Bruce W. Maki,
The installation of an exhaust duct for a bathroom fan is about
the same as the process for installing a dryer vent duct. This
- Locating a suitable area to cut through the outside wall. The
best location is one that has as few bends as possible. Every
turn in the duct increases the resistance to air flow, and sharp
turns are much worse than gentle curves.
- Determining the exact center point of the hole.
- Drilling a small pilot hole through the wall.
- Cutting the large hole from the outside (typically) based on
the exit point of the pilot hole.
- Installing the duct outlet hood and sealing it from the
- Attaching the ductwork upstream from there, and connecting to
Location Is Important:
By far the most important aspect of vent ducting is figuring out
where to put it. Four-inch vent ducts can easily fit in the typical
"joist bay" (space between adjacent joists), but under no
circumstances are such ducts allowed to cut through a joist that
spans any significant distance. The required hole would so badly
alter the structure of the joist that it would be at risk of
breaking. If the duct has to cross a joist it must pass below
In this project I installed a used bathroom fan in my basement so
I could remove fumes from paint, stain, and urethane. It's cold in
the garage and too expensive to heat the place just for drying paint
and urethane finishes. It's quite possible that the air flow rate
(measured in cubic feet per minute, or CFM) is not adequate
to create a completely safe atmosphere. Fumes may still get into the
house, but this is better than what I had before.
In my case I chose a location for the fan based on an easy and
straight run of duct out the wall. This isn't always possible for
other fans or dryer ducts.
|The joist bay where I decided to install the fan
and duct (arrow).
I had to be certain that I would be able to cut through the
outside wall in this bay. If I cut through the next bay to the
right, the hole would go under the laundry room, which juts
out from the house behind that wall.
||The fan housing and a piece of duct.
I measured the distance from the left side of this box to
the center of the duct outlet. I needed to cut the duct hole
in the right location within the joist bay, or else the duct
pipe would not be aligned with the fan outlet.
|Once I determined the best location for the duct
to break through the outside wall, I drilled a small hole
through the rim joist.
I used a spade bit with a couple of 12-inch extensions,
which made a drill bit about 2½ feet long. I did this because
there was a drain pipe in my way and I was too lazy to get a
taller ladder so I could get really close to the action.
Sometimes it's easier to drill holes from far back.
Besides, the foundation on this house is almost 2 feet thick,
which means you just can't get close to the edge of the house to
drill holes like this.
Note: Before drilling through a wall, it's very important
to look around on the other side and make sure there is
nothing in the way. There are few things that can ruin a pleasant
Saturday afternoon quicker than the words "um, honey... I just
drilled through the gas pipe".
||I went outside to see where the hole broke
through. At this point I realized that the duct was going to
cut right through the water table molding (that piece
of wood below the yellow siding... with all the paint
missing). That's not a problem, just something I'd prefer to
|Using the 4 inch diameter duct as a guide, I
drew a circle big enough for the duct.
||I drilled a series of 5/8" holes around the
I would normally use a 4½" hole saw for this, but my hole
saw requires a ½" drill, and my only ½" drill has a
broken gear, so I had to do this the low-tech way.
|I used my reciprocating saw (with a narrow wood
cutting blade) to cut out the circle.
Years ago I cut out a few holes like this, but I didn't yet own a
reciprocating saw. I marked the circle and drilled a ring of holes
tightly packed together. Then I used a keyhole saw to cut between
the holes. That takes more time and effort, but it's certainly
worthwhile if a homeowner has no desire to buy a reciprocating saw.
A jig saw should also work here, providing the blade is long
enough to reach all the way through the rim joist. That could be
well over 2 inches on many houses.
||The hole wasn't very circular, but that didn't
|The duct discharge hood fit just fine.
||I marked the wood where it needed to be cut away
to let the hood sit flush.
|My initial plan was to cut the siding away so
the white plastic would sit flush (or below) the wood siding.
But when I discovered that the duct would cut through the
water table trim, I changed my mind.
It turned out that the siding and the white-painted board
below the water table (whatever that piece is called) formed
an almost-flat surface. The red arrow shows the small gap at
the top when the bottom was pressed flush.
||Or, when the top was held flush with the wood,
this small gap appeared at the bottom. I figured I could cover
this with caulking.
|This dryer duct just a few feet away is an
example of how not to install a duct hood. Rain can get
behind the white plastic piece and enter the building.
This is the laundry room annex that needs to be completely
rebuilt, inside and out. That's why there is no paint on the
siding... there's no point in painting something that will
soon be removed.
Unless, of course, you are like me and take FOREVER to get around
to these projects. In hindsight, painting this annex would have been
a good idea because it's been an eyesore for many years now.
Which illustrates a point: Many people, myself included, defer
repairs and maintenance on things they intend to remodel soon. But nothing
ever goes as planned, I always say. New projects pop up, or some
factor complicates the intended remodeling project and it gets
pushed farther back. Suddenly, it's three or four years later and
nothing has happened, except damage from time and weather. For
example, it would be tempting to delay re-shingling a roof because
you planned on building an addition. But when the addition gets
delayed repeatedly, the roof can leak and cause structural damage,
creating more headaches and expense. But, I digress...
||I drove in some 1½" stainless steel
flat-head sheet metal screws to hold the duct hood in place.
|I caulked around the perimeter with siliconized
acrylic latex caulk.
||The outside work was complete.
|On the inside, I sprayed expanding foam
insulation around the irregular-shaped hole to fill the gaps.
Of course, I sprayed too much foam and it oozed out all over
||This plastic ring is meant to cover that
irregular hole on the inside. After I scraped off some excess
foam, I stuck this ring in place and screwed it to the wood.
|I cut a piece of 4" duct with tin snips.
||Round ducts just snap together. It's harder than
it appears... I always seem to struggle with these things.
|I applied a few pieces of metal foil tape to the
seams, just to prevent them from popping open later.
||All of the round ducting I've bought at Home
Depot in the last couple of years has had this problem... the
crimped end wasn't crimped heavily enough and won't fit inside
an uncrimped end.
So every time, I've had to improve on their crimping by
making a series of twists with a pair of needle-nose pliers.
|For example, the duct on the left is a piece I
crimped myself (because I cut off the crimped part and used it
somewhere), and the other duct is factory crimped.
I should buy a crimping tool, but I don't do enough
ductwork to justify it. My home-made crimps work okay, but
they probably increase the resistance to air flow slightly.
||Before installing any duct pipe I placed some
fiberglass insulation against the rim joist.
|I installed a piece of duct and the fan housing.
The fan was mounted to the joists with small sheet metal
screws at the ends of four steel telescoping brackets (red
I also taped the duct joints with metal foil tape. If you've ever
used duct tape on heating ducts, you know that it doesn't last long.
It does last a bit longer on cool ducts like this, but I only use
foil tape on ducting. In fact, there's some university research web
site out there that points
out how poorly duct tape performs and says that duct tape should not
be used for ducts.
So why do they call it DUCT TAPE then? They should call it
||I connected the supply wires.
|If you've never done any maintenance on a fan,
this might be news to you. Most fans (especially Broan and
NuTone) have a removable motor and squirrel-cage unit that
simply plugs into an outlet that is built into the fan
The light is usually made the same way, plugged into a separate
outlet. So when a bathroom fan is wired, all you are really doing is
wiring one or two outlets.
||The fan unit simply slides into place. It's held
on one end by two tabs in two notches, and at the other end a
single screw secures it to the housing. Quick and simple.
|The completed fan. Normally there would be
drywall around the fan, but this is just an unfinished
What About A Dryer?
Connecting to a dryer is similar, except that usually a piece of
flexible corrugated ducting is used to connect the dryer to the fixed duct.
This allows the dryer to be pulled away from the wall and pushed
back while staying connected. The type of flexible duct to use is
the all-metal kind. Every dryer I've seen recommends against
using flexible plastic duct, because they can overheat and ignite.
They also develop small holes and leak.
I noticed a dryer duct product that looks interesting:
Tjernlund Products, Inc. (1-800-255-4208) This device is
supposed to boost the air flow in a long or restrictive duct.
- Cordless Drill/Driver
- Basic Carpentry Tools
- Reciprocating Saw
- Tin Snips
- Duct Hood, 4"
- Duct, 4"
- Metal Foil Duct Tape
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