than that, allot 50 cfm for each
has been turned off. Or put the light and the fan on separate switches. You can also
fixture (toilet, lav, shower), and
connect the fan to a humidistat, which is a moisture sensor that will turn off the fan
allow 100 cfm for hot tubs.
when a preset moisture level is reached.
Noise won’t be much of an issue. Whereas
cheap bath fans are rated at 3 or 4 sones, some
energy-efficient models run at less than 0.3 sone.
Remote inline fans are another option: They’re
larger and noisier, but because they are typically
located in attics or crawlspaces, they’re out
of earshot.
To see how simple whole-house ventilation
can be, let’s look at installing a bath fan.
Installing a bath fan. Place the fan near the
shower—over it if possible—and run a duct from
the fan out the roof or through a gable-end wall.
Soffit and sidewall vents aren’t as desirable
because expelled moisture could get drawn up
into the attic by a soffit-to-ridge updraft. Keeping
moisture out of attics and wall cavities is crucial,
and you can help achieve it by caulking the fan
housing to the ceiling and sealing each duct joint
with foil duct tape, not cloth-backed duct tape.
Before screwing a bathroom fan box to a ceiling joist, apply several parallel beads of silicone caulk
Position the fan on the ceiling. If there is an
between the box and the joist to minimize vibration. The scrap of drywall to the right of the box
unfinished attic above, drive a screw up through
ensures that fan-box flanges will be flush to the finish drywall ceiling.
the ceiling to approximate where you want to put
the fan. Make a cardboard template of the fan
box (housing) and then go up into the attic. Once
you’ve located the screw (you may have to move
insulation to find it), place the fan template next
to the nearest joist—most fan boxes mount to
ceiling joists—and trace around it. (If the fan box
has an adjustable mounting bar, you have more
latitude in placing the fan.) Use a jigsaw or recip-
rocating saw to cut out the opening in the ceiling.
To keep the drywall cutout from falling to the
floor below, screw to the drywall a piece of scrap
wood slightly longer than the cutout.
Mounting the fan. To mount the fan box, you
may need to remove the fan-and-motor assembly
first. If the housing flange mounts flush to the
underside of the ceiling, as shown in the photos
at right, use a piece of drywall scrap to gauge the
depth of the unit relative to the finish ceiling. But
whether the housing flange sits above or below
the ceiling drywall, caulk the flange well with
After mounting the fan box, attach ducting to the fan box’s exhaust port. Keep duct runs short, and
seal metal duct-fitting joints by wrapping them with self-adhering foil tape.
Energy Conservation and Air Quality
401
polyurethane sealant to create an airtight seal
circle onto the roof. If the circle would cut into
between the two materials. To further secure the
the tabs of any shingle—roughly the bottom half
fan and anchor the edges of the drywall opening,
of a shingle strip—use a shingle ripper to remove
run blocking between the joists—along two sides
those shingles before cutting the vent hole in the
of the opening—and screw the drywall to the
sheathing. Be gentle when removing asphalt shin-
blocking. In some cases, you’ll be screwing
gles and you can probably reuse them. Use a
through the fan’s housing flanges as well.
hooked shingle knife to cut the circle into any
P R O T I P
Follow the wiring diagrams provided by the
remaining shingles and the roofing paper.
manufacturer. In general, it’s easier to run elec-
Flash the fan’s roof vent as you would any
Moisture can condense in
trical cable through a switch box first because
other roof vent: Feed its upper flange under the
bathroom-fan ducts that run
junction boxes inside fan housings tend to be
shingle courses above and over the courses below.
through unheated areas. To
cramped. Bathroom fans should be protected
Caulk or nail the flange edges per the installation
reduce in-duct condensation,
by a ground-fault circuit interrupter (GFCI); see
instructions and renail the surrounding shingles.
insulate the ducts; keep the duct
chapter 11 for more information.
Once the roof vent is flashed, go back under the
runs short; and if you’re venting
Installing the roof vent. Keep duct runs as short roof and attach the free end of the duct, also seal-
the duct through a wall, slope
as possible to reduce air resistance. After attach-
ing that joint with metal duct tape.
the duct downward toward the
ing the lower end of the flexible duct to the fan’s
wall cap so the moisture will run
exhaust port and sealing the joint with metal duct
out if it does condense.
tape, hold the free end of the duct to the under-
side of the roof sheathing (or gable-end wall) and
trace its outline onto the sheathing. Drive a screw
through the middle of the circle. Then go outside
REmotE Inline FAnS
and locate the screw, which represents the middle
Even a well-made bathroom fan will be relatively
of the vent hole you need to cut. Sketch that
noisy if its motor is in the bathroom ceiling 2 ft.
from your head. But if you install the fan some
distance from the bathroom, you’ll reduce the
Roof vent
noise considerably. That remote location may
zzzzzz A Bathroom Fan
mean longer duct and wiring runs, but routing
them is rarely a problem. In fact, with a large-
enough fan motor and multiple ports, you can
vent two bathrooms with one fan. Because lon-
Flexible metal duct
ger duct runs can mean greater air resistance,
consider installing rigid-metal or PVC ducts,
whose smooth surfaces offer less resistance,
rather than flexible metal ducts. Alternatively,
you could oversize the fan slightly: Remote fans
are noisier than standard bathroom fans anyway.
Tucked away in an attic or crawlspace, who’ll
hear them? Better fan makers such as Fantech
Metal foil tape
and American Aldes® offer acoustically insulated
Silicone caulk between
cases to deaden sound further.
flange and drywall
Fan box
Ceiling drywall
To keep moisture from leaking into the attic, apply silicone
caulk between fan-box flanges and mating surfaces, such
as drywall. To ensure airtight joints, use metal foil tape to
seal ducting to the fan’s exhaust port and to the roof vent
or sidewall vent.
402 Chapter 14<
br />
A BAlAnCEd VEnTIlATIon SySTEM
Adding a whole-house exhaust fan and a handful
Heat-recovery ventilators (HRVs) can be incorporated into existing forced
of passive intake vents will get rid of stale air and
zzzzzz Balanced HRV System
hot air ductwork cost-effectively. HRV systems extract heat from outgoing
admit fresh air for a modest investment. But the
exhaust air and use that heat to warm incoming fresh air—thus saving
cycle still wastes money. Stale though that
energy. In hot climates, an energy-recovery ventilator (ERV) uses outgoing
exhausted air is, it has been heated or cooled at
stale air to cool incoming fresh air; ERVs can also temper the humidity of
great expense. And fresh intake air still needs to
warm, incoming air. Because either system has both intake and exhaust
be conditioned. So mechanical engineers next
vents, there is no danger of depressurizing the house.
devised a heat-exchange system that would use
exhaust air to temper incoming fresh air. At the
heart of this balanced system is a heat-recovery
Exhaust air ducted
Remote fan with
to HRV
multiple ports
ventilator (HRV) in cold climates or an energy-
recovery ventilator (ERV) in warmer climates.
Typically, an HRV/ERV system has two fans,
one to bring in fresh air and one to expel stale air.
Return
Its heat exchanger recovers 75% to 80% of the
air
heat in the outgoing air and uses it to temper
incoming air. As both air streams move through
the exchanger, it can filter pollen and dust from
Vent the range
incoming air and remove excess moisture. And,
hood separately.
of course, by equalizing air pressure in tight
houses, the system prevents negative house pres-
sure and backdrafting. And the system can be
installed cost-effectively because it can be tied
Fresh air
into existing duct work. On average, HRV/ERV
intake
elements can be retrofitted for $1,500 to $3,000.
The principal expense is not the equipment, but
the labor required to cut into (and repair) finish
surfaces to add ducts.
Exhaust
air vent
Control ing Moisture
and Mold
Return-air
HRV uses exhaust air
plenum
to temper incoming air.
Mold needs three things to grow: water, a tem-
crawlspaces; (5) inadequately vented bathrooms,
perature range between 40°F and 100°F, and
kitchens, laundry appliances, water heaters, and
organic matter, such as lumber or paper. If your
furnaces; and (6) leaking HVAC ducts.
renovation includes gutting finish surfaces,
By far the largest source of interior moisture
replacing standard drywall with paperless dry-
comes from kitchens and bathrooms. But as
wall (p. 422) is a smart move in high-humidity
we just looked at installing an exhaust fan and
areas. Otherwise, the best way to thwart the
treated the other sources elsewhere, let’s next
growth of mold is to exhaust excess moisture—
consider sealing crawlspaces.
send it outdoors, that is.
Interior moisture generally isn’t a problem
SEAlInG CRAwlSpACES
unless it’s excessive and sustained. (Relative
Most crawlspace or basement wetness is caused
humidity indoors should be 35% to 40% during
by improper surface drainage—especially
the heating season.) Signs of excessive moisture
clogged gutters and downspouts. For more on
include condensation on windows, moldy bath-
reducing dampness caused by exterior water, see
rooms or closets, soggy attic insulation, and exte-
chapter 10.
rior paint peeling off in large patches. Figure out
Crawlspaces are well named: They tend to be
where the water’s coming from, and you’re half-
dark, dank, dirt-floored areas only a few feet
way to solving the problem.
high. To disperse moisture, building codes typi-
cally prescribe 1 sq. ft. of screened vents for each
SouRCES oF EXCESSIVE MoISTuRE
150 sq. ft. of dirt floor, or 1 sq. ft. of vents for
Common sources of excessive interior moisture
every 1,500 sq. ft. of floors covered with a mois-
are (1) air infiltration; (2) poorly installed
ture barrier. Problem is, open crawlspaces mean
roofing or incorrectly flashed windows and doors; cold floors and heat loss in winter, and in sum-
(3) improper surface drainage; (4) unsealed
mer, warm, moist air entering through vents
Energy Conservation and Air Quality
403
invariably condenses on the cooler surfaces of
becomes more difficult if there are masonry
the crawlspace—leading to mold and worse. So it piers or wood posts present. In that case, use
makes more sense to seal and condition crawl-
two pieces of polyethylene to cover the floor, with
spaces. Otherwise, mold spores growing in the
each piece running roughly from the base of a
crawlspace will be sucked into living spaces by
post to the crawlspace perimeter. Slit the plastic
bath and kitchen exhaust fans and carried all the
and run it up 6 in. to 8 in. onto each pier; caulk
way up to the attic by the stack effect of rising
and tape the plastic to the pier. If wooden posts
heated air.
rest directly on masonry pads, jack each post
To seal crawlspaces, first rake the floor to
enough to slide a piece of metal flashing or heavy
remove debris and sharp rocks, which could
plastic underneath; otherwise, moisture will wick
puncture a plastic moisture barrier. Heavy sheet-
up through the post and eventually rot it.
ing will last longer: 6-mil polyethylene is mini-
The wall portions of the sheeting will be less
mal, but commercial waterproofing firms, such
likely to pull loose if you mechanically attach
as Basement Systems®, use 20-mil polyester
them. If you use sheeting as heavy as a pool liner
cord–reinforced sheeting, which can withstand
(20-mil), you can drill holes through it into the
workers crawling and objects stored on it. Seal
concrete and drive in nylon expansion fasteners.
vent openings by gluing 2-in.-thick EPS foam
Lighter grades of polyethylene can be wrapped
panels over them, using a spray foam or a poly-
several times around furring strips and then
urethane sealant such as Vulkem 116, which
attached to walls with powder-actuated nails.
adheres well to masonry.
(For this operation, eye and hearing protection
In a 1,000-sq.-ft. rectangular crawlspace with-
are essential.) If condensation persists in the
P R O T I P
out jogs, it typically takes five large sheets of
sealed crawlspace, insulate the walls with EPS
polyethylene to isolate the space: a single floor
foam panels. Alternately, if basement walls are
If you’re unsure whether
sheet that runs about 1 ft. up onto walls and four dry, you can spray foam insulation onto walls in
basement-wall wetness is caused
wall pieces that overlap at the corners and the
lieu of foam panels. Ultimately, it may be neces-
by moist interior air condensing
floor by 1 ft. and run up the walls to a height 2 in. sary to add a dehumidifier to condition humid
or ground water seeping
through, try this: wipe dry a sec-
to 3 in. below the mudsills. (Leave mudsills
crawlspace air.
tion of wall, then duct-tape a
exposed so they can be inspected periodically.)
1-ft. by 1-ft. piece of aluminum
Because the sheets are heavy, cut them outside
ClEAnInG up Mold
foil to the dried area. In a day or
on a well-swept driveway, roll them up, and then
Mold can’t grow without moisture, so first iden-
two, remove the tape and note
unroll them in the crawlspace. Overlap seams
tify and correct the source(s) of the excess mois-
which side of the foil is wet.
roughly 1 ft., caulking each overlap with polyure-
ture before you start cleaning up. Otherwise,
thane sealant, and then taping the seams with a
the mold can return. If mold is extensive, hire a
compatible peel-and-stick tape such as Tyvek
professional remediation company.
tape. Use polyurethane caulk to attach the tops of necessary precautions. Limit your exposure to
sheets to the crawlspace walls; if the walls are
mold spores by wearing a respirator mask with
dirty, wire-brush them first to ensure a good seal. N95 filters, rubber gloves, eye protection, and
Because the moisture barrier must be contin-
disposable coveralls, which you should discard at
uous to be effective, sealing floor sections
Poor ventilation and a damp dirt floor helped this gaudy
Poorly supported, shedding insulation, and leaking air, this
fungus blossom on a crawlspace joist. The same conditions duct should be replaced. Low spots in sagging ducts can
encouraged mold to flourish behind the baseboards on the become reservoirs for condensation and mold.
floor above.
404 Chapter 14
find stained or rotted wall plates and extensive
mold colonies.
Throw out moldy drywall. On the other hand,
moldy lumber and engineered wood products
Renovation 4th Edition Page 94
