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Over the years a handful of readers have approach me for advice about
moisture buildup on the inside of properly installed vapor barriers. I
suspect this problem is more widespread that it seems on the surface because
the only way you'd notice it would be if the drywall was missing on an
otherwise finished wall. But despite its low-key visual presence, moisture
buildup within stud frame wall cavities is important because the stakes
are so high. Any long-term moisture buildup within a stud frame wall can
fester for years before making itself known. And by the time it does,
it's far too late to do anything. If you're planning renovations to your
home, or building a new one, it pays to understand how moisture can get
into wall cavities that are equipped with a sound valor barrier.
Basements Require Special Treatment
The finishing of even a dry basement demands special measures to prevent
damaging levels of moisture from entering the finished wall structure
and causing decay. In addition to the usual source of airborne moisture
within the warm, moist air of any heated basement during winter, the poured
concrete or blocks that form almost all basement walls are quite porous
and can allow lots of water valor to pass through them from the soil into
the basement. Even basement walls that have been treated on the outside
with tar during construction can let moisture through due to shoddy workmanship,
damage while backfilling or migration through the tar layer. Water valor
passing through unfinished masonry walls isn't usually a problem because
it gets carried away by the air. Problems will arise, however, when that
same masonry wall is covered with an insulated wood frame and sealed under
a valor barrier. When this happens the moisture has nowhere to go, eventually
condensing as liquid water within the wall and causing rot.
The important thing to remember is that the inside surface of any basement
wall must be thoroughly waterproofed before any stud frame wall goes up,
even if your basement appears dry. Don't rely on an exterior layer of
waterproofing that may or may not be present underground. Even if it's
there, it's probably not enough to do the job. You can prove this to yourself
by sealing a 2" x 18" x 18" piece of rigid foam somewhere
on your unfinished basement wall, about 12" below ground level. Use
a bead of caulking all around the edges of the foam and leave it there
for several months, preferably from fall through winter. Chances are quite
good that when you remove it you'll see evidence of moisture build up;
either beads of liquid or dampness. This is exactly the kind of thing
that would build up continuously under any stud wall you set in place
without the precaution of thorough waterproofing.
The most potent masonry waterproofing product I know works on the principle
of crystallization. Xypex pioneered this product and it's now available
at every hardware store in Canada; at least one chain now carries their
own, similar house brand. All versions of the product come in a powder
that's mixed with water into a soupy liquid that's sloshed onto a moistened
masonry surface with a long-bristle brush. The treated area must be kept
moist for several days as the product seeps into the masonry's pores and
forms a waterproof crystalline structure. In about 2 weeks the surface
is completely impervious to liquid water and valor I've seen this treatment
completely stop actual streams of liquid water entering a basement through
cracks and pores. It's surprisingly effective.
Air Conditioning Can Cause Condensation
The whole point of a valor barrier is to stop warm, moist interior air
from seeping into wall
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cavities during winter and condensing there as it cools. But what happens
when the cold air is inside the house, and the warm moist air is outside,
as on sweltering Canada Day weekend when your air conditioner is running
full tilt? Liquid water forms inside wall cavities, that's what happens.
In this case the source of water is from outside the building, so a conventional
valor barrier can't solve the problem.
Experts haven't arrived at a consensus on how to sidestep this problem
through residential design and construction, because several conflicting
aims are at work. The problem is especially acute in places like Canada
where huge temperature swings are experienced each year. You might think,
for instance, that installing a valor barrier on the outside of the wall
frame, as well as the inside, would do the trick. This is true, in theory,
but only if you could be absolutely certain that rainwater would never
get past the exterior wall surface and into the wall cavity. This is an
unwise assumption because water will eventually get in, even in small
quantities. What about housewrap products? These are designed to stop
liquid water infiltration, yet allow water valor to pass back out if need
be. As such, they can't stop warm, moist air from sneaking in through
brick, siding or stucco on a humid summer day because they're not designed
to. The ideal answer is not clear.
While there is no general consensus on how to stop air conditioner-related
condensation within walls, there are some strategies that can lessen or
eliminate the problem in some cases:
Use your air conditioner sparingly: Summertime wall condensation occurs
because of the temperature difference between cool indoor and warm outdoor
air. The lower this difference, and the shorter time period over which
the difference occurs over, the lower the tendency to form condensation.
Turn down your air conditioner and you'll save money on electricity and
keep your wall cavities drier.
Insulate the inside of wall surfaces. Canadian building codes allow for
as much as 1/3 of a wall's insulation to be installed on the warm side
of a valor barrier, though it rarely ever is. Under this approach, 1"
of rigid foam insulation could be safely installed on the inside of 2x6
wall studs insulated to the standard value of R-20, before drywalling.
The foam would increase the temperature of internal wall cavities during
the cooling season, thereby reducing the tendency for condensation to
form inside.
Make sure exterior wall vents are clear during winter: Many exterior
walls include vents to allow liquid water to dry should it leak in during
a rain. Missing mortar joints in brick walls and small, round louvers
in various kinds of siding are two types of common wall vents. The time
for internal wall moisture to dry is during the cool and cold times of
the year. Be sure that any wall vents are free from ice or snow blockages
that could prevent this essential drying action.
For the very latest on residential home construction in Canada, take
a look at the information products offered by
Canada Mortgage and Housing Corporation
800-668-2642
www.cmhc-schl.gc.ca
This government-run organization is a world leader in providing useful
home-building and renovation info especially for Canadian conditions.
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