Leaky Housewraps
Results from the most recent Building Materials Laboratory
study show some housewraps may do little to block rain intrusion.
by Paul Fisette - © 2000
Liquid water is without question the most destructive force
acting on light frame structures. As builders, we are mesmerized by
vapor diffusion and vapor barriers. But it is still good old
fashion water leakage that rots walls, sills and other building
elements. There is no shortage of pathways leading water inward.
Any discontinuity in a wall surface will do nicely: seams, windows,
butt joints, knots, and siding overlaps are thirsty conduits. The
trick is to build energy-tight walls that drain water leakage
outward. Installation of high-tech housewraps is the most common
attempt to have it all.
I find myself inspecting a growing number of failures related to
water intrusion and remain convinced the leading cause of water
damage is poor design. Proper flashing and the integration of
flashing with an able housewrap are perhaps the most critical
details enabling walls to function as weather barriers (see
http://www.umass.edu/bmatwt/walls.html). However, I have found that
even properly flashed walls sometimes leak. Field investigations
and conversations with other building scientists nudged me to
question the viability of housewraps as weather barriers. It is
widely known that water leaks through virtually every type of
siding. But are all housewraps equally able to shunt water once it
gets past siding?
One of the primary ways that rain is driven into walls is by air
pressure differentials. In a word - wind. During the fall of 1998 I
brought samples of 15 pound felt, Tyvek, Rwrap, Typar, Amowrap,
Pinkwrap, and Barricade into the University's Building Materials
Laboratory to see how these popular housewraps stand up to
pressure-driven water. The housewraps were subjected to stress
imposed by columns of water and the results were reported in the
November 1998 issue of JLC (see http://www.umass.edu/bmatwt/weather_barriers.html).
Non perforated wraps (Tyvek and Rwrap) and 15-pound felt paper won
that round by a knockout. Perforated wraps leaked like sieves. I
also did some exploratory work to determine the effect of soaps
(from powerwashing) and water soluble extractives (like those found
in cedar and redwood) on water permeability of housewraps and
determined that soaps and to a lesser degree extractives can cause
wraps to leak. As promised in the November 1998 issue of JLC, I
have extended the study to look at the ability of housewraps to
block the capillary flow of water.
Siding provides discontinuous coverage. True, overlaps and seams
are slender ports, but narrow breaks are large enough to pass
significant amounts of water. Some of the most common wall soakings
result from splashback, inadequate overhangs, blocked gutters,
roof-wall intersections; and exposure to prevailing wind. Capillary
suction is a strong force. Even under conditions of light or no
wind pressure, water can be sucked through seams, cracks, joints
and upward behind the overlaps of horizontal siding. Add to that
fact, most wood siding is not back primed, so its backside sucks
water up like a sponge. The face of wood siding is usually painted.
When painted wood absorbs water, it takes a long time to dry. Paint
holds water in. As a result, wet siding is held tightly against
housewrapped sheathing in most homes. The question we tried to
answer is: Do housewraps suck?
The Test
I am leery of extending lab results into predictable
field performance. However, lab testing can help clarify our
understanding of field-use potentials. The typical wall system
considered for our experimental design is built this way (starting
from the outside): wood siding (unprimed and unpainted), housewrap,
and sheathing. We imitated this lay-up in our experiment.
First we cut samples of 1/2-inch bevel cedar siding into 2-inch
squares and soaked them in water. Saturating the siding was an
attempt to mimic what happens to untreated wood siding or wood
siding that has not been backprimed when it is exposed to a period
of rain. Next we placed a square of saturated siding into each of 5
petrie dishes that were half-filled with water. The top half of our
siding samples remained well above the water line. Water in the
dishes served to charge our samples, maintaining saturation
throughout the test period. Next, we placed a 4-inch square of
housewrap over each piece of saturated siding so that its edges
extended well beyond the perimeter of the dish. On top each square
of housewrap we positioned a 2-inch diameter disk of blotter paper.
The blotter served as a visual indicator to tell us if any water
leaked through the housewrap. Finally, a 2-inch square piece of
1/2-inch plywood was placed on top of the blotter paper. The entire
sandwich was held together firmly with an elastic band. The
sandwiches were unbanded and checked every 2-hours for 2 days (9:00
am - 5:00 pm --- no checking at night). This test was performed for
Tyvek, Rwrap, 15-pound felt, Amowrap, Pinkwrap and Barricade.
The Results
Tyvek, Rwrap and 15-pound felt showed no signs of liquid
water leakage throughout the 2-day test period. However, the
blotter paper on the Tyvek and Rwrap samples felt slightly damp to
the touch by the end of the period, suggesting that there had been
some transfer of water vapor by diffusion. Blotter paper disks on
the 15-pound felt paper samples remained bone dry to the touch.
Typar samples all leaked within the first 2-hours of the test. Two
of the Typar samples leaked immediately - as soon as the blotter
paper was placed onto the wrap. Blotter paper indicators for
Barricade, Pink Wrap, and AmoWrap (all perforated wraps) became
saturated before the layers of the test setups could be secured
with an elastic band.
Before we packed up the testing gear, we decided to look at one
more detail. What happens when you nail the sandwich together as
you do on a real wall? We repeated the experiment only this time we
drove a nail through the siding before setting it into the petrie
dish. We forced the square of housewrap and disk of blotter paper
over the nail. Then we drilled a tight-fitting hole (slightly
larger than the nail) through the plywood so that we could slide
the plywood on and off the nail to inspect the blotter paper for
leakage. No surprise here: all wraps leaked around the nail within
the first 2-hours of the test.
End Points - What it all means
As I said earlier, I am reluctant to predict field
behavior from lab bench results. However, it is abundantly clear to
me that perforated wraps, in their current state of development,
can leak when placed in contact with water and/or wet wood. So why
use them? You can choose a product that won't.
Unprotected wood absorbs water. Most wood siding is not
backprimed. Let's stop this practice. Backprime and end coat wood
siding before it is installed. A water repellent treatment is best.
This one step alone will not only minimize rot, but will also
prolong the service life of the exterior paint. Over the life of a
house this represents a significant savings! If the budget permits,
build a vented rain screen. Building an airspace directly behind
the siding lifts the siding away from the drainage plane. This
encourages free drainage, air-pressure equalization (reducing
wind-driven leakage), and allows the back of the siding to dry more
easily. It also reduces the likelihood that water will leak through
nail holes that perforate the housewrap.
Use a housewrap as a secondary weather barrier under all siding.
For my money, I'll choose a housewrap that shows the most promise.
Test results narrow my field of potential candidates to Tyvek,
Rwrap and 15-pound felt. Felt paper leaked after 30 minutes in the
column test, but still makes my list as a strong candidate.
I think that felt paper blocks the flow of liquid water under
pressure (column) for a respectable length of time. When it leaks
under pressure, it seams to leak very slowly. It blocks capillary
flow for days. Felt is inexpensive. What is also appealing is that
felt is forgiving. If water gets on the wrong side of a
felt-wrapped wall, the felt can absorb the water and over time
allow drying to the outside of the structure. Plastic housewraps
don't move water this way. Plastic wraps are non absorbent. They
rely on vapor diffusion to move water that gets on the wrong side
of the wrap. Diffusion is a slow and weak force. The perm rating of
felt is much lower (less permeable) than the plastic housewraps
when it is dry. But this does not mean that it traps water vapor in
the wall cavity. As the relative humidity rises, the perm rating of
felt rises too- eventually above that of Tyvek. Felt is dynamic.
Having said that I also like Tyvek and Rwrap. They cost more than
felt, but these products stand out to me as being very strong
products. They roll out as large continuous sheets. They have shown
an ability to reduce air exchange and energy consumption in some
studies. And they have demonstrated to me that they can block the
flow of liquid water, while allowing diffusion of vapor.
Thumbnail Photos, click on photo to select enlarged
view:
Figures 1 - 4 depict the study described in the article found at
http://www.umass.edu/bmatwt/weather_barriers.html
Figure 1. The column test apparatus.
Figure 2. Typar leaking under the force of a 3-1/2 hydro head.
Figure 3. Owens Corning PinkWrap leaking under the force of a 3-1/2 hydro head.
Figure 4. AmoWrap leaking under the force of a 3-1/2 hydro head.
Figure 5. Placing unpainted cedar siding specimen in water.
Figure 6. Covering saturated cedar square with wrap and then
blotter paper.
Figure 7. Covering the blotter paper with a dry piece of plywood.
Figure 8. Note leakage of water moving upward, marking blotter
paper by capillary suction.
Figures 9 - 12 show tests that explored the effect that nail
insertion had on capillarity. All wraps leaked upward around
nail.
Article