a drainage plain in between them. Building paper also allows stucco to dry to the outside. In case your OSB or plywood sheathing gets wet, it can dry to the outside. Again, the devil is in the detail and execution. Make sure you have good moisture management details (i.e., like weep screeds, flashing, window & roof details, etc.) in your plans and they get done correctly.
I do not believe with a 2x4 double wall w/ 5” space (thermal bridging) and 12” of blown insulation you will have any condensation on the inside of the sheathing. I think you have a good wall assembly there.
Doug: Stucco choice was simply personal preference for aesthetics. An earlier incarnation of our wall plan was an R-48 2x6 wall using 4" exterior poly-iso and construction techniques similar to the REMOTE methods developed in Alaska. This version of the design would have used fiber cement siding on 1x furring, but our personal preference was always stucco. We switched to dbl wall after costing out both options with our builder, and went back to stucco at that time. This part of the country has a lot of very talented, experienced stucco contractors so we are comfortable of getting a qualtiy install.
Armando: thanks again. we do plan to include those details with the significant details I've included on air sealing.
Jim
J.Riggins, I'll add to the opinions that you shouldn't get condensation as such on the sheathing surface in the winter. Bear in mind that molecular diffusion of water vapor through a 1 perm vapor retarder layer is indeed a very slow process. If your inner wall is tight, with no air currents carrying moisture by leakage into the cavity, you're fine. Do the calculations, and you'll see that even over several months of bitter cold weather, the little water vapor that does diffuse through and doesn't get passed on through the sheathing to the outside air is safely
All good suggestions. What I have not seen discussed is the lack of moisture buffering capacity. One post referred to the clay plaster's properties in that regard. I would fill the cavity with dense-pack cellulose instead of fiberglass as you then have a vapor diffusion open, albeit slow, assembly that also has moisture buffering capacity to deal with the seasonal swings in moisture.
As of late we have encountered a few well insulated houses that have moisture issues despite using adequately sized and operating mechanical ventilation systems. I am not a fan of relying on a active system when you could use design to get the job done with a passive one.
I concur with Niko on the moisture-buffering advantage of cellulose, which will be far greater than a think layer of American Clay. The clay may offer some diurnal storage, but the cellulose will offer seasonal storage and release as well as active hygric redistribution to minimize any potential moisture damage to framing or sheathing.
As additional benefits, cellulose treated with borates is toxic to all common household insects, repels rodents, and offers a significantly higher level of fire-resistance and mold resistance than any form of fiberglass.
It may be dry in CO, but your indoor RH assumption of 30% may be unrealistic, depending on the size of the house, the number of occupants, lifestyle variables in terms of water use, and CFM volume of whole-house air exchange. 40% is more realistic, as well as healthier for human occupants.
Don't know much about your climate. Did you try to do a thermal-hydro analysis on WUFI 5? I probably already know this, it's a free software you can download from Oakridge National Lab, just google "Wufi". We are designing a net-zero-energy house/passive house here in the D.C. area and I found this software very illuminating, especially when you try to model a "bad wall". Remember to enter your wall assembly in the correct order though, I modeled a wall that I thought would be great with the Gypsum on the outside and it took me an hour to figure out why there was so much moisture in the house.
From my modeling experience those "diffusion open" walls with little or no vapor barrier that Europeans advocate so much really perform like a champ.
Check out ours here if you are interested:
http://www.facebook.com/profile.php?id=570665515&v=app_2309869772&ref=notif#!/pages/Burke-VA/Southern-Exposure-Homes/136492286371086?ref=share&__a=3&ajaxpipe=1
We are at an earlier stage of planning than yours and can certainly use some wisdom.
Roger
Superior Walls http://www.superiorwalls.com or AAC http://www.aacpa.org would be good products to consider if they are available in your area.
Superior precast insulated concrete steel-stud walls have very high embodied energy, require trucking from PA, are meant to set on a bed of compacted crushed stone, and are not particularly energy efficient - certainly no comparison to a double-wall frame.
AAC walls are definitely the most cost effective if using a stucco system. Also it it the safest bet by far as in regards to moisture concerns, as there are none if the material is monitored before finish installation. That coupled with the thermal mass always Make AAC the choice if you are looking for a stucco finish. Note: ( make certain to use only AAC compatible finish products from STO, Elite Cement or Sider Oxydro. http://tetra-con.com/aac/hebel-tm
I still don't understand how people do comparisons and pick spider spray over cellulose for a wall like this. Especially when moisture buffering is mentioned. Especially for an OSB sheathed wall that is far removed from interior heat.
-Rob
James, how did this wall assembly turn out? I am considering a similar approach.