Mechanicals-Bathroom Ventilation

Bath fans serve a few different functions.  Removing moisture, removing odors, and if noisy enough, to drown out sounds.  For the health of the home, removing excess moisture is most important.  This blog will be all about selecting, sizing, installing, controlling and testing a bath fan.

Walking through any of the big box store’s bath fan section can be a little overwhelming.  Dozens of choices, just a fan, a fan with light, a fan with a light and night light, a fan with a light and heater.  There’s even a fan, light, and a speaker.  That should drown out noises!  Ceiling mounted, wall mounted, …lots of choices.  I’m not going to help you with style, that’s up to you.  What I hopefully can help with is sizing, controlling, installing, and venting the unit.

Bath fan choices at a big box store.

Let’s start with sizing.  A bath fan is rated by how many CFM or cubic feet per minute of air it can move.  The 2012 International Residential Code states:

M15007.4 Local exhaust rates. Local exhaust systems shall be designed to have the capacity to exhaust the minimum flow rate determined in accordance with table M1507.4

The table states that bathrooms and toilet rooms require mechanical exhaust capacity of 50 cfm intermittent or 20 cfm continuous.  Depending on the size of the bathroom, 50 cfm will work well if installation is correct, meaning the ductwork isn’t impeding air movement.  More on that later.  If the bathroom is large, a rule of thumb is 1 cfm per square foot.  An 8 x 10 bathroom might have a bath fan capable of moving 80 cfm.  A 5 x 7 bathroom is only 35 square feet, but according to the 2012 IRC, cannot have a fan smaller than 50 cfm.

If you are planning on having a fan that operates continuously, 20 cfm meets code.  This is creating an exhaust only situation within the home, which I’m not a fan of in my climate.  Depending on where the bath fan is located within the home, upper floor or basement, that continuous operation may increase stack affect during the winter months.  Exhausting more air out the upper area of the home will mean outside air will be drawn in the lower areas of the home.  If it were me, I would use the fan only when needed.  If continuous ventilation is needed within the home, install an HRV.  (Read about HRV’s here.)

I have heard some discussions about oversizing bath fans to quickly remove any moisture in the space needing ventilation.  To me, this makes sense. Moving from a 50 CFM fan to an 80 CFM fan may have another advantage. Overcoming duct restrictions in a “not perfect” installation.  I’ll get to ducting a fan in a bit.

Installing a bath fan in an existing home can be a challenge.  Is the fan location between two floors or in an attic?  How far is it to an outside wall or roof? You’ve not only got to think about the location within the room needing ventilation, but also how you are going to run the ductwork and get power to the unit.  Sometimes the ducting will need to be installed before the fan.

Let’s start with a between floor installation.  This installation is typically going to be more difficult than installing in an attic because the work is completed from the room where the fan is being installed.  Limited space between floors makes connections difficult.  There are “remodeling” exhaust fans on the market that are designed to be installed without a solid connection to the framing.  These units are supported from the drywall or other ceiling materials. You still need to be able to get power to the unit and exhaust to the exterior, but the hole cut in the ceiling will only be as large as the fan itself.  Installation of this type of fan is limited to a select few “remodeling” style fans.

Sometime running ductwork and/or getting power to a fan requires removal of more ceiling.  At this point, any fan can be used.  You will be able to attach the fan to building’s framing members, which is a typical installation.  There’s more room to install the venting and power the unit, but the ceiling will have to be repaired after the installation.

Installation in an attic usually will be easier.  Most work can be completed in the attic, as long as the installation isn’t too close to the roof eave.  Any bath fan can be used because working from the top side, you will be able to attach the fan to the roof trusses or other wood framing.  Ducting and finding power for the unit is also much easier.  My advice is to locate where the fan will be in the attic and remove the insulation from this area.  Cutting the hole first may result in a lot of insulation dumping on your head, especially if it is blown fiberglass or cellulose.  (Always check the type of insulation present in the attic before starting the installation.  If vermiculite is present, assume it contains asbestos and handle accordingly.)  If there is spray foam installed in the attic, plan on spending some time digging it out.  I’ve had to remove just enough in remodeling projects to know it does not come out easily.

How about a wall installation?  Not recommended in a northern climate if installed in an exterior wall!  Some fans are designed for straight through installation while others require ducts to be installed.  Most of the units designed for wall installation will displace a lot of insulation, reducing wall R-values.  Units installed in an exterior wall cavity may become a condensing surface in a room with elevated humidity levels.  There may also be moisture damage or rot within the wall cavity where the fan is installed.  If a wall installation is needed, install the fan on an interior wall, ducting should be installed vertically, then out a sidewall, rim joist or roof.  The large holes required for the ductwork (3-4 inches) will make horizontal ducting nearly impossible.

You’ve got the fan installed, now you need to figure out how to seal the fan frame to the air control layer and assure there is enough insulation around the fan.  Do you know where your air control layer is located?  In my climate, it’s usually polyethylene sheeting (Minnesota is one of the last states still using poly).  It could also be drywall, wood sheeting, spray foam, or some other materials.  Tapes, caulks and spray foam will all work to air seal the fan frame.  Also be sure there is enough insulation over the fan in attic installations.  I’ve seen foam boxes installed over the fan with cut-outs for the electrical connection and venting with mounded loose-fill over the top.  Sometimes there isn’t a lot of room for insulation.  If the fan is installed in a vaulted ceiling or close to the eave, insulation values will be less.  This heat loss may result in spots on a roof with no snow coverage during the winter, possibly causing ice damming.

OK, you’re ready to install the ductwork for the fan.  What are the best materials to use and what are the code requirements?  The best duct is going to have low air flow resistance and the best installations are going to have as few bends and shortest run as possible.  This is true of all ductwork, whether a bath fan, dryer vent, or a forced air heating system.  If possible, use solid metal ducts. They are going to require the most time and effort to install, but they will move the highest amount of air.  Sometimes it’s not possible to use hard-pipe ductwork, a high-quality flex duct will be your second choice.  This style of ductwork looks like a giant slinky wrapped in a foil or plastic covering.  It bends (kinks and crushes) easily, the length of the duct can be adjusted, and usually the duct will be one continuous run.  Again, be sure to use the fewest number of bends and the shortest length possible.  Every extra foot and bend will reduce the amount of air moving through the duct.  If possible, slightly slope the duct towards the exterior to drain any condensation that may form within the duct.  Secure the duct to both the fan and outside wall cap using a quality foil tape (not standard duck or duct tape). Good foil tape will have a paper backing that needs to be removed to expose the “sticky” part of the tape.

A roof vent kit that is supposed to contain all needed materials. The flex-duct is not insulated and does not meet code for my climate.

Minnesota requires that all ductwork in unconditioned spaces be insulated to at least R-8, meaning if your bath fan ductwork is in an attic, it needs to be insulated for its entire run.  This insulation is typically some sort of fiberglass.  Closed cell spray foam is also allowed to insulated ductwork, but at a higher expense.  If the ductwork is between floors and exits a rim joist, the first 3 feet from the exterior connection will need to be insulated to R-3.3 (it will probably be easier just to insulate the entire run).  In both attic and between floor installations, a vapor barrier will need to surround the required duct insulation to reduce the risk of condensation forming on the ductwork.  If using flex duct, you can purchase the duct, insulation and vapor retarder in one assembly.  If installing rigid metal duct, the insulation with vapor barrier will be purchased separately and installed during the installation of the ductwork.  The outer layer of insulation will also need to be sealed at both ends with that high quality foil tape.

The outside wall cap requires a damper to limit the amount of air (and bugs during the summer) that can move through the ductwork and enters the home.  Most wall caps will have a gravity or spring system to keep the dampers closed when the fan is not in operation.  Do yourself a favor and spend a little extra money to buy a decent wall cap.  I’ve installed cheap units that break and fall apart during installation or leak a lot of air during the heating season.  A few manufacturers to look into are Seiho International and Primex.  Thanks to @terrawattblake on Instagram for the info, and also Dryer Wall Vent, thanks to @nielsencristcarpentry and @pytiakdb, also both on Instagram.  Location of the wall cap is also important.  Keep them away from windows and other wall caps that intake fresh air.  Last thing you want is that moist (and possibly bad smelling air) that was just exhausted from the home to end up in your fresh air intake of an HRV, ERV or entering back into the home through an open window.  I also recommend keeping the wall cap as far away from a roof eave as possible.  Many roofs are ventilated, air moves from the soffit, into the attic and finally out a ridge or static vent located at or near the top of the roof.  Allowing moisture from an exhaust fan to enter a ventilated roof system isn’t a good idea.

You could also use a roof mounted damper system, but I recommend only as a last resort.  Sometimes it’s the only option available, I’ve installed many of them.  Just be aware that the vent may end up covered in ice and snow, reducing its effectiveness during the winter, when you need it most.  My last home had two roof mounted dampers that I could hear flopping during windy conditions.

Metal wall cap with foam air gasket. About twice the price of the plastic versions.

Lastly, be sure to do a good job sealing the vent to the wall or roof, making it water-tight.  Roof mounted damper systems will need to be flashed correctly with the roofing material.  Wall caps should be, if possible, sealed to the weather resistive barrier and siding.  Air sealing should also be completed on the interior of the wall cap.  All this is much easier to accomplish in new construction.

Always inspect the wall cap or roof vent for proper operation after the fan is operational.  Turn on the fan and look to be sure the dampers open and close correctly.  I have seen vents stuck in the open position without the fan operating.  Even though there is usually a second damper at the bath fan, this damper creates a poor seal, air will move past and end up in the bathroom.

The required electrical connection will depend on the type of fan.  The National Electric Code does have code requirement for bathroom wiring. GFCI (ground fault circuit interrupter), and now AFCI (arc fault circuit interrupter) protections may be required.  If installing a bath fan unit that also has an electric heating element, a second electrical circuit may be required.  I don’t recommend unexperienced people to try to install or modify electrical circuits within there home.  Electricians are expensive, but way cheaper than an injury to yourself, a family member or guest in your home.

Part of the electrical connection for a bath fan is the control.  A simple wall switch is easiest but is often not turned on.  Wiring the fan to operate with the bathroom lights can be an option.  Timers are another choice, again they require someone in the bathroom to turn them on.  Simple occupancy sensors work well.  Some are programmable so that they remain on for a period of time after a person leaves the bath.  There are controls, and even some bath fans that monitor humidity levels within the room and automatically turn on and off.  Installing a bath fan is a lot of work, make sure it is being used by choosing the best control for your situation.

Testing a bath fan with an exhaust fan flow meter.

You’ve gone through all the work to install a bath fan, testing the fan for proper air movement is the last step.  My preferred testing method requires a flow pan and digital manometer.  Only costing around a grand, I’m sure everyone reading this blog will order one today!  Ok, maybe not.  There are a couple of low-cost tests that can be used, the first will show that the fan is moving air, the second will give a semi-accurate flow rate.  The first test: tear a single piece of toilet paper off a roll and with the fan running, see if it sticks to the fan shroud.  If it falls off, you’re not moving enough air.  If it sticks, air is moving.  How much air?  You have no idea.  A second test requires a garbage bag, a cardboard box or metal coat hanger that is large enough to fit over the bath fan, and some duct tape.  If using a box, cut the bottom of the box out and secure the garbage bag to the inside of the cut box, you may have to use duct tape to secure the bag to the box.  If using a metal coat hanger, bend the hanger so that it is round and attach the garbage bag.  Duct tape again might be needed.  Expand the garbage bag so that it is fully inflated, place over the bath fan, pressed against the ceiling or wall to create as tight a seal as possible.  Turn the fan on and count how long it take the garbage bag to deflate.  You will need to know the size of the bag being used, a smaller garbage bag, 26 x 36 inches, which deflates in 2 seconds is moving approximately 75 cfm, 4 seconds, 40 cfm, and 10 seconds, 20 cfm.  A larger bag, 31 x 47, 2 seconds is approximately 210 cfm, 4 seconds is 105 cfm, 6 seconds is 75 cfm, and 10 seconds is 40 cfm.  A website that better describes the garbage bag air flow test is:

This test isn’t exact, but it’s better than the toilet paper test.

If the test is a success, congratulations, your done!  But what if you’re not moving enough air?  These trouble shooting suggestions will also work for an existing fan.  Is the outside damper opening?  Is the ductwork connected to the fan and wall cap or roof vent?  Is the ductwork straight, and as short as possible?  How many elbows, bends or kinks are in the ductwork?  Is the fan motor running?  Is the squirrel cage that moves air spinning?  If an existing fan, is the ductwork clear of debris?  (I’ve heard of bee’s nests and mice nests in bath fan ducts.)  Is the decorative fan shroud full of dust? Is the inside of the bath fan clean?

Last piece of advice, read and follow the manufacturer’s instructions.  I don’t know how many times I’ve had to do something twice because I was sure I new how to install or operate something, only to find out the instructions required something different.  Read before you start.

As always, leave a comment here or DM me on Instagram, I want to know what information you’re looking for along with ideas and suggestions that will make this website better. Thanks for reading!

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