Zip vs. OSB, House Wrap, and Tape
mills9867665789760
| Posted in General Questions on
I understand that ZIP sheathing is both a WRB and an air barrier (when it’s properly taped and installed), while OSB + house wrap is an effective WRB but is not an effective air barrier.
My question: is it just the tape that makes ZIP an effective air barrier, or is ZIP’s green coating also important? If I sheath with cheaper OSB and then use high quality tape to seal the seams, would that be an effective air barrier?
Setting aside labor costs, does OSB + tape + house wrap have all the benefits of ZIP?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
OSB has been shown to leak air right through the field of the material, enough that it does not qualify as an air barrier. The green coating on the ZIP gives it both water and air resistance.
Brian,
As Peter said, OSB is problematic. It would be fair to say either AdvanTech or plywood + tape + house-wrap has all the benefits of Zip. One other advantage of going that route is that the flashing of things like windows and other penetrations relies on laps (gravity), rather than tape for it's integrity.
Got it - thanks for the replies, Peter and Malcolm
For 20+ years, I’ve used OSB (mostly) and Zip sheathings, both taped, rolled and sealed, to great success when installed correctly. We consistently get between 0.8-1.2 ACH50. We also use taped and sealed rigid foam, hence, I want my wall assemblies to dry in/out from the rigid foam. The key words here are… ” installed correctly” and decide how your assembly dries.
Most of the Builders I work with decide which product to buy depending on their budget and availability. Labor is the same for both.
Also, a key part of the equation is our HVAC systems, which are designed to maintain a RH ≤50%.
5/8” OSB is a Class II vapor retarder with 1-3 perms when <30% RH. The perm goes up to 2-4 when RH goes up to 60%-80%, and is prone to rot.
Per Huber: The permeability of the entire panel is based on the permeability of the OSB (1-3 perms depending on thickness), while the permeability of the integrated protective overlay alone is 12-16 perms.
Per BSC RR-0403: Air Barriers - Rigid materials such as gypsum board, exterior sheathing materials like plywood or OSB, and supported flexible barriers are typically effective air barrier systems if joints and seams are sealed.
https://www.greenbuildingadvisor.com/article/study-finds-osb-sheathing-not-a-reliable-air-barrier
“They used a standard testing method (ASTM E2178) to check air-leakage rates under pressures that ranged from 25 to 300 Pascals (Pa)”
“What constitutes an air barrier? For purposes of this study, RDH used a standard criteria of 0.02 liters per second per square meter at 75 Pascals of pressure (lps/m2). If a material allows more air than that to get through, it’s not effective as an air barrier.”
I’m stand by my experience. Our Blower-door tests are taken at 50 Pascals. Let’s compare apples to apples. That picture doesn't say what Pascals were used at that point.
Armando,
You may be right, and your experience lends itself to that conclusion. I recall other bloggers (like David Goodyear) have used it successfully too. I only know about the possible problems with using OSB as an air-barrier from what I've read here on GBA
@Malcolm – Think about this for a minute and let it sink in… RDH calls OSB a bad air barrier by their report made “at the behest of Huber, which manufacturers Zip products”, and using “a standard testing method (ASTM E2178) to check air-leakage rates under pressures that ranged from 25 to 300 Pascals”.
If the definition of an air barrier, per RDH, is to use in their study the standard criteria of 0.02 liters per second per square meter at 75 Pascals of pressure (lps/m2), and anything more than that, it’s a failure, then why all HERS raters test homes at 50 Pascals? Isn’t ACH50 the Code measurement for building air tightness? Where in the Code says that the Passive House Standard is the measurement for a tight building envelope?
I don’t have a problem with Huber and RDH having their study, but I do have a problem with the conclusion that it’s their way or failure. These same sentiments are also expressed, by several folks, on the comments of the original article, very disappointing!
Exactly this type of argument is what turns Builders off about the sustainable and green building movement. It implies that if the building system a Builder uses is not with Passive House parameters, it’s a failure... and that it’s an insult and a disservice to the rest of the industry that works hard building homes better than code, following the NAHB program, LEED for Homes, Energy Star, ZERH or any other good program. I also think it creates an unfair resentment towards the Passive House program.