Along the coast where wind-blown rain is a regular weather feature,
it's no surprise to builders that the exterior of a home must be
carefully detailed to manage water. Properly installed housewrap
and flashing are the key elements of an effective water-resistive
barrier, which is essential to protecting walls (see "Common
Callbacks," January/February 2007; www.coastalcontractor.net). But
rainwater is not the only moisture source. The air itself,
particularly the humid air associated with coastal regions, carries
a significant moisture load. This means that sealing up air leaks,
not just water leaks, is a vital step to managing moisture,
especially for homes in hot, humid climates.
Pressure at Work
Understanding the dynamics of moisture and air movement is the
first step for proper moisture management. In hot, humid climates
the outdoor air is obviously warmer, but it also typically contains
much more moisture than the indoor air. When this hot, humid
outdoor air gets pulled inside by an AC system's huge blower and it
hits the cold indoor surfaces, there is a very high potential for
that moisture to condense.
A difference in air pressure between the inside and outside is the
primary reason the outdoor air gets pulled inside. A leaky air
conditioning system tends to put a home under negative pressure.
The culprit is leaky ductwork, particularly in the supply ducts
running through an attic or crawlspace, which lie outside the
building enclosure. Since most AC systems tend to have more linear
feet of supply ducts than return ducts, there are many more
opportunities for leaks. And all those leaks are pushing
conditioned air out of the system, where it is lost into the attic
or crawlspace. An AC system is designed for the return side to pull
the same volume of air as the supply side is supposed to deliver.
When the supply side doesn't deliver that air into the rooms, but
the return side is still pulling that much air from the rooms, the
indoor volume of the home is put under a negative pressure, and
this will forcefully pull air — hot, humid air —
through all the cracks and gaps in the building enclosure.
As soon as all that hot, humid air is pulled through the cracks and
gaps, it hits the cool indoor surfaces of the conditioned interior
and has tremendous potential to condense. Even by itself, humid air
isn't great. Dust mites thrive at high humidity levels, and people
don't. Our comfort zone is somewhere between 30% and 50% RH. But
once moisture vapor has an opportunity to condense, turning to
liquid water, it causes even more problems: it soaks into building
materials and becomes a prime resource for mold and rot.
Now, let's put all this in the context of a coastal home in a hot,
humid climate. Frequent rain and high humidity mean exterior
building materials get wet often and tend to dry slowly. Plus, the
humid air is pulled into the home, where it can wet the interiors
of building assemblies too. A leaky building that is put under a
huge pressure differential by a leaky HVAC system becomes a
superhighway for moisture to infiltrate on the back of the incoming
In this unforgiving climate, mold, rot, and decay will run rampant
if we don't address all of these issues during design and
construction. We need to create a forgiving enclosure by defending
the home on two fronts: (1) we must manage bulk moisture with a
good water-resistive barrier (WRB), flashing, site drainage, and a
sensible roof design; and (2) we need to manage air leaks —
those through the enclosure, which bring humid air inside, as well
as those in the HVAC system, which create conditions that
forcefully drive air through the enclosure.
There is no magic bullet for reducing air leaks. A high-quality
housewrap on the walls, properly lapped, taped at seams, and sealed
around wall penetrations, can reduce air leakage in a typical home
by 15% to 35%, on average. This can go a long way toward improving
a home's energy efficiency and reducing the chance of moisture
But the housewrap is just one component. To be effective, it should
be used in conjunction with other air-sealing techniques to advance
the total performance of the home. We want a "belt and suspenders"
approach to stopping air infiltration that addresses air leaks in
walls, attics, crawlspaces, ductwork, window and door openings, and
all penetrations through the enclosure. Viewed this way, an
effective air barrier on a home means not only housewrap but also
tape, spray foam, sill seal, and even sheathing and blocking that
is strategically placed to limit airflow through building
Here are some of the most important places to focus on air
Rim joist. The rim joist is a common area
for air leakage. It's nearly impossible to get a continuous seal
from the inside, which is one of the best reasons for using a
well-detailed housewrap on the exterior. In addition to being
vulnerable to air leaks, the framing is a strong thermal bridge.
Best practice calls for filling this cavity with a closed-cell
spray foam (Figure 1). Be sure to check local fire code
requirements for any exposed insulation.
Figure 1. Fiberglass is not an air stop and
cannot be relied upon to seal the rim joist (top). Best practice
calls for sealing this area with spray foam (bottom).
Wall penetrations. Where pipes run
through exterior walls, seal them to the WRB with contractor's tape
to keep water out, and foam around the pipe from the inside to
further stop air leaks. The same goes for AC line sets, light
fixtures, and vent terminations that pass through a wall.
Attic bypasses. Plumbing vents, ducts,
wires, and can lights routed through the attic end up riddling the
top wall plates of partition walls with holes that allow air to
leak into the attic. The best solution is to use a foam sealant to
seal wiring and pipes through the top and bottom wall plates
(Figure 2). If you must use recessed lighting, be sure to use
Figure 2. Every pipe and wire that passes
through the top plate is an air leak that siphons conditioned air
away from the living space. And because of the tendency of warm air
to rise (called the "stack effect"), the air leaking out of all
these wiring holes is driven by positive pressure, causing
conditioned air to escape from the home even faster. The solution
is to seal each penetration with a spray-foam sealant.
Combustion appliance flues. Water heater
and fireplace vents are typically installed in chases that open
directly into the attic when the fire blocking is left out. The
result is a wide-open conduit for air leakage though the ceiling.
The solution is to block the top of the chase with a sheathing
material, then cut a hole for the vent, maintaining the
code-required distance from the heat vent and combustible. This gap
can then be sealed with a metal collar (Figure 3).
Figure 3. The top of a vent chase can dump a
lot of conditioned air into an attic. Prior to installing the vent,
cap the chase with OSB and seal the edges with a foam sealant, then
seal the vent to the OSB with a metal collar.
Tubs and shower units are often installed right over open framing
without any insulation or air barrier. Combined with the oversized
cutout in the floor sheathing for the tub drain, you often end up
with a huge raceway for air that is uninterrupted from the rim
joist, through the floor system, and into the walls. The solution
is to insulate the wall cavity and sheathe it before the tub gets
installed (Figure 4).
Figure 4. Before installing a tub unit, the
wall cavity must be insulated and sheathed to stop the free
transfer of air and energy. Thermo-ply or a similar type of
sheathing is a good choice for this application because it doesn't
pack the wall out too far.
Window and door openings. Use a low-expansion foam to fill the gap
between window or door units and the rough openings. Many builders
stuff this gap with fiberglass insulation, but such an approach
won't work. Fiberglass will not stop air. Caulk may not be the best
option, either. The ASTM 2112 window flashing standard recommends
the sill flange along the bottom of the window be left open to
allow water that leaks through the window unit to drain outside.
When this is done, the caulk will never provide a perfect air
HVAC ducts. It's not uncommon to find homes with duct leaks
amounting to 300 to 400 cfm. If this amount of air is escaping the
system and putting the home under a negative pressure that draws
the same amount of outdoor air inside, it can amount to almost half
a cup of water per hour being deposited inside the walls on a
90°F day at 90% RH.
The best product for sealing the seams in metal register boots is a
water-based mastic accompanied by a fiberglass mesh on seams and
larger holes. These mastics have the consistency of mashed potatoes
and can be easily spread over joints in the ductwork with a
paintbrush or gloved hand. The seams in all register boots should
be sealed with this mastic, applied thick enough to prevent the
mastic from cracking when it cures.
Flex duct that is joined to the register boots should be secured
with both duct tape and drawbands (Figure 5). But if you want a
duct tape that actually stays on ducts over time, look for the UL
181 designation. UL 181 duct tapes usually have an acrylic- or
butyl-based adhesive, which is less likely than rubber adhesive to
dry out over time.
Figure 5. The photo on the left shows a poor
(but unfortunately, common) duct connection to a boot. It should be
executed as shown in the photo on the right: Peel back the cover
and fix the liner to the boot with acrylic tape and a draw band, as
shown. To complete the connection, pull the cover back down. Tape
and secure this with a drawband as well.
HVAC ducts also get routed poorly, which can intensify air leaks.
Crushed and mangled supply runs will restrict airflow and increase
the pressure inside the ducts, causing air to be forced out any
leaks that do exist at a higher rate (Figure 6). To avoid problems,
all ductwork should be supported well and runs kept as straight as
possible. Sweeping curves are better than sharp bends (Figure 7).
Do not kink, crush, or otherwise restrict the size of the duct when
running it through framing or around plumbing and wiring
connections (Figure 8). In addition, it's important to caulk around
the register boot where it meets the framing.
Figure 6. The kink in this duct will severely
restrict airflow. Like a kink in a hose, the reduced size increases
the pressure in the duct, causing any leaks to shoot air at a
Figure 7. Fortunately, the HVAC installer
didn't cut out the truss members to make room for the ductwork.
However, this sharp bend could have been easily avoided by
rerouting the duct from the start. Codes require that duct be
installed fully extended, not compressed.
Figure 8. Coordinate the mechanical trades:
This conflict between the HVAC duct and a plumbing drain could have
been avoided by both trades meeting and reviewing the plans prior
to the crews' arrival on site.
Putting Diffusion in Perspective
So far, I have focused only on water and air leaks — the two
biggest moisture movers. But there is a third way moisture can move
through homes: by diffusion. Diffusion is the absorption of water
vapor into a material, followed by the release of that moisture
vapor to a drier side. Vapor retarders — materials designed
to reduce moisture flow by diffusion — were originally
developed for use in colder climates. There they are placed on the
warm interior to prevent moist indoor air from diffusing outward.
But vapor retarders should not be used on the warm side of walls in
hot, humid climates.
When building materials get wet, we want them to dry out as fast as
possible. The only way wet building materials can dry is by
diffusion or evaporation. Therefore, we want to avoid the use of
low-perm products on the exterior, where they can inhibit drying. I
also think it's important to avoid low-perm wall coverings, such as
vinyl wallpaper, in wet areas, such as bathrooms. Instead, opt for
a breathable latex paint over a glass-mat wallboard, which
eliminates the paper face that provides a perfect food for mold.
Steve Easley is principal of Steve Easley
Associates, a company based in Danville, Calif., that provides
building-science training and quality assurance for builders
nationwide. All photos by the author.