Which OSB materials are airtight?
BrunoF
| Posted in General Questions on
Besides zip and Advantech sheathing, which other OSB products are reliably airtight?
thx!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
If you're worried about the airtightness of OSB, but don't want to use Zip or Advantech, I would just step up to CDX.
At least in NJ, the only non-commodity OSB for wall sheathing I frequently see is Zip. On a hunch, I would suspect that the high resin (i.e. long warranty) floor sheathing OSBs from LP and GP are also pretty air tight, but they're not something you'd be sheathing a wall with.
BrunoF,
How air-tight are you aiming for? One of the takeaways of this article appears to be that the problem is really only for projects trying to get below 1 ach50.
https://www.greenbuildingadvisor.com/article/is-osb-airtight
I've paid close attention to the discussions on airtight sheathing for the 10 years or so since the issue was discovered. All sheathing appears to be fairly airtight, but if you want to get below 2-3 ACH50, there is a chance that you could have trouble when using commodity OSB made in northern states. If you're in southern US states, or you are using Huber's Adventech or Zip sheathing, I don't think you have to worry. There are premium OSB brands other than Huber but I have not heard of any airtightness testing involving them.
Michael,
Unless the price difference makes it prohibitively expensive to use CDX, can you think of any reason to use OSB instead?
Malcolm, I like that OSB typically lays flat, while CDX seems to resemble Pringles chips more each year. CDX veneer quality varies; I've heard of issues with delamination, and especially about having to take extra steps at the seams when the face veneer is missing. I still prefer CDX in most cases but understand when builders would rather not work with it.
I have seen the delamination issue first hand, luckily with a non-critical application (the roof on the doghouse I built for my dog). One of the plys delaminated and "rippled" on some of the material, but not all. One of my subs has said he's seen that too, and it's more common with box store material.
You can do better to buy plywood from actual plywood dealers, who apparently area aware of this issue and try to source material that doesn't have this problem. The other thing to do is pay a little extra for plywood with more plys, so instead of 3 ply get the 4 or 5 ply version. More plys tends to make a better quality sheet of plywood in most cases.
Bill
FYI, OSB in my area is $13 / sheet, zip $32 / sheet and CDX is $40 / sheet so it looks like if I anything above OSB it might as well be zip.
I am targeting ATAP, as tight as possible but I am not building a passive house and don’t have an unlimited budget.
I may need to start another thread on what I could expect to achieve with various levels of tightness and would it would cost to get there, but the more I think about it, the more I am questioning the super tight house from an overall cost / value standpoint. It is clear that it will cost extra money to have a tight house, then it will cost even more money to have an ERV or other method of ventilation…neither of which would be necessary with a “normal” house. It is just starting to seem crazy to put a bunch of money into sealing up a house, just to open it back up with another device.
I’m in Central NC, zone 4a, so a mild climate that is hot and humid in the summer and cold and dry(ish) in the winter. Maybe I would be ok to just go use regular OSB with tape and sealant, using the drywall as a secondary barrier and run the range hood (which will have makeup air) if / when things get stuffy.
> I am targeting ATAP, as tight as possible but I am not building a passive house and don’t have an unlimited budget.
With all respect, "as tight as possible" is not a practical metric, because you are going to run into other walls (i.e. budget) before you get to the true "as possible". It's like walking into a car dealership and asking for a car that goes "as fast as possible".
To points made by others, these seem to be issues when you're getting down really low chasing ACH. In climate zone 4A, chasing air leaking through OSB is probably really far beyond the point of depreciating returns if the details are otherwise executed well.
OSB airtightness is not something I worry about. For many years we've achieved ≤1ACH50 on almost every single job. The key is all about including air-sealing details on plans, and have the williness to follow with good quality installation.
The greatest air leackage on all building envelope occours through unsealed holes, the bottom and top sill plates, around all openings, and interceptions and joints of two planes, as in walls to walls, walls to roofs and viceversa. That's why good installation guidelines and practices, tapes, seals, caulks and gaskets are your best friends.
Here's a simple article to explain differences in wall sheathings:
https://asiri-designs.com/resources-1/f/osb-vs-plywood-vs-zip-system-vs-solid-wood---which-is-best?blogcategory=Materials
Many will point to articles about OSB vs. Zip airtightness research done with an ASTM E2178 standard (@75 pa) by ABAA, Building Science Corporation, Huber, RDH Building Science and many other folks, however these tests are conducted in a controlled environment lab with winds equivalent to 173 mph, or hurricane category 5 winds. Any of the following ring the bell? Ivan, Katrina, Wilma, Maria and Ian!
FYI, the 2021 IECC requires air-tightness testing @50 pa!
So, make sure you understand the difference between an ASTM test and a real-life application.
Armando, you work primarily in the south, where the OSB airtightness issue is not known to be a problem. As I've mentioned before, my experience with leaky OSB was first-hand, on a real project--we routinely got to 0.40 ACH50 using a panelized system with standard details, but on one project we could not get below 1.0 ACH50. That's with doing everything else we could to plug any holes; based on results from other projects, I bet the house would have tested at 0.10 ACH50 if not for the OSB issue. It's a rare problem, and seems to only apply to northern states, and it won't matter for most projects, where getting to 3.0 ACH50 is considered a feat, if it's considered at all. But if it happens to you, you'll change your tune.
Michael, is the North vs South issue something to do with the wood used to make the chips in the OSB, or problems with processing? I wouldn't have expected so much in the way of regional variations. I also wonder if a simple step of priming the panels prior to installation would be sufficient to seal any small pinhole voids to make the panels more airtight.
Bill
Bill, from what I understand, it's likely the fiber species--northern OSB is usually made from Aspen, vs. SYP in the south, and apparently they behave differently. There is no airtightness requirement for structural sheathing so it's not a defect, just a material quality. Priming would probably help but you would go through a lot of paint. On the project I was involved with, we used a fluid-applied membrane on the interior.
Miguel - I stand by my advise to the OP, who lives in NC and is not looking to achieve PH standard. It is more important to air seal holes, joints, sill plates, openings and interceptions than worry about airtightness in OSB.
Based on my experience in 20+ years of working with high-performing homes, I'll be willing to bet that any project with good airsealing can achieve between 1-3ACH50 anyday, and that will be better than most, regardless of the OSB.
IMO, the idea that all projects to be good need to achieve ≤1ACH50 is unrealistic, and I think you agree as well.
Armando, thank you for your perspective especially as related to my region. I am going to have a heavy hand in these “belt and suspenders” efficiency improvements and your experience really helps.
If it is likely that I achieve 1-3 ach with osb, tape and diligent sealant then I just need to determine if a 1-3 ach home in central NC will need an ERV or some other ventilation setup.
Thx!
Deleted
OP - All homes I design have ERVs as part of a comprehensive HVAC system.
Michael - I tend to look at it as an open discussion of ideas. I was trying to answer to the OP about his project in his CZ.
Armando, thank you! Have you ever tried an HVAC supply side ventilation option before arriving at the ERV solution? If so, what made you go with the ERV?
Years ago, we used to do some supply only ventilation. We changed because it was harder to control humidity and temperature during the shoulder seasons, and too much humidity during the summer to control with just the AC and air-handler. Now we install balanced ventilation in all States, and dehumidifiers in CZs 2A-5A.
Armando, thank you!
Armando, do you also install a dehumidifier when building with a sealed crawlspace?
Yes, always in CZs "A".
thx!
I would take a look at LP smart
EddieH,
LP smart for ???
Sorry, LP weatherlogic
EddieH,
I had forgotten it. Wonder why it hasn't gained market share? Seems very similar to ZIP.
Its a great system. I'm not sure. I figured I would recommend. its great for water as well. Not just air
Deleted