Introduction: How to Replace a Bathroom Exhaust Fan

About: Avid DIYer, hobby chef and gluten-free food blogger. Would have become a chef years ago when I went back to school to get my chef's certificate but no one trusts a skinny chef!

They say nothing is certain but death and taxes, but I’d like to add a third: repairs! In one week alone, three things broke down on us that had to be repaired. When you’re a do-it-your-selfer and a blogger, you have to look on the bright side and call that a good week: repairs alone can give you a ton of things to write about!

For the Home Improvement contest, I’m sharing a DIY on how to replace a bathroom fan. 'Hidden' repairs are ones that go unnoticed, but they can be the most important DIYs you'll do because they protect the investment in your home - and your health. A well running (and installed) bath exhaust fan will help prevent mold growth which can trigger asthma and allergies. If you notice moisture stains on your walls or ceilings, metal corrosion, visible mold, peeling paint or wall paper, cloudy windows and high levels of humidity, it's time to change that fan. Other than the tight and awkward quarters you have to work in, it's a fairly straightforward repair - and well worth the effort to know that it's done right.

We weren't sorry to see our old fan go; It was so loud that my wife could barely hear me singing rubber ducky to her just outside the bathroom door. We really should have replaced it long before it conked out though: as (bad) luck would have it, ours stopped working when the hottest and most humid stretch of weather hit us. Ironically, that was the same morning that our air conditioner broke down too, so by the afternoon our house was as hot as you know what and the attic was even hotter! Talk about sweat equity!!

Step 1: You Will Need

  • replacement fan
  • ducting (various lengths and configurations)
  • tyvek coverall
  • safety mask
  • tuck tape
  • duct insulation (only if you can't reuse the old stuff)
  • gloves (to wear while moving insulation)
  • drill
  • drill bits
  • sheet metal screws
  • tin snips
  • electrical bushing
  • cardboard box to help transport supplies all at once
  • portable light (and extension cord to run to nearest electrical outlet)
  • ladder
  • plywood (to put between the joists to help you walk around the attic)

Step 2: Remove the Old Fan

To start, I cut the power to the bathroom so I could disconnect the wiring safely without risk of electrocution! I then removed the old fan so I could find a replacement that would fit without having to cut a bigger hole into the drywall. I temporarily taped a plastic bag over the hole after removing the fan.

Since the fan is also connected to the light, and there isn't a window in the bathroom, I set up a work light outside the door.

I protected all the surfaces in the bathroom by taping plastic to the walls and on the floor to catch any insulation/mess that might drop down when I was working in the attic later.

Step 3: Inspect What You Have So You Can Purchase Accordingly

I inspected the condition of the old ducting in the attic and discovered that the original 4″ pipe attached to the vent was poorly installed and there were a lot of gaps. I also discovered that the builder cut too big a hole into the roof – which further explained the gaps. The picture shows what the old duct (and fan) looked like.

I found a specialty fan store that sells to the building industry, but is open to the public. I took the old fan with me to get the same dimensions to fit the hole in the ceiling, but I upgraded to a whisper quiet fan (only 1 sone). Any fan under 1.5 sones is considered to be quiet so keep that in mind when shopping.

Another thing to keep in mind is the diameter of the duct connector on the new housing. To maximize performance, try to match your duct diameter to the new fan. Our duct was originally 4″ wide but because of the larger hole in the roof left by our builder, I opted to use a 5″ gasket in order to bridge the gaps at the roof vent and replace the 4″ ductwork with 5″ fittings. However, the replacement fan was 4" so I bought a duct reducer (installing the 4″ end onto the fan and the 5″ end onto the new ductwork). There’s nothing wrong with increasing the size of the ductwork, but don’t ever do the opposite or you will restrict the exhaust from the fan!

All-in, it cost about $125 for the fan and supplies.

Step 4: Determine Your CFM Rating

With respect to performance, a fan's ability to move air is measured in cubic feet per minute (CFM), so look for a CFM rating that will meet your needs by moving enough air for the size of your bathroom. To determine your CFM rating, use the following formula:

Length x width x height of room x .13 = the minimum CFM rating

In addition to the fan, I purchased a variety of new fittings. Get more than you think you need and return what you don't use; there's nothing worse than being stuck in the attic and then realizing that you have to run out to buy something you didn't get!.

Step 5: The Real Work Begins

I suited up in a white Tyvek coverall, like the one pictured, to protect against the scratchy insulation. I also wore a heavy duty mask: if you've ever had mice in the attic, breathing in small particles from the mess they leave behind can make you sick. You need to take the precaution of wearing a mask so you don't breathe in any toxins.

This is off topic, but if you do find signs of mouse activity when you get up there, you can toss bags of warfarin pellets (if you can still purchase them) around the perimeter of the attic to get rid of them (that's all extermination companies do). If you don't use it all up be sure to lock it away where kids and animals can't gain access to it for obvious reasons.

Step 6: Gather Supplies

I placed a ladder beneath our attic access and removed the panel (ours is in the bedroom closet). I took all my equipment up in a box to keep it all together and do the trip only once. This included a drill, screws, screw driver, tin snips, duct fittings, fan, electrical bushing, silver tuck tape, etc. I also took a bright light on an extension cord up with me to see (the light was run to another electrical power supply that was still working).

If you haven't previously done work in the attic, you should place some runner strips of plywood across the joists so you can walk around without risk of falling through the drywall! I had already ripped down some plywood for this purpose a few years ago so I was good to go. Once in the attic, I pushed aside all the blown in insulation so I could locate the electrical wiring and hole in the ceiling of our bathroom (this is where covering the hole with a brightly covered bag came in handy; it was easy to spot). Don some gloves when handling the insulation - it can be itchy.

Step 7: Installation

I removed the plastic bag, positioned the new fan body over the hole in the ceiling and then screwed it into the joists. A metal strip (shown in the first picture) was attached to the back to help secure it further to the joist (it can reduce side to side vibration). Depending on where your hole is positioned between the joists, you may have to install anywhere from one to 4 of these strips to secure it.

Our fan was positioned right beside the joist so I only needed one new strip at the back.

Next, I installed the electrical bushing onto the fan (it protects the wire) and then fed the wire through and connected it. I used tuck tape to seal all along the edges of the fan.

Step 8: Connect Gasket and Dry Fit

At the roof line, I used a 5" gasket with a seal around it for the connection to the roof vent. This is a much better solution than the straight run with release cuts the builder previously installed because it seals any gaps. I used tin snips to cut away one side of the gasket to fit it flush against the joist in order to line it up with the roof vent. Once fit, I peeled the tape off the gasket and pressed it up onto the underside of the roof. I pre-drilled and inserted screws all around the gasket.

I dry fit the metal ducting, starting with the reducer at the fan, until I eventually got it all to line up with the roof vent.

As you can see in the last picture, one of the pieces of ducting is articulated so it can be twisted into just about any position to line the duct work up with the roof vent.

Step 9: Connect to the Roof and Seal

With all the dry fitting complete, I pre-drilled a hole into each duct joint and installed a few 8 x 1/2"screws to hold the sections into position. Then I wrapped each joint with silver tuck tape to seal it.

Step 10: Test Electrical Connection and Insulate

Before finishing the insulation, I turned the power back on to make sure everything was running smoothly. Then I turned the power back off again as a precaution and went back into the attic to wrap the pipe with insulation and tie it on with cord (I reused the old insulation that was originally there).

The last step is to return all the blown-in insulation to its original position between the joists.

Step 11: You're Done

I brought all my tools back down and closed the attic access panel.

The last step is to install the plastic ceiling cover over the fan to finish it off; it attaches with metal clips. Now the fan purrs like a kitten; bring on the rubber ducky!

Step 12: Please Vote

If you found this Instructable helpful, please vote for it in the home improvement contest!