ASHRAE Journal - March 2015 - 48

COLUMN BUILDING SCIENCES
Joseph W. Lstiburek

Evolution of the Residential Air Barrier

Forty Years of
Air Barriers*

PHOTO 1 Plastic Vapor Barrier. Not a "production friendly" method of achieving
airtightness and it was not "robust" in terms of surviving the construction process
and in providing performance over the service life of the building. It was also climate sensitive. It made no sense for assemblies that saw air conditioning. It was a
vapor barrier on the wrong side of such assemblies.

BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Things have evolved considerably since
the Eisenhower and Diefenbaker years.
Hutcheon† taught us about airflow that
decade but it took more than a half century
to get it right. We needed air control. We
needed an air control layer - an air barrier.
We started off with locating it on the inside
and finally ended up with it on the outside.‡
We started by combining it with a vapor
barrier on the inside then we finished by
combining it with a weather resistive barrier
(WRB) and continuous insulation on the
outside (Figures 1 through 5).
(Figures
Throughout the postwar years practitioners were
taught, incorrectly, that vapor barriers were necessary in
cold climates to protect wall assemblies from moisture
damage and that it was necessary to install these vapor
barriers on the interior of cavity insulation. The industry
saw the introduction of kraft facings and foil facings on
batt insulations as a result. These vapor barriers were by
their very nature discontinuous and they proved ineffective in protecting wall assemblies from vapor. Vapor
is principally transported by airflow not by vapor diffusion. We needed air barriers not vapor barriers to control vapor flow. It took decades for that distinction to be
appreciated.§

The first attempt at addressing the issue was to take
a vapor barrier material and turn it into an air barrier. The result was coined an "air-vapor barrier." Sheet
polyethylene was already being installed on the interior
of insulated wall assemblies as a vapor barrier (Photo
1). It was given a second function-that of air control.
Easy to do conceptually-very difficult to do in practice.
With painstaking effort and attention to detail, extraordinary levels of airtightness were achieved using this
approach-less than 1 ach at 50 Pa. Canada's R-2000
program was based on this approach. The sealant used
was "acoustical sealant" as it would remain flexible over
the service life of the building. It soon got the moniker
of "black death" as it got on to everything including the
installer. It became clear that the approach was not a
"production friendly" method of achieving airtightness
and it was not "robust" in terms of surviving the construction process and in providing performance over the
service life of the building. It was also climate sensitive.
It made no sense for assemblies that saw air conditioning-it was a vapor barrier on the wrong side of an airconditioned wall (Figure 1).

* This is a play on the title of Professor Hutcheon's classic paper "Forty Years of Vapor Barriers." Read down a couple of footnotes and enjoy....
† Hutcheon, N.B.; "Fundamental Consideration in the Design of Exterior Walls for Building." NRC Paper No. 3087, DBR No. 37. Division of Building Research, National Research Council of Canada, Ottawa, 1953.
‡ At the turn of the 20th Century it became common to install a layer of rosin paper over wood board sheathing and under clapboard siding (see "The Evolution of Walls," ASHRAE Journal, June 2009) to
reduce drafts. This rosin paper was an early "air barrier." Having pointed this out, I am going to conveniently ignore this fact because it messes with my narrative.
§ "It was 30 years from the time of Rowley's paper before it was clearly established and widely accepted that the leakage of air from inside a building through constructions, and not vapor diffusion alone,
was often the principal means by which water vapor moved to cold surfaces. The concept of vapor diffusion was not wrong, but it was not the only way. It is incredible, in retrospect, that it should have
taken so long to reach this conclusion...." N. B. Hutcheon, from "Forty Years of Vapor Barriers," Canadian Consulting Engineer, special publication on Moisture Control, Ottawa, 1978.

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