SINCE 1992


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General Properties of Housewraps
The manufacturers of housewraps have for years promoted the fantasy that water
vapor in wall assemblies only moves one way – from the inside out. So, therefore,
walls should be designed to “breathe” outwards. So, therefore, you should buy a
highly vapor permeable housewrap. So, therefore, you should buy “our product.”
Now housewraps are amazing materials. They have two seemingly contradictory
properties: they retard the passage of water in the liquid state while freely allowing the
passage of water in the vapor state. How they accomplish this is elegant simplicity that
takes advantage of the differing behavior of water molecules in the two states. Water
molecules in the liquid state cluster together creating “water polymers” that act as large
molecules. However, water in the vapor state is actually individual water that may be
close together or far apart depending on their concentration. The holes in housewraps
are large enough to permit the passage of individual water molecules in the vapor state
but small enough to retard the passage of the clustered molecules of liquid water. But
housewraps are not a “magic wonder material” that should be used everywhere
without thought. Like all materials, you have to pick your spots.
Although housewraps restrict or permit the passage of water molecules based on size,
they cannot control the direction in which the water vapor molecules move. That is,
the housewraps do not know the difference between the inside and the outside.
Housewraps are not capable of letting water vapor move in only one direction – say
from the inside to the outside. Therefore water vapor can easily move in either
direction through a housewrap. In summary, housewraps are selective with respect to
the size of molecules that can pass, but not the direction in which the molecules move.
Water vapor also doesn’t know the difference between the inside and the outside.
Water vapor only cares about concentration gradients and thermal gradients. If a
concentration gradient or a thermal gradient exists from the inside out – water vapor
moves from the inside out. If the gradients are reversed – water vapor moves the
other way. Those who doubt this should consult the Second Law of
Brick Veneer and Housewraps
Brick veneers pose a special challenge in wall design and performance because they are
reservoir cladding systems. When they get wet from rain they act like a reservoir or
sponge. Wet brick can then get heated above ambient temperature by solar radiation
creating a powerful inward acting concentration gradient and thermal gradient. These
gradients are exacerbated by air conditioning (cooling and dehumidifying the interior).
A good analogy would be to consider the brick as a moisture capacitor that is charged
during a rainstorm and that subsequently discharges in the direction of the temperature
gradient and the concentration gradient. The magnitudes of these forces are huge. In
fact typical psychometric charts will not work – you have to go to the Steam Tables.
See the Moisture Control Handbook (pg. 65). This was first documented by Grant
Wilson’s work with the Division of Building Research, National Research Council of
Canada in the mid 1960’s and has been referenced by every ASHRAE Fundamentals
Volume over the past 35 years (pg 24.8, 2001 ASHRAE Fundamentals Handbook).
Brick, Stucco, Housewrap and Building Paper
You would have to be insane to recommend a vapor permeable assembly behind a rain
wetted reservoir cladding system experiencing these kinds of drivers. Now, in fairness
to a 50 perm housewrap, it works well when it’s installed over a 1 to 2 perm OSB
sheathing behind a brick veneer. It’s another story when the 50 perm housewrap is
installed over a 15 perm fiberboard or a 25 perm gypsum sheathing. With the OSB,
the water vapor is stopped at the outside surface of the OSB and then migrates back
outwards through the housewrap into the airspace behind the brick veneer when the
gradients reverse. With the fiberboard and gypsum sheathing the water vapor blows
right through the assembly into the wall cavity where it causes mischief.
Now let’s get a little more complicated. The #30 felt doesn’t cover itself with glory in
the same application described above. It changes its permeability as its moisture
content goes up. It is a hygroscopic material. Depending on how well vented the
brick veneer cavity is (actually, the correct phrase is depending on how poorly vented
the brick veneer cavity is), the #30 felt increases in permeability as it picks up moisture
to match the permeability of the housewrap. So installing #30 felt over fiberboard or
gypsum sheathing is not necessarily a big improvement over installing housewrap.
However, installing #30 felt over OSB works well (and so does the housewrap).
Brick veneers should be well vented. They should have a drainage plane. The
drainage plane-sheathing complex behind the brick veneer should have a low water
vapor permeability. This can be accomplished with a high perm housewrap over a low
perm sheathing (OSB) or it can be accomplished with a low perm drainage plane such
a bituthane or foam sheathing with joints treated to function as a drainage plane.
Housewraps can be used as both drainage plains and air barriers. Installing foam
sheathings over housewraps also provides a resistance to inward vapor flow.
Impermeable foam sheathings work well with permeable housewraps. It’s hard to beat
that combination behind a brick veneer.
Stucco, Housewraps and Building Papers
Stucco claddings are significantly different from brick veneers in their impact on
housewrap and building paper performance. Impregnated or coated building papers
are not designed to stay wet. If they stay wet they will rot. Water must be allowed to
drain from the surface of the building papers. In a hardcoat stucco application, a
drainage space must be provided between the hardcoat stucco and the building paper
drainage plane.
Traditional impregnated cellulose based building papers were originally robust enough
to debond from the back of hardcoat stuccos as they dried and experienced shrinkage
creating a drainage space between the building paper and hardcoat stucco.
Water penetrated the stucco, ran down the face of the debonded building paper and
out of the wall at a flashing location. The building paper didn’t stay wet long enough
to get into trouble. Drainability was improved by providing two layers of felt or by
going to coated papers (coated papers don’t bond as well to stuccos as do impregnated
papers) or by doing both. Two layers of coated paper are the norm in most parts of
North America.