Interior sheathing as an air and vapour barrier

3/4 inch OSB sheathing on the interior can be an excellent air barrier and a great vapour barrier as well.

Interior sheathing wood frame construction
Interior sheathing wood frame construction © Ecohome

We are currently in an evolutionary period of home construction. The old standby wall recipe of 2x6s, glass fiber insulation and polyethylene saw us through the last few decades, but doesn't measure up anymore with building codes, nor the trend towards high performance housing.

One aspect of wall assemblies that is garnering some well deserved attention lately is how we control the migration of moisture. A polyethylene vapour barrier is one way to do it, but it's the old way to do it, and by no means the best for this climate. In fact, for homes with air-conditioning it is closer to being the worst than the best. There are many alternative vapour barriers that  can perform  better than poly, one of those is Oriented Strand Board sheathing (OSB).

Whether or not a material qualifies as a vapour barrier is determined by the amount of moisture that passes through it, and it is given a rating. Any material that allows less than 60NG (nanograms) of moisture to pass through under specific conditions, is considered a type 9 residential vapour barrier by the National Building Code. 

Including a vapour control measure on the warm side of the insulation is essential for preventing moisture movement through walls in winter, and the ensuing damage that comes with it. In summer however, with the combination of hot, humid days and air-conditioned interiors, the vapour drive reverses and forces moist air inwards through your insulation where it can condense on a cold and impermeable vapour barrier.

Ideally we'd have no vapour barrier in summer; but short of that we should at least have one that allows as much drying to the interior as possible without sacrificing its winter performance. So the closer your vapour barrier is to 60NG, the better. For the sake of context it should be noted that polyethylene is rated at 3.4NG.

Rated at 40NG, 3/4 inch OSB sheathing can be one of the better vapour barriers for home construction in most of Canada. But in order to act in that role, it needs to be on the inside.

Sheathing provides a necessary structural strength to house frames, but it is written nowhere except in our minds that it has to be on the outside. When installed on the inside it still provides the structural strength, but can additionally act as both an air barrier and vapour barrier.

There is no question that this technique also presents a new challenge to the builder, namely the fact that you have exterior rather than interior cavities to fill with insulation. But this can easily be overcome with foresight and planning,

The accompanying framing images and the following wall description are of a project in Val des Monts, Quebec, built by Wakefield Construction.

Interior sheathing wood frame construction
Interior sheathing wood frame construction © Ecohome

Interior sheathing                             from the inside out:

  • Drywall
  • Horizontal 2x4's (on edge) as strapping, to allow for wiring chase without penetrating the air barrier
  • 3 / 4 inch OSB sheathing (with taped joints)
  • 2x8 studs with mineral wool batts in cavities (R28)
  • 4 inch wax-impregnated wood fiberboard on the exterior to provide a drainage plane, break the thermal bridge (R13.4)
  • Vertical strapping (if your cladding requires horizontal strapping, be sure to do a vertical layer first to allow for drainage)
  • Cladding

This is not some theoretical untested wall system, and we are not selling snake oil. The technical specifications of 3/4 OSB meet the requirements of building codes for both air and vapour permeance. By moving the sheathing to the inside, you are simply letting it live up to its full potential as an air barrier and vapour barrier, and eliminating the need to install a separate product to do that job.

One of the benefits of OSB as an air barrier is that it is solid. A polyethylene or foil air barrier can be easily pierced by the slightest touch with a sharp tool, without even realizing it. In contrast, it is unlikely you will put a hole in an OSB air barrier that was not intentional, or at least unnoticed.

Blower door tests

Blower door test
Blower door test © Celine Lecompte

The air seal of a building is measured in ACH (Air Changes per Hour), and is determined by a blower door test where the building is depressurized by a fan in the door, and air leakage is measured.

The average home built to code using conventional building practices is expected to have air leakage rates of 3.5 ACH, which under normal air pressure conditions means the entire volume of air in a home will have leaked out and been replaced 3 or 4 times every day. Using this technique of interior sheathing on previous buildings, Wakefield Construction has achieved ACH results that are a fraction of that, as low as 0.4 ACH.

There is a common misconception in the building industry that a house can in effect be too tight. That is completely false; the tighter the better. You need fresh air, but it should be supplied by properly balanced heat recovery ventilation, not arbitrary holes in an air barrier. Always seal your home as tight as you possibly can and let your air exchanger do the job it was designed to do.

Ordinarily there is a lineup of drywall crews, plumbers, cabinet installers, electricians, heating and cooling contractors, all waiting their turn to punch holes in your air barrier, perhaps not realizing the importance of properly sealing those breaches afterwards. Because of that unfortunate reality and a general lack of prioritizing air barriers in the industry, under normal air pressure conditions the average brand new home can expect to have its entire volume of air leaked out and replaced 3 or 4 times per day.

Solvent-free building tape
Solvent-free building tape © Ecohome

Along with focusing on techniques for preventing air leakage, some thought needs to go into products as well, and how they are best applied. Worth noting is that most commercially available building tapes contain solvents that evaporate and eventually become brittle and peel. The most durable tapes on the market contain no such solvents, so they actually do the job they were intended to do. They won't come cheap, but they work.

Of all the things you can do to ensure a successful air barrier, one of the more important ones could simply be to not install polyethylene. I say this because we get lazy with poly and tend to see its simple existence as the cure-all for preventing air leakage and moisture diffusion, which it is not. And without it you actually have to put some thought into an air barrier, and that is the first step towards success.