Plywood versus OSB for exterior walls?
I am planning an addition to my house which will have a 500ft basement with 2 floors above it. The exterior walls are 16 ft by 22 ft. The basement will be concrete, and will have brick cladding for the above grade portion (as an aside, the single layer of brick cladding has no gap but is directly applied with mortar to the concrete – what do youthink of that?). The first and second floor will have stucco cladding. Now, my architect has chosen this for the exterior: 3 coats portland cement plaster (stucco), metal lath, 2 layers of 60 minute grade D building paper, 1/2″ external grade plywood sheathing (specified no OSB) applied horizontally with 1/8″ minimum space between adjoining sheets, wood stud framing (2×4 and 2×6).
I was surprised to find plywood being specified, since I have seen multiple references on Building Science to plywood as a thing of the past (“sadly those days are gone forever” would be a typical reference to plywood sheathing). Am I right that this is a cost issue? It’s all about OSB and foam now is what I find. It seems my architect specified plywood due to the use of stucco “in accordance with the requirements of ASTM C 926 and recommendations of Portland Cement Association (PCA).”
So here is my question. I am on a tight budget, replacing plywood (about 900 sq ft of sheathing in total) with OSB and rigid foam combination might be a big cost saving, plus be giving me very desirable exterior insulation for the frame. (This would be either foam and OSB all around or perhaps 2 layers of foam changing to one inch of foam and one inch of OSB on the corners for shear strength, as I read on Building Science). I also read EIFS is coming back having solved the vapor and drainage issue. What would be your suggestion as to the best approach? Do the new foam/OSB/EIFS options have the durability of plywood? I’d obviously want the most economical way, so long as it is durable.
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Most people agree that plywood is superior....
It is surely better than OSB on the Wuzwood Scale
If you do choose OSB...
Be Sure to "Mind the Gap"
http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh/?topic=/new-documents
John,
Thanks. Yes, this "mid the gap" article says what I was referring to. Joseph Lstiburek writes:
"We no longer build as many of these things out of plywood as we used to because plywood is being replaced by alternatives. This is most evident in housing where plywood is pretty much gone. In housing we see two alternatives - oriented strand board (OSB) and foam boards."
Lstiburek then implies that the problem with sliding stucco is fixable with the air gap, as you point out:
"Turns out that to help out with the drying thing we need an air-gap between the cladding and housewrap/OSB interface. Especially with airtight claddings like stucco... The air-gap is simple, elegant and unbelievably effective in helping out drying. Back-vent your cladding and be happy."
So if the air gap is "unbelievably effective," then what is the plywood getting us? Would it be limited to structural shear strength? The article offers that OSB will be sufficient, whether in corners or whole house or with additional metal inset (I live in Baltimore so seismic and wind are not so great). All in all, this is Lstiburek's final take (I note he says that the gap "prevents" the sliding stucco problem):
"I don’t think that I am going out on a limb when I say that most builders are going to construct houses with wood framing and OSB sheathing at corners for shear strength. Everything will then be covered with foam sheathing – including the OSB sheathing at the building corners... There will be a few houses where entire walls will be sheathed with OSB – not just at the corners – and again these OSB sheathed walls will be covered with foam sheathing. And finally, there will be a few houses where the entire building will be sheathed with OSB, and in turn this OSB completely over-sheathed with foam sheathing... Providing an air-gap between the stucco cladding and the exterior face of the foam sheathing prevents the debacle we got with “face-sealed” exterior-insulated finish systems (a.k.a. “EIFS,” a.k.a. “synthetic stucco." With the air-gap we get drainage and the control of hydrostatic pressure. "
So he says we have "control" of the problems. "Relax and be happy."
If this were your house, would you go with OSB and rigid foam and "relax and be happy"?
Richard,
You're focusing on the wrong issue — plywood versus OSB. You should instead be asking your architect a much more important question: what details will be used to be sure there is a ventilated air gap between the stucco and the sheathing?
900 square feet does not sound like a lot of area....
I would guess the upgrade is $300 to $500?
just wild arse guessing on price
For $400 ... I would go plywood .....relax and be more happy.
Your Architect may be more happy with you also.
Plywood is more forgiving than OSB when it gets wet (it doesn't deform like OSB). Like John said, it doesn't seem like the extra (one-time) cost is that much more for what could be some cheap insurance.
I agree with Martin. Plywood versus OSB is not the issue.
We just need to do a better job at protecting the moisture sensitive materials in our homes.
Let's NOT all go out and start specifying plywood for our projects.
OSB is less taxing on forests!
OSB vs plywood IS a valid question. The simple truth is we don't need either. Rough-sawn board sheathing performs as well or better for most purposes and is more durable than either.
Even if OSB comes mostly from "waste" trees and carefully-managed forests (but do we know that?), if plywood is more durable (and it is), then it may be more environmentally responsible to use a forest product once for the life of the house than a less impactful forest product that requires replacement and major renovation during its life cycle.
I do not use OSB in anything I build. The price savings is simply not worth the loss of quality and durability.
The question of OSB or Ply under stucco is a good one. You may want to refer to the APA (The Engineered Wood Assoc) http://www.apawood.org and look for Q370F. It's info on stucco over wood sheathing (both ply and OSB). The APA is a great resourse for structural panels and they act as 3rd party inspection and grading for both the plywood and OSB insdustry. I do agree with everyon that the 1/8" spacing is essential to a good framing job and actually is a requirement for both ply and OSB. Running the structural panels vertically can eliminate the horizontal joints and using longer length OSB (9' or 10') and overlapping the band joist can dramatically increase the strength of the wall to reduce racking as well as capacity, stiffness and displacement. Above all enjoy your new addition.
Thanks to everyone who answered. It would seem that if plywood is really not that more expensive than OSB, plywood would be the way to go (though I'd have to look in on rough sawn board). I guess a 1,000 sq ft addition is considered small, and all the references to plywood alas being too expensive and thus a thing of the past are aimed at full large homes.
As regards the gap behind the stucco, the plan is as I outlined: 2 layers of building paper applied to the plywood. The gap is between the building papers. Is the space between the building papers equivalent to the 3/8 of an inch that is recommended as a gap?
I'd like also to confirm where the gap is if I use an external rigid foam. The "mind the gap" article says "the gap can go behind the foam sheathing, and refers me to photograph 9, which has the caption: "Stucco can be applied directly to the foam, or any other cladding." So the combination of cross-cut grooves and the wrinkly paper on OSB (or just one) is giving us the gap. But if I use 1" external rigid EPS foam on plywood, am I correct in interpreting that the cross-cut grooves would be suffiient, i.e.., stucco-direct on rigid foam, cross grooved back of foam direct to plywood?
Richard,
Like you I have been going through a build project and can share some insight. Here are some points to consider:
1). Get educated because it is your house
2). Pay a little more to do it right
3). Watch the labor as they may or may not understand why/what they are doing
4). Plan before you start - I did it on the fly and it became pretty stressful as time became an issue on things. Don't assume because it is made you can get it!
5). If there is a product for something such as window flashing units use it. Because if you assume the labor will know how/what they are doing you may be wrong
6). Get involved and hire a good builder. My builder has been super suportive in everything I recomended.
7). Read up on how the manufactures suggest installing a product and ask questions that the sub should know. Example Icynene recomends painting a vabor barrier for open crawlspaces in humid climates. Ask what the subs plans are if they don't know or ignore the manufactures recomendation move on.
Good Luck.
Robert
Richard,
I would never install stucco over plywood with just two layers of asphalt felt. That's not a ventilated air gap as far as I'm concerned. I've reported on too many failure stories — rot behind stucco stories — to imagine that crinkles between two layers of felt will save my house.
In the "Mind the Gap" article, Lstiburek wrote, "Turns out that to help out with the drying thing we need an air-gap between the cladding and housewrap/OSB interface. Especially with airtight claddings like stucco and manufactured stone veneer; and especially where the cavities are insulated with those new fancy spray insulations (like cellulose, or spray foam, or blown-in-batt). The air-gap is simple, elegant and unbelievably effective in helping out drying. Back-vent your cladding and be happy. How much of an air-gap? We know from experience that 3/8 of an inch is a pretty safe dimension with stucco, manufactured stone veneers, wood claddings or other claddings like fiber-cement that lie flat against the housewrap and OSB."
So If I were you, I'd use a three-dimensional plastic mesh product (Cedar Breather, etc.) like Lstiburek shows in the photos. But hey -- it's your house.
If you end up using OSB and two layers of felt with no air gap, be sure your contractor has really really good liability insurance.
Martin,
Thanks for the great advice. This is why I am asking these questions. I am telling you what my architect and a consultant structural engineer have put down, and am double checking with you.
The reason I get confused is when I read another article in Building Science on "stucco woes" that says that two layers is a good solution (http://www.buildingscience.com/documents/insights/bsi-029-stucco-woes-the-perfect-storm). In this article Lstiburek gives a picture of stucco-2 layers of paper-OSB. Am I missing something? Still, I would rather be conservative and go with the mesh to supply the gap.
Richard,
If you read further down in the article that you just linked to, you'll see that Lstiburek returns to the question of an air gap:
"One solution to this problem is also pretty straightforward—the use of a ventilated air gap between the stucco and the building paper/OSB sub-assembly. The air gap does not have to be particularly big to be a big deal—3/8 inch (9 mm). One method of getting the air gap is to use a drainage mat between two layers of building paper (Photograph 8). The gap allows redistribution of the moisture in both the stucco and the OSB sheathing. The gap also does something else—if it is wide enough it becomes a ventilated space making the “Goldilocks” vapor curve argument moot. Once we have a ventilated space (with meaningful air movement) the permeability of the traditional building papers and plastic building papers almost does not matter."
He seems to be saying that with plywood you can maybe get away with two layers of asphalt felt, but with OSB you really need the 3/8-inch or larger gap. At least that's how I read it.
If it were my house, I'd go with the true air gap with either OSB or plywood.
Richard,
I think you should make your decisions based on where you're building (correct if I am wrong Martin).
High moisture climate- Cedar breather as the air gap between stucco and OSB/plywood.
Low-Mid moisture climate- One layer of Tyvek Drainwrap. and one layer of building paper as your bond break between stucco and the Tyvek. This system will give you better drainage than the 2 layers of building paper.
I'm not sure why even for new homes plywood would be considered too expensive. Regardless of the scale of the project, the cost difference between plywood and OSB is still a small percentage of the total project cost.
I guess OSB is money in your pocket if you're the builder and plywood is cheap insurance if you're the homeowner.
A 1,000 SF house is considered luxurious in most parts of the world and for a the couple of decades of prosperity following WWII, a 1200 SF 3-bedroom home was the middle-class dream.
But with all due respect to Martin, two layers of grade D 60 minute building paper with a weep screed at the bottom is the standard accepted stucco method nationwide and specified by the IRC for stucco. "STUCCO UNDERLAYMENT: The weather resistive barrier shall include a weather-resistive vapor barrier permeable barrier with a performance at least equivalent to two layers of
grade D paper." (IRC703.6)
This is what Joe Lstiburek has been recommending. He's getting more conservative because everywhere he looks today he finds rotting OSB. So don't use the OSB, use the industry-accepted installation method, and you'll be fine. Heavy 2-ply kraft paper is preferable to single-ply. Paper-backed metal lath is common for the outer layer.
I would not substitute polymeric housewrap (such as Dupont Stucco-Wrap) for the second barrier as it's generally much too vapor permeable for a reservoir cladding like stucco. Grade D papers have a perm rate of 10-15, compared to the 25-60 for plastic WRBs.
Richard,
The brick cladding should have a drainage space behind it, and be attached with brick ties. The foundation should be waterproofed before cladding. There should be weep holes in the first course at the brick ledge.
Another application on 1/2 " cdx: I'm up in the Pac NW on a wooded lot and have been advised to use 15 lb felt paper (tried and true) on the cdx and then install the cedar shingles to the upper exterior, and cedar planks to the lower exterior (w/ no added rainscreen measures). Wrapshield by vaproshield seems a better alternative than the felt: air/rain barrier but with a 50 perm for drying to the exterior. Wrapshield (taped at the seams) OR the 15 lb paper?
Frank,
The high-perm membranes are appropriate with a rainscreen system to prevent inward vapor drive from wet cladding. Without a rainscreen, a lower-perm membrane such as #15 felt allows drying to the exterior while somewhat restricting inward solar vapor drive.
Too much perm is not necessarily a good thing, unless there is another way for the moisture to escape. 5 perm is sufficient for exterior surfaces (5 times the 1 perm code requirement for interiors).
Richard: If it freezes where you are you must have a gap between the brick and the foundation behind it. Brick does virtually nothing to keep moisture out, and liquid water in little pockets will freeze and expand causing all kinds of problems.
Will you see problems the first year, the fifth, or the tenth? Who knows. The one thing you can count on is that water will win: that's why they call it the universal solvent. If you are as concerned as all of us should be about building durability, you need to guard relentlessly against threats from water in every single instance.
Since everybody else is citing Dr Joe, I might as well jump on the bandwagon. He says over and over again that every siding material except vinyl performs better with a rain screen installation. The 3/8" gap includes a little something extra . . . what the folks in New Orleans call "lagniappe"--to make it impossible for water to move laterally or upward by capillarity.
Finally, when you specify your plywood ask for douglas fir . . . it is the most dimensionally stable species for this application.
Ed, Thanks so much for your post. It's good to see that at least some things are universally accepted in the "building science" community. Thank you for the douglas fir recommendation. i had not heard of that before.
If you're referring to the rainscreen, its certainly NOT universally advised in the building science community.
The HUD PATH (Partnership for Advancing Technology in Housing) tract on Moisture Resistant Homes - Best Practices Guide recommends a rainscreen approach only for the most severe exposures, based on climate severity, overhang ratio, and site exposure. For most climates and most building sites, a drainscreen or a concealed barrier (WRB) approach is more than adequate.
A rainscreen has some hidden liabilities, or unintended consequences, as well. These include moisture introduction into soffit vents and fire channels under the cladding that can lead a ground fire directly into the roof.
Richard, the best thing you've done is ask your questions! Keep it up..ask the installer and the builder if they have liability insurance coverage for stucco hard coat or EFIS and ask if your homeowners will cover it too if something goes wrong. Then double check the work. When we do a job with this finish I enlist the manufacturers representative and an outside consultant to doube check the design and field installation. You will have lots of support to show it was done right when the time comes to sell! The suggestions for an air gap made above have several benefits, just make sure there's a bug screen intact on top and bottom. You don't need bees in the wall! Good Luck.
I've seen a few references to the contractor having good liability insurance - this is a false sense of security. Most liability policies only cover contractor negligence during the construction process - once the house is complete - liability insurance does not do any good - even if something was installed improperly. Best bet is to get your contractor to purchase a third party warranty. Homeowner's insurance typically only covers catastrophic leaks, like a one time roofing leak or a water pipe breaking - a slow leak over time like a drip in a water line or a small leak in a poorly flashed wall assembly will not be covered either. If there is rot, there is typically mold and most policies exempt mold as well - so basically all of the insuarnce policies will not protect you at all - better get it right the first time.
According to the APA - plywood and OSB are interchangeable. Seems to me more of a personal preference - some of the old time architects and builders think plywood is superior mainly due to the original particleboard days - they are not very informed on the new products. I am constantly amazed at how the upper end, seasoned architects in town are clueless when it comes to green building, energy efficiency, etc. They have been stuck doing the same thing all their life and do not have a very open mind to new ways of doing things. As long as you have a proper drainage plane and proper flashing details - both will perform fine. I made the switch from plywood to OSB about 15 years ago and have had zero problems (Although I still use a sheet of pressure treated plywood as my first course of sheathing). From an installation standpont - I like OSB much better. If plywood gets a little wet and then gets a little sunlight - it warps like crazy. OSB will remain pretty stable unless it gets a lot of moisture and even then it will just swell slightly. Due to this swelling, I will not use OSB for floor sheathing - stick with a product like Advantech - but it is fine for walls and roof sheathing. Most of the wood made these days is more new growth wood - they do not let the trees grow as long as they used to. This is true for plywood as well - the veneers they use are not as good as they used to be - at one time plywood may have been superior but as OSB technology and adhesives have improved, plywood has slowly been losing some of its superior qualities bringing the two closer together. I've met several "old time" builders who have actually made the switch and say they have less nail pops in their walls due to the fact that the OSB does not warp the way that plywood does.
Danny,
If you would simply state your preference (or bias) rather than attack those whose experience, knowledge and study have proved otherwise, then you would not open yourself to a counter-attack.
If some one as knowledgeable, experienced, and informed as Joe Lstiburek of Building Science Corp believes that OSB is an inferior product to plywood, then perhaps it's not simply "old-timers" disease which leads other experienced designers and builders to prefer plywood.
With increasing processing of wood products, the vulnerability to moisture, mold and decay organisms increases. Manufacturer's can try to compensate to some extent with "improved" resins, but the additional chemical constituents carry their own liabilities.
If you prefer processed wood to veneer wood materials, that's your choice. But don't confuse your own bias for misinformed judgements on the part of those who understand these issues far better than you.
I normally sit on the sidelines for these types of questions, but here is what 25 years of plywood and OSB claims have taught me; 1: OSB deteriorates faster in moist conditions. I have seen bad stucco jobs completely destroy the structural integrity of OSB in under 1 year, even with a stucco wrap used. 2: Plywood quality is not as uniform as OSB, and even in the absence of a lot of moisture, a bad run of plywood will delaminate. 3: Stucco is porous, and those that say a drainage plane/rainscreen is necessary are exactly right. Flashing details and termination points on stucco walls are of utmost importance. The reason your architect specified ply probably is related to OSB's utter and total failure when introduced to bulk moisture. So, avoid bulk moisture, not OSB products. One of the best products that I can think of for the application would be Huber's Zip system. Over that, you'll still need at least one more weather resistive barrier by code, and I always recommend a drainage plane/ rainscreen. In the absence of an OSB product with a well applied weather resistive barrier, a good quality ply would be my choice with two layers of felt . The cost difference for your addition, figuring your projection from your existing home is 16' and width of the addition is 22', means that you would need no more than about 38 pieces of 4x8 product. using 1/2" fir ply, zip, and OSB ans comparison, osb would cost around $450 today, Zip would add $100 more, fir ply $100 above that. Hardly a large cost, considering that your weather barriers and rainscreen products will likely cost more than the substrate difference by a multiple of 2. This is less about the substrate and more about controlling moisture, which is the real issue with stucco. The devil is in the details.
TC - double all your numbers - OSB just went up again today...I think I am switching to plywood.
Robert - certainly did not want to come across as "attacking" anyone as I enjoy this website and value everyone's opinions on it so I apoligize if it came across that way. When it came to the "clueness" remark I was actually referring to green building knowledge in general not based on their preference for OSB. I recently had an architect go behind my back and tell a homeowner that I was using an inferior product (OSB) and that he was embarrased to have it on his jobsite and I was a bad builder for using it. This same architect has a vapor barrier drawn on the inside of the wall which everyone in NC has known not to do for 15 years. I should not have generalized the entire older population but I see the ignorance every day and am forced to defend myself due to their lack of knowledge when I am the one studying, reading and taking classes to learn more and build the best house I can.
My main point was simply when installed properly with a proper WRB and proper flashing details - they are equal products. The fact that plywood may perform better under a bad stucco job should really be indifferent in my opinion - kind of like saying that slab construction is better than crawl space construction because mold grows in crawl spaces when they get wet (I know, not the best analogy but best I can do right now). Judging a product for a condition that it should not be exposed to in the first place does not justify condeming the product and all that use it. Just for the record - I have no problem with plywood whatsoever. Thanks for the rebuttle.
In regards to permeability: VaproShield breaths in one direction. From the inside out. It does not allow moisture through it from the outside. It works like gore tex. It is a vast improvement over typical wraps.
Always use a properly installed rainscreen!
Steve,
Nothing breathes in one direction only. Neither housewraps, nor GoreTex. Both are designed to prevent liquid water diffusion in both directions and allow water vapor diffusion in both directions.
You get damp inside a GoreTex jacket in the rain because once the outer nylon shell gets saturated with water (after the initial water repellent wears off), it can no longer pass water vapor (from insensible perspiration).
Polymeric housewraps will allow vapor diffusion from the inside out in the winter and from the outside in when the pressure gradients reverse in the summer. They will keep liquid water outside and prevent condensed vapor on the inside from diffusing outward, trapping it in the sheathing.
If the sheathing is OSB, it will rot much more quickly than even the worst plywood. This is commonly found with bituthane adhered directly to OSB as window head flashing. The OSB rots away because the membrane doesn't allow drying to the outside.
Any membrane that's designed to keep water away from the sheathing will also keep water in the sheathing when (not if) it accumulates, either from the inside or from exterior leaks. For anyone to suggest that OSB and plywood perform equally or are interchangeable simple defies both science and experience. The each have their unique liabilities, but OSB is far more vulnerable to mold and rot.
Builder use OSB for the same reason they still mostly use fiberglass insulation - it's cheap. They are both, however, low quality products and both diminish the long-term livability and durability of a home.
There are a lot of opinion, and the right answer may be that ALL of them are right on certain levels and within certain points of reference. Some of the information is pertinant on a regional and climatic level. Mos people agree that OSB turns to mush if subjected to continual moisture, and gapping plywood should alleviate any warping problems that may be perceived. The reason that a double layer of paper is specified behind stucco, is for drying purposes. Stucco has a more difficult time drying-out when applied over a plywood substrate, because there is no effective back-area for drying as would be found over conventional stud bay open spacing. This condition has a tendency for creating more tendencies for cracking of teh stucco finish. By applying two layers of paper, the condition of cracking is alleviated due to the opportunity for a lottle bit of substrate slippage to absorb the movement that causes cracking. Regionally, as I alluded, there are differences in temperature changes, rain quantities, and accompanying dew point situations that may cause moinsture to turn-up where unwanted no matter how tightly the house may be sealed. The other problem, that I did not notice being mentioned is the toxicity of products that have become accustomed to be used in our homes, which is exacerrbated by the tight-house syndrome. OSB is made of a large percentage of glue, compared to plywood, and products like this, such as MDF is used in a lot of cabinetry as well. These products outgass fumes that may be harmful over the long haul to the occupants. These factors should be considered, along with the consequences that may have a further impact on even the financial considerrations.
Be very careful of using rough planks for stucco substrates...many building codes will not allow because there is no shear value...that's why plywood is used. This is equally imnportant for high wind areas as for seismicly active areas. Here in California, structural calculations in many areas for homes result in wind loads being a more critical factor than seismic evaluation, surprisingly enough.
Thanks to all who continue to post here. I keep learning. Overall, it seems that plywood gives me a sheathing that is a little more robust with moisture control, affords a little more shear stress and avoids the glue offgassing of OSB. Thank you Walt for making the comparison to rough planks.