Architectural Record 2015-04

156 EFFECTIVE AND AFFORDABLE HOUSE WRAPS EDUCATIONAL-ADVERTISEMENT
That goes for all claddings—reservoir claddings
like stucco as well as wood, insulated vinyl,
or fiber cement all have the potential to hold
trapped moisture.
Over the years, it has become common
practice to install a weather-resistant
barrier (WRB) between the sheathing and
the cladding as a second line of defense in
moisture protection. A WRB, or house wrap,
is a lightweight synthetic material which, if
properly designed and installed, will protect the
structure from weather, moisture, and other
environmental damage over time. Specifically,
a house wrap has a threefold task: stop liquid
water intrusion; be sufficiently vapor permeable
to promote drying of the wall cavity while
allowing water vapor to escape outside of the
structure; and prevent air infiltration into
the structure. In other words, an effective
house wrap will assure that bulk water is not
admitted to the wall cavity, and any water that
does breach the barrier is quickly drained,
allowing the wall system to dry out, and drafts
are avoided—all of which work to favorably
impact structural integrity, the service life of the
building, indoor air quality, and energy costs.
Permeability is a key consideration as, if water
is left to collect within the walls, not only is
the possibility of mold increased, but the built
up moisture will almost certainly compromise
the R-value of the insulation. To be considered
effective, a house wrap product should possess a
relatively high moisture vapor transmission rate.
If they are properly sealed, house wraps
can also serve as an effective air barrier.
In fact, preventing air leakage can be a key
part of a house wrap’s contribution to
sustainable building.
According to Franklin & Associates in
a study prepared for the American Plastics
Council, house wraps are “quite effective
at reducing air infiltration.” As the U.S.
Department of Energy (DOE) maintains,
because some 50 percent of heating and cooling
energy for homes stems from air infiltration, it
follows that blocking that infiltration has the
potential to reduce energy usage and related
greenhouse gas (GHG) emissions. While the
study acknowledges that the evidence is “highly
variable and dependent on many factors,” the
amount of air house wraps can likely block falls
in the range of 10 percent to 50 percent of the
infiltrated air, with “the reduction in energy
consumption of a typical house in the U.S. as
a result of applying house wrap is estimated to
be 12 to 60 million Btu per year. Over a period
of 30 years, these values become 360 to 1,800
million Btu.” 1
The DOE, in fact, cites house wraps as the
most common air barrier material, and notes
that sealing the house wrap joints with the
manufacturer’s tape can boost the material’s
Photo courtesy of Kimberly-Clark
performance by some 20 percent. 2 House wraps
can also help the home to qualify for and
maintain ENERGY STAR® certification.
Most structures, regardless of whether they
are stick built or metal framing, commercial
or residential, single or multi-level, should use
some type of weather-resistant barrier. House
wraps are necessary protection in all woodframed
residential structures and most are
applicable in multifamily residential structures
up to five stories high—beyond that height limit
a commercial-grade wrap is needed as typical
house wraps are subject to tearing from wind
forces. The exterior of the structure should also
be taken into consideration when selecting a
house wrap. Reservoir claddings that absorb
and store water such as stucco, brick, and stone
all pose serious concerns in regard to water
penetrating into a wall assembly—walls should
be built with an air space behind these types
of cladding.
TYPES OF HOUSE WRAPS
There are several types of WRB—asphalt felt,
Grade D building paper, plastic house wrap,
and liquid-applied WRB, among others. Tar
paper, or felt as it is now referred to, was the
preferred choice of WRB many years ago. Felt
has evolved from a heavy, unwieldy material to
a lighter, water-resistant material. While some
builders prefer it, felt is still relatively heavy, and
is susceptible to punctures, and the fact that it
traditionally comes in 3-foot-wide rolls makes
it somewhat cumbersome and time consuming
to install. Advocates note that if wet, felt is
absorbent and will gradually dry. Most often
used in the western U.S. and under stucco,
grade D paper is a lighter-weight, lower-cost
asphalt-saturated paper. Grade D paper has a
minimum water-resistance rating of 10 minutes,
with many manufacturers far exceeding the
minimum requirements. Still, if Grade D paper
becomes and stays wet, rot may occur. Liquidapplied
WRBs are another option. Sprayed or
rolled on sheathing, they form a coating that
is air tight and resistant to water penetration.
One drawback with liquid-applied materials is
their high cost relative to other alternatives. A
consistent thickness must be achieved, adequate
attention paid to detailing, the proper cure time
observed, and protective clothing worn during
the application process. Air and water resistant,
liquid-applied materials vary as to vapor
permeability effectiveness.