Image Credit: Energy Vanguard
Image Credit: Energy Vanguard Building Science Corporation's OSB and plywood permeance graph, from Joseph Lstiburek's article, "Mind the Gap, Eh?"
Image Credit: Building Science Corporation Chris Timsuk's OSB research looked at the variables shown in this table, which was taken from his doctoral dissertation.
Image Credit: Chris Timusk Professor Timusk's OSB research measured the permeability of OSB with sample cups like the one shown here.
Image Credit: Chris Timusk This table shows the main results of Professor Chris Timusk's research on the permeability of OSB. Higher density and more resin content leads to lower permeability. Moisture cycling leads to lower permeability.
Image Credit: Chris Timusk Wetting and drying cycles and their effect on permeability. The more a piece of OSB has gone through such cycles, whether with liquid water or water vapor, the more permeable it becomes.
Image Credit: Chris Timusk A vented rain screen, using furring strips in this case, can help wall assemblies dry out.
Image Credit: Energy Vanguard
More Building Science
Oriented strand board (OSB) gets blamed for a lot of problems that are really the fault of the designers and builders. Part of the problem, of course, is the perrenial confusion between correlation and causality. OSB hit the market as we really started getting serious about insulation and air sealing. As I described in the story of painters refusing to paint insulated homes, building assemblies that keep the exterior sheathing and cladding colder restrict its ability to dry. Is that the fault of the OSB? Or the insulation? Or the lack of a vapor barrier? Or something else altogether?
How is OSB different?
Oriented strand board is that flaky plywood you see on most residential job sites these days. (That’s a closeup of a piece in the photo above.) I toured an OSB plant several years ago and got to see how it’s made, from beginning to end — logs to 4’x8′ sheets. It’s pretty cool! To get a sheet that’s 1/2″ or 3/4″ thick, they start with a layer of chips (called strands) that’s about 6″ thick.
OSB replaces the previously dominant sheathing material, plywood. The properties of these two materials, however, are not the same. Joseph Lstiburek, PhD, PE, wrote about the importance of recognizing the differences in his article, Mind the Gap, Eh? The article included a permeance graph (see Image #2, below).
Joe’s recommendation: “The key seems to be an air gap between cladding and sheathing and an ability to redistribute moisture – typically facilitated by – yes, you guessed it – a gap.” Read his article for the full scoop on that; but in short, this is why we need vented rainscreens on homes sheathed with OSB.
Permeance and permeability
Permeance, as shown in the graph above, is a number that tells you how much water vapor will move through a particular piece of material. Permeability tells you how much water vapor will move through a particular type of material. Permeance depends on thickness; permeability doesn’t. (For the science geeks out there, permeance is an extrinsic property, permeability intrinsic.) Another way to think of them is that permeance is like mass, and permeability is like density.
The units of these two quantities aren’t very pretty, but I suppose you want to see them, don’t you? Here they are in imperial, or inch-pound (I-P) units:
[permeance] = grain/square-foot·hour·inch of mercury, which we normally shorten to perms
[permeability] = grain·inch/square-foot·hour·inch of mercury, which we shorten to perm-inches
Don’t say I didn’t warn you!
The quirkiness of OSB
The graph depicted in Image #2 (below) shows that the permeance of OSB is really low when the mean RH is low and that it doesn’t even reach 10 perms when the mean RH is 100%. That means it’s still a class III vapor retarder at best, whereas plywood, which gets up over 30 perms, becomes quite permeable as the air on one side gets more and more humid. So, plywood dries better than OSB, but how well do we really know OSB?
At Building Science Summer Camp this year, Chris Timusk, a building science professor at George Brown College, gave a presentation called The Quirkiness of OSB (pdf). He did his doctoral research on OSB and how well it deals with moisture. I’ve spent the past few days reading as much of his dissertation as I could, and I have to say, it’s a fascinating and well-written document. (Click to download pdf version.) Chapters 2-4 give the general background information on wood, OSB, and moisture and, I believe, are readable even by those without science degrees (although that would certainly help).
Timusk saw the existing data on OSB and realized that we really didn’t know much at all about how well it handles moisture. For example, if you look at a graph of OSB permeance like the one above, you need to know what type of OSB it was. OSB comes with a range of densities, resin type and content, and surface treatments.
So that was the first thing he wanted to look at. The table in Image #3 below (Table 5.1 in Timusk’s dissertation) shows the sample types that Timusk chose to study.
His research project looked at two important moisture properties of OSB: permeability and sorption. The former, as stated above, tells you how much moisture moves through a material under a given set of conditions. The latter tells you how much moisture the material can hold as you vary the relative humidity. In this article, I’m going to discuss only his permeability results and will come back to sorption in a future article.
The photo at the top of the page (Image #1) shows one of the samples he studied. The researchers manufactured the OSB themselves just for this experiment but did so in a commercial OSB plant, not in a lab. Then they cut it to various sizes and shapes and sealed the samples to the tops of cups, as shown in Image #4 below. They placed the cups in a chamber where they could control the temperature and humidity on the outside of the cup.
In each cup was either water, for the wet cup results, or a dessicant, for the dry cup results. They would periodically weigh the samples and cups to see how much moisture they gained or lost. That allowed them to calculate permeability.
Permeability results
Here’s a quick look at the results of all the permeability tests they ran. The graph in Image #5 below (Figure 7.11 in Timusk’s dissertation) shows that there’s quite a lot of variation from the least permeable to the most permeable sample. Here are the main takeaways:
- Higher density is correlated with lower permeability
- The OSB core is much more permeable than either the top or bottom surfaces.
- The more resin in the OSB, the lower the permeability.
- The more a sample has cycled through wetting and drying, the more permeable it is.
- There’s a huge difference (~6x) between the most and least permeable samples tested.
Now, let’s look at the most interesting part of this research.
Relative humidity cycling and permeability
In addition to the samples with various densities, resin contents, or surface treatments, Timusk looked at two other variables. One was the result of taking the samples through repeated cycles of soaking in water and then drying. The other was cycling the samples through exposure to high relative humidity (100%) and then low RH (42%).
About the latter, Timusk wrote in his dissertation, “The findings from the relative humidity cycled specimen are perhaps the most significant of all the tests conducted.” Why? Because repeated exposures to high relative humidity aren’t that uncommon.
Image #6 shows the results for both the liquid and vapor wetting tests.
As you can see, in both cases, the permeability of the OSB increased with repeated cycles of wetting. For soaking, it increases by nearly a factor of four. For exposure to humid air, it nearly doubles. Wow!
How should we build with OSB?
OK, so the main takeaway here is that we don’t really know how permeable the OSB we’re using is. If you’ve got a type that has higher density or more resin, your OSB may have a lower permeance than the material specifications say. If it has a lower density or has been soaked and dried or repeatedly exposed to high humidity, it probably has higher permeance than stated.
What we do know, however, is that it’s still not as permeable as plywood. That means you should assume it needs some help drying out, so always follow the two main rules of moisture management:
- Don’t let susceptible materials get wet.
- Allow for materials and assemblies to be able to dry out after they do get wet.
Flash the windows properly. Use overhangs. Install your cladding over a gap with a vented rain screen.
Remember, there are three things that can cause a building to fail faster than anything else: rain, moisture, and water. Respect the H20 molecule!
________________________________________________________________________
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.
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27 Comments
Avoid OSB
Avoid OSB
(would be my two word guest blog.
Martin, toss my name in the hat for a two word blog....)
Bruce Brownell=good
OSB=not so good
Very interesting subject
Since the 90's, most of the builders used the osb as an all around material, roofs, floors, joist or external and internal sheathing, why? Because it was cheaper than plywood. In most of application, OSB was a pretty good choice but , here in Quebec, it was a questionable choice. A lot of buildings built with OSB as external sheathing are now showing signs of failure and, as we can understand in Timusk research, OSB is not as bad as our experts are saying about.
It look that most of the building failure are cause by the ignorance of the two main rules of moisture management. Finally OSB is not the main culprit but bad assembly, poor rain and moisture management are.
Now, rules have changed, because new energy code is asking effective R value thermal bridge insulating material on new buildings, a lot of builders moved from OSB to air barrier laminated EPS and steel wind braces. There is still a lot of work to do on water and humidity management but it's a good beginning.
I have the chance to meet Both, Chris Timusk and his father at this year spring training camp, we sat at the same table for dinner and i remember how Chris seems to be worried about doing a WUFI presentation on some wall assemblies in front of a colored and a bit drunken audience, but i know that most of its fears were coming from the fact that Joe Lstiburek was sitting in front of him, at this time, Joe wasn't the most loyal fan of modeling, but i think he slightly evolved since on this sensible subject.
Finally everything went well and it was a very interesting presentation.
OSB is a class-III vapor retarder, not class-II
"..he permeance of OSB is really low when there's little RH difference from one side to the other and that it doesn't even reach 10 perms when the RH is 100% on one side and 0% on the other. That means it's still a class II vapor retarder at best..."
It is above 1-perm under most real-world conditions, which is class-III ass seen in figures 7.11 and 6.6
https://www.greenbuildingadvisor.com/sites/default/files/osb-research-chris-timusk-permeability-summary.png
https://www.greenbuildingadvisor.com/sites/default/files/osb-research-chris-timusk-cyclic-soak-rh-cycling-permeability.png
And that's good- in most US climate zones you don't WANT anything as tight as class-II vapor retardency on the exterior of a wall assembly, and class-III vapor retardency is just fine. Consider that #15 felt is also a class-III vapor retarder (which also varies with humidity), as is standard latex paint. Class-III vapor retarders are not exactly moisture traps- they're more like moisture speed-bumps, which can be a good thing.
The real issue with OSB isn't low vapor permeance relative to plywood, but rather it's higher concentration of wood-sugars relative to plywood, which makes it more susceptible to mold/rot than plywood (or plank.)
Response to Dana Dorsett
D'oh! I've fixed that little mistake. Of course it's a class III vapor retarder (1-10 perms). Not sure why I wrote class II. And yes, thinking of it as a speed bump for moisture transport is a good way to look at it. You've got to handle the moisture management details properly.
Comment to Dana Dorsett
Dana,
It is my understanding that the 'wood-sugars' differences between plywood and OSB are minimal and that, if anything, plywood would have a greater amount (due to less processing). The increased density, waxes, and resins found in OSB actually lend it a better degree of protection to mould growth than plywood under constant RH and T boundary conditions, due mainly to a difference in sorption isotherm and thus, water activity.
The reason for the disparity with field forensics is that these conditions do not typically persist for long- things dry out. Plywood can dry out much faster than OSB, and is thus subject to shorter durations of elevated humidity. Black and Straube did a series of experiments on full scale wood framed walls with OSB and plywood, with and without biocidal treatment. The plywood tends to grow mould first and to a greater rate than the OSB.
nice info mr Allison ...
En suite de ...
Why is OSB used instead of regular plywood ??
$$$$$ seems to be the only reason
But if you ask me, when you are trying to shave 100$ off a 225 000$ building by using lower grade materials ...
what is the difference in adhesive content of OSB VS ply ?
Anyone that has left some OSB and plywood under the rain for a few weeks up to a few years,
know which of the 2 he would prefer to use without hesitation.
$$$ is a significant reason for OSB versus plywood
This post is very timely for me as I have just started re-looking at using OSB (and similar products) for my roof and wall assemblies. Around Chicago, 1/2" 4x8 OSB is now about $12 per sheet versus $20 per sheet for 4 ply plywood. I build fairly simple 24'x40' 2 story homes with gable roofs and go through 124 sheets for both walls and the roof. $8 more per sheet x 124 sheets is almost $1,000, not $100 as Jin implied.
I have both read about and used Zip Wall from Huber and recall hearing and experiencing that it is actually more of a strand board than chip board and that it is denser and has a high resin content than commodity OSB. I'm not sure if the paper's author Chris can't or won't mention if zip wall in particular was tested but I am very curious how it performed against commodity OSB if it is in fact denser and/or has a higher resin content.
Zip wall won't be significantly less expensive than plywood for me but at least I get the benefit of less environmental impact since it can be manufactured from smaller trees and the benefit of a potential second air barrier if I decide to tape the seams in addition to using a separate WRB.
Response to Scott Sanders
The research project I wrote about here was done on OSB that Timusk had specially made in a commercial OSB plant. They specified the densities, resin contents, and surface treatments, so they didn't test Zip or any product you can buy from your supplier.
The advantages of OSB are more than just being able to use smaller trees. In his thesis, Timusk said they use small trees, fire-damaged trees, trees killed by insects, or scraps of wood from other uses. Also, if you haven't seen it, I wrote about some testing I did a few years ago showing that
Zip is a great air barrier against infiltration and exfiltration. House wrap works well against infiltration and not as well for exfiltration.
Scott Sanders
I am sorry i might have exaggerated my pricing ..
Up until recently , for 12-13mm 4ply we paid 13-14$ when purchased in bundles
and 9-10$ for OSB.
Now OSB is more like 12-13$ and 4 ply gone up to 16-17$.
Cheap houses are cheap houses.
I still consider alsphate shingles poor's man roofing method that is not very ecological at all
Using OSB under it is even a worse idea, but i understand the economic factor.
I still see people using the black condensed paper sheathings around here to save even a few bucks more ( lol ...should've taken a pic of a neighbor that built his own house ..it looked like a chekered flag going like : OSB|black sheathing|osb|BS etc... at least he saved himself at least 100$ on his nice house ..what a pitty )
One could use OSB but at least cover it up with some quality membrane,
or use a metal roofing product.
Again, we are talking about saving less than 1000$ on a 200 000$+ building.
At least one could propose client and up-cost to plywood.
And the argument about saving trees .....COMMON
wood industry chips and damaged wood parts all get recycled and used nowadays.
It's not like it is all going to waste.
A green building is one that stands through time.
Doing over a few times is not economically and ecologically smart.
Response to AJ
AJ,
You have twice commented, "Paid for ad for OSB."
I can't tell whether you think the comment is a clever quip (and therefore worth repeating) or whether you are genuinely cynical enough to think that GBA accepts money from manufacturers to influence our reporting.
If you actually think that GBA is corrupt, here's my answer: we're not.
If you think that the quip is clever enough to post twice, I respectfully disagree with your judgment.
Problem is straight forward Martin
Those of "us" that have been around thousands of sheets of OSB IN USE (not in tests) well many of us are willing to truthfully say out loud, that OSB is.... CRAP.
This blog does NOT mention anything about users running into bad OSB time and time again... yet plywood is just not being found in the field in a rotted crapped out state like OSB is.
This blogger is AVOIDING or not informed like those of us who actually USE the stuff.
Get my point now??
Would be nice if the PHD had some in field use comments included of the downsides of OSB that can be avoided by NOT buying the stuff instead of just saying strap it.
Response to aj builder
Echoing what Martin said, I'll jump in and reiterate that there's no corruption here and it's not a paid (or unpaid) ad for OSB. If you don't like OSB, that's fine. But a lot of it gets used and people need to know about the science behind it, which was the sole purpose of this article. I believe you'll see if you read it carefully that I didn't advocate for OSB or against plywood anywhere in the article. I was reporting on a presentation I saw about the moisture properties of OSB.
In full disclosure, Huber Engineered Woods does advertise in my blog (blog.energyvanguard.com), but they have not once asked me to write anything that will help promote their products or denigrate their competition.
I'll close with a quote that I meant to include in the article itself:
"Building materials do not fail. When building materials are assembled together and deleterious microclimates are created, it is the hand that has designed and assembled the materials that has failed."
~ Professor John Timusk (Chris's father)
"Paid for ad" for OSB
Well, looks like it is a "paid for ad" for OSB
This post has been slightly Editied for Martin... The reason for me posting this Ad remark twice is because the blogger did not mention the bad of OSB in actual use verses tested use and... has not commented yet that there are some bad points in real field use as to why to strap and I believe this blog misleads by saying the plywood is as bad or even worse then OSB when in actual use in the field many of us always find LESS problems with plywood and WAY more problems with OSB.
Even the "miracle" product Advantech OSB goes bad if wet for far longer than a few months which may happen if a building going up runs into a delay snag. Happened where I live and two years later the entire deck structure built with Advantech flooring was rotted, holes people breaking through the floor, ripped down and demoed. Off to the landfill with more "low cost" OSB.
OSB is not all good news, I tell it like I see it, not how I test a little square in a lab.
Media spin is spun positive for it's own needs
Here's a product I would like to find someone to post some positive spin about...
Buy a Lada
If you know anything about Lada's then you know my point. If not... pay no heed.
A free ad for OSB
You know, I am not so crazy about OSB either and value many of the better features of plywood. But put in the context of the building system, the issue is relative.
Nobody here is building with OSB anymore in a way that its disadvantages are a problem and in fact, most of the design advice here protects the sheathing effectively. On the other hand, plywood has one important down side that cannot be avoided and that is why I prefer OSB to ply: dimensional stability.
Plywood twists and turns, pulls nails and even screws, lifts and warps off of plates and separates at seams and butt joints. Ply can even delaminate if you glue it since the opposing forces can cause the layers to separate. Stability-wise, my experience is that ply will be more negatively affected by humidity than OSB That is a huge downside for an air sealing, infiltration-conscious aficionado.
OSB, if you keep it dry forever, will work better forever.
If you keep it dry
Wishful
If you keep it dry
Wishful thinking in the real world...
Here's reality.... Guy goes to store sees lowest price is 7/16" OSB great buys pits on trusses set 24" OC. Snow load, what the heck is that? Well it happens to be 90 pounds per sqft there. Nice. Oh and the roofer weights 320... Ah but no problem... Gotta just remember to step from truss to truss that's all... True story real life people and saving maximizing saving money with OSB.
OK we're not building multi hundred year homes here like the crazies in silly Europe. We're Mericsns after all, the exceptionals.
Prosoco?
What about applying a WRB like Prosoco onto the OSB? If you can prevent the OSB from absorbing water/moisture, it should have a lifelong service life.
Response to Peter L
Peter,
OSB wall sheathing can get wet from either direction. The exterior side of the sheathing can get wet if there is a flashing flaw or a WRB flaw. The interior side of the sheathing can get wet in winter if the wall cativies are filled with air-permeable insulation.
A liquid-applied WRB can protect the exterior side of the OSB from liquid water, but it won't prevent the OSB from getting damp in February due to moisture reaching the OSB from the interior side.
"What we do know, however, is
"What we do know, however, is that it's still not as permeable as plywood. That means you should assume it needs some help drying out, so always follow the two main rules of moisture management:
•Don't let susceptible materials get wet.
•Allow for materials and assemblies to be able to dry out after they do get wet.
Flash the windows properly. Use overhangs. Install your cladding over a gap with a vented rain screen.
Remember, there are three things that can cause a building to fail faster than anything else: rain, moisture, and water. Respect the H20 molecule!"
After being hit by a few rains during the construction stage, osb makes excellent land fill. 1/2'' O soggy board is blows up to 5/8" -3/4" on the edges. It would be interesting to see what the engineers have to say about 3'' oc edge nailing under this condition.
"•Don't let susceptible materials get wet." This may work in a perfect world. Wet happens. Contruction rain or covering osb with thick foam that slows down drying caused by a rain leak, could be a problem.
I agree, OSB doesn't make decisions so its the builders and designers mistake in specing susceptible materials.
My experiance shows Taped Zip sheathing handles contruction rain without the above issue.
Osb and plywood are not all made the same. I have a two sq ft piece of plywood covering a outflow pipe in my pond. Plywood is half out and half in the water. After over twenty years the plywood has not delaminated, warped or rotted. Interesting.
Alain Hamel
There are alot of activities in my region ( lanaudiere ) and i have yet to see a single new house in MRC d'autray that is built with external insulation.
( i believe i saw 1 built by the homeowner himself a few years ago, they used isolopan products with tuck tape )
100% of what i have seen in the last 2 years is still going for 100% interior insulation,
using OSB as exterior sheating covered by tyvek or similar products. ( locally )
J'imagine qu'ils rencontrent les normes en ajoutant de l'isolation a l'interieur comme plusieurs ont l'habitude de faire.
Vous etiez a la conference de mr Lang si je ne me trompe ?? :)
Anyone knows the % of wood vs binders used in OSB and plywood ?
couldn't find the info after 30min of lookup ...
Jin Kazama
Hi Jin
I know there's a lot of activities in Lanaudiere, i use to live there (Repentigny) and no, i wasn't at gunter conference. Since August 2012, there's a new addition to the 2008 QCC, the part eleven is about energy efficiency and it's mandatory to respect these minimal values. If you look closely, you will find that all the R values in it are the previous version of Novoclimat prerequisites except that there is no inspection or blower doors testing anymore.
Contractors can use to insulate the structure for thermal bridging by the outside, inside or both sides. Since there is absolutely no enforcement, most of the builder didn't change their construction techniques, this is the biggest outcome in this new regulation.
Most of the builders are insulating by the inside to cover the thermal bridges and continue to use OSB outside with not so good air barrier strategies, if you look closer there is poor water and humidity management around the openings, so without a good blower door test and low permeability material on both side of the structure and add to this mix the temperature drop inside the wall caused by the inside rigid insulation panel and you are good for some troubles.
One of the biggest flaw in the new regulation is the no enforcement situation but the other one is the lack of knowledge, why? Because when a builder was part of the Novoclimat program he had to follow a two days course and all off the houses he built were inspected, ventilation system were calibrated and the blower door test were done just before the installation of gypsum. The inspector or , you can call him the rater, was not only enforcing, he educate the workers about air flow and heat flow and their connection to humidity transfer.
The fact is that now, all the builders or self-builders have to follow these rules without any guidance or overseeing, all the condition are in place for the perfect storm.
This is unfortunate and extremely irresponsible.
For the binder i think it's a bit lower than the particle board who's around 25%.
A+
Is this a paid for ad for OSB?
It does not rot so often especially sandwhiched to rigid foam that this blog should warn that plywood is worth the upgrade cost?
Been around lots of rotten OSB. So far I can remember only one bad sheet of plywood and that was because they waited a decade too long to replace a half lap crap flat boathouse roof.
Sandwich some rigid like we all are supposed to do now over OSB. Add ten years. Walla, lots and lots of work for us in the trades. Or just don't go there to start with.
Rainscreening is mandatory if it rains where OSB is to be used. Plywood is worth the upgrade most of the time.
I would like to see a study as to Roxul breathable rigid over sheathing, building science maybe...
Martin, does such a study exist?
Anyone out there have a twenty year old home outsulated with Roxul rigid high perm insulation over sheathing, OSB, plywood?? With in wall batt insulation?
Joe L, Straube.... what say the building science pros?
Permeance and Permeability
I'd first like to say that I appreciate a lot of the articles on this website and respect the authors. I've learned a lot. Although I've asked questions in the forums, I don't believe I've commented on an article. One of the reasons being that the authors do a pretty good job at explaining things. I did see something in this article though that my scientific mind can't let go. I may be misunderstanding, but the author compares permeance and permeability to weight and density, respectively. I think this is a bad example because weight is a force, an object's mass multiplied by the acceleration the object is experiencing, and density is the ratio of an object's mass per unit volume, which is not the same relationship. The author could have used resistance and resistivity, but that might not help the layman.
Response to Trevor Bowden
Although there are different kinds of density—including weight density, which varies with location—you make an excellent point about the potential for confusion here. If I'd paid attention when I wrote, I would have used the term mass from the beginning. I've revised the article to substitute mass for weight.
I think my confusion began when I did a quick check on the words extrinsic and intrinsic. The Wikipedia page I looked at referred to weight vs. density, not mass vs. density. But when I checked just now, I noticed that that page also links to another page on intensive and extensive properties, which I guess are the terms I really should have used. They list several examples of such pairs, including heat capacity/specific heat capacity, enthalpy/specific enthalpy, and a partner of the example I used, volume/specific volume.
Thanks, Trevor! You should comment here more often.
response to Robert Lepage (in response #7)
In re: " Black and Straube did a series of experiments on full scale wood framed walls with OSB and plywood, with and without biocidal treatment. The plywood tends to grow mould first and to a greater rate than the OSB."
Curiously the skunk works at Waterloo also came up with the experimental result shown in Photograph 1 of this document, which shows a very different (and strongly opposite) result:
http://www.buildingscience.com/documents/insights/bsi-027-material-view-of-mold
(I'm not sure whether Straube or Black was part of that exercise.)
The link in the main article should be redirected to the pdf in BSC's website:
https://www.buildingscience.com/sites/default/files/01.05b_Timusk%20Thesis.pdf
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