ASHRAE Journal - November 2013 - 58

Column Building ScienceS

Cladding

Building Paper

Cladding

Furring

Cladding

Furring
Stone Wool/Mineral Fiber
Rigid Insulation
Building Paper

Building Paper
Fiberboard
OSB Sheathing
Fiberglass Batt/
Cellulose

OSB Sheathing

Fiberglass Batt/
Cellulose

2×4 Wall Framing

Fiberglass Batt/
Cellulose
2×6 Wall Framing

2×6 Wall Framing

Polyethylene Vapor
Barrier

Gypsum Board

Polyethylene Vapor
Barrier

Gypsum Board

Interior Latex Paint

Drying to
Exterior
Figure 2a The "Old Plastic Wall." The wall that

kinda worked with fiberboard but not with OSB.
Don't use this wall with air conditioning.

Drying to
Exterior
Figure 2b The "New Plastic Wall." The wall that

works with OSB because of the ventilated cladding. Don't use this wall with air conditioning.

July 2005). There is a pattern here-every decade or so
we tend to revisit the same principles. Air leakage was,
and is, more important than vapor diffusion. Things
have not changed. Architects beware. If you forget this,
the ghost of Hutcheon** will haunt you like the ghost of
Banquo haunted Macbeth.
Folks used and some are still using plastic vapor barriers to solve what was and is really an air leakage problem. You do not have to use a plastic vapor barrier to
solve an air leakage problem. This is obvious, but yet, we
still have folks swearing that it is necessary. We learned
long ago that we can address air leakage with other
materials that are not also vapor closed. The most common being drywall.†† Drywall used to control air leakage
has the advantage of being vapor permeable-a wall can
still dry to the interior. Check out Figure 1b. The drywall
is caulked or glued or gasketed to the framing. It has all
of the advantages of Figure 2a without any of the disadvantages. Recall, in Figure 2a, the polyethylene is there to

Drying to
Exterior
Figure 2c High Performance Plastic Wall.
This wall works well. Just don't use it with air
conditioning.

control airflow not vapor diffusion.
A couple of things to note in Figure 1b: an unfaced fiberglass batt or netted or blown fiberglass cellulose insulation are used as cavity insulation and OSB sheathing is
used in place of fiberboard. We don't need the kraft facing on a fiberglass batt to control vapor diffusion as the
latex paint on the gypsum board performs pretty much
the same function. But we do have to compensate for the
reduction in vapor permeability of the exterior sheathing in some way. OSB is very different than fiberboard
and very different than plywood (we were already here
before, check out "Mind the Gap, Eh?" ASHRAE Journal,
January 2010). The real key to this assembly is the air
space between the cladding and the OSB sheathing. The
air gap significantly improves outward drying, controls
rain-induced hydrostatic pressure and facilitates redistribution of absorbed water in the OSB sheathing.
We didn't build wall 2a for very long in Canada.
We replaced the fiberboard with OSB sheathing and

**Hutcheon, N.B., "Fundamental Consideration in the Design of Exterior Walls for Building." This was the legendary paper that started it all for many of us.
††"Drywall air barriers" were my entry into the big leagues. Gus Handegord told me that plastic vapor barriers to control air leakage were a dumb idea when I was starting up what was to become the
R-2000 program. He told me about some crazy Quebecois building high rises in Edmonton by the name of J.C. Perreault who was using drywall as an air barrier and that I should consider it as an option
for houses. This lead to the Airtight Drywall Approach and my Master's and, of course, meeting and worshiping "J.C." like everyone else who met him. Jean-Claude was a legendary figure in the construction
industry in Canada. His bastardized English-Quebecois way of speaking was charismatic. His practical knowledge was without peer. He was famous for saying "Cherchez le trou." To get the humor that is
coming you have to understand the derivation of "trou"-it is Quebecois for "hole." J.C. would bring down the house saying, "Dat's where da air goes trou da wall...."
58

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ASHRAE Journal - November 2013

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