August 29, 2017

Leaky OSB: A Game of Chance

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?

Air permeability requirements for air barrier materials in passive houses – Comparison of the air Permeability of eight commercial brands of OSB – By J. Langmans, R. Klein, S. Roels, 2010

Air permeability requirements for air barrier materials in passive houses – Comparison of the air Permeability of eight commercial brands of OSB – By J. Langmans, R. Klein, S. Roels, 2010

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.

Our Pro Clima SOLITEX and INTELLO membranes, TESCON and CONTEGA tapes, caulks and ROFLEX & KAFLEX gaskets are lasting products that form complete and reliable systems of airtightness.

Repeating ourselves: don’t depend on OSB for airtightness.


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4 Responses to Leaky OSB: A Game of Chance

  1. Scott September 13, 2017 at 1:27 pm #

    Just a thought- why not paint the OSB with an exterior paint? The paint should seal the wood and act as a moisture barrier. Spraying OSB would be relatively quick and easy after install. One question would be taping the seams, before or after the paint. If the paint would harm the tape if sprayed on top. I would think the tape would stick better to the painted OSB than unpainted OSB.

    • Ken September 13, 2017 at 4:01 pm #

      Thank you for the comment. It’s certainly possible that painting the OSB can work – but like the OSB, the paint is not designed to be an air barrier and would act as a vapor retarder or barrier. Of course there are liquid air barriers that can be applied as there are also OSB boards that have such products pre-applied to them – like ZIP – all designed to be air barriers. As airtightness is so important it’s critical that we use materials designed and tested for that purpose. Of course we offer both exterior air barriers (SOLITEX MENTO) and interior air barriers (INTELLO) that are tried and tested as absolutely airtight – and offer the best vapor variable/control possible.

  2. Mike March 13, 2018 at 7:53 pm #

    I’m considering using OSB with taped seams with Tescon Vana or INTELLO as the air barrier for my ceiling. I’m building with ICF walls and besides the doors and windows the ceiling is my biggest concern for air leakage. I will be installing blown cellulose insulation (R-60) in the attic and using a 2×6 service chase on the ceiling. So, it will be trusses, INTELLO or taped OSB, 2×6 service chase, then drywall. My concern is the supporting the weight of the blow cellulose in the attic with INTELLO. I see what did and it makes me want to cry. The thought of installing INTELLO, taping the staples, attaching 1×4, then 2x(4 or 6) seems to be a lot of material and work.

    Is there a better/easier way to prevent sag on the ceiling?

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