Make a Bathroom/utility Fan Quiet With Sorbothane

This project has been a long time in the making. Since I bought my house almost 6 years ago, I've been trying to figure out a good way to add a bathroom vent fan to the first floor full bath with the least renovation possible. I finally took the plunge into the project this spring. I decided to install the Broan 512M 6" Through-the-wall Fan. I'm pretty sure this is actually a utility fan, but it works as a bath fan too. It's simple and cheap and it would fit perfectly in to the bathroom soffit in my case. At 3.5 sones, this is not a very quiet fan, so I needed to come up with some options for improving the sound level. Using some Isolate IT! Sorbothane noise absorbing sheet rubber I was able to reduce the noise by 1 sone and eliminate the vibration that many people complain about with this fan. Here's how I did it.

In this case, I cut a 6.75" hole in the soffit. You can see it in the top corner of the whole bath photo (just above the shower door). I ran 6" flexible duct over the shower enclosure to the outside vent. I also included an in-line butterfly backdraft damper to make sure that I don't get any chilly air coming back in even on the windiest days. All that is already in place. So now it's time to fit the fan. To cut down on the fan noise, I decided to ring the outside of the housing with ultra-soft Sorbothane rubber. There is a lot of research on how Sorbothane can cut down noise and deaden housings. One study found that 1cm of ultra-soft 30 durometer Sorbothane can cut down 10 dB of noise. In this case, I am limited by the space between the mounting studs and the fan, so 1/8" was all I could fit. Also, I wanted to be able to easily stick the Sorbothane to the housing so I selected an Isolate IT! Sorbothane 1/8" x 6 x 12" Acoustic Sheet with 3M pressure sensitive adhesive (peel and stick). I only ended up needing about 1/3 of the sheet, so keep an eye out for more projects. This sheet can be cut with scissors. It was really easy to work with.

I cut 12" long strips that were about 1.25" wide. The Isolate IT Sorbothane comes with a plastic on it to prevent it from sticking to itself and picking up dust. I decided to leave this on which isn't a problem since it won't be visible anyway. Peel off the 3M adhesive release paper and stick it to the fan housing. Be careful not to cover up the electrical access plate. You could cover more of the housing, but 1.25" fit perfectly with where I wanted to attach the flexible ducting.

Since I wanted to be able to remove the access plate during installation and for any future work, I added 2 little pieces of Sorbothane to it. Again, I left the plastic on the exposed side of the rubber.

I made sure to have at least 2 of the 4 mounting screws anchored in studs. This made sure the fan mounted firmly and helped keep vibration down. I added some wood blocks behind the sheetrock for the other 2 screws to give them something to really bite into. If you really wanted to cut down on vibration, you could add some Isolate IT Sorbothane bushings to fully decouple the fan from the wall. For this project, just the acoustic isolation sheet was enough for my purposes.

Final notes: I used 40 durometer Sorbothane for this project. They don't sell the 1/8" sheet in 30 durometer because it is too soft and deforms during production. The 40 durometer (Shore OO scale) worked just fine. Sorbothane is really sticky/tacky especially in the soft 30 and 40 duro versions. You might be able to do this project with the non-3M PSA sheet, but I wanted to make sure that it stayed on the fan for years to come.