Everyday at 475 we have dozens of conversations with builders all over North America. We often generate the ideas for this blog based on common trends, questions, and concerns… basically, things we find we’re repeating. And it’s worth repeating and repeating: don’t depend on OSB for airtightness.
- OSB is not manufactured to be an air barrier.
- There are no manufacturing standards to assure that specific levels of airtightness are being achieved.
- There is no quality assurance.
- There is no reliability.
A simple analogy should suffice:
You’re planning to spend a weekend out on the lake, and you need to choose a boat. Do you go with the model that lacked manufacturing controls to ensure it stayed afloat? Or that has literally never been tested for buoyancy and leakage? Unless you’re interested in swimming home, you go with the one designed to keep the water out.
The hazard is known
It’s all been reported before, (Martin Holladay has a fairly exhaustive post on the subject from 2014 in GBA here) but we continue to hear of Passive House projects that are failing their blower door tests because the OSB is their only air barrier, and it’s leaking.
How is that possible you ask? Because OSB leaks too much.
You have a budget
Of course OSB has been used as an air barrier for some time – so what’s the deal? How’s it been working? First published in 2010, this informative graph below demonstrates that – it depends.
We like to think of airtightness as a budget. And for Passive House the budget limit is 0.6 ACH50. If the material you are using is not much tighter than the budget limit, then you have almost no wiggle room for poor workmanship – everything has to be perfect. Are you perfect?
If you are working with brand A or brand D, it is unlikely you can hit Passive House airtightness, as there simply isn’t enough of a buffer for unforeseen leakages – you’re not that perfect. But if you you have brand B – wow, you have great tightness that gives up almost nothing out of the gate.
How do you know what you are getting? You don’t.
Vegas is not the model
Performance shouldn’t be a game of chance. For your air barrier choose materials that are manufactured to stringent airtightness specifications.
Repeating ourselves: don’t use OSB for airtightness.