Two of my bedrooms share a wall with my garage attic. One of the bedrooms started smelling like gasoline after my snow blower started leaking in the garage. After long investigation, I found out the smell is coming from an electric outlet which backs to the garage attic.
So the question is how to seal this electric outlet so my bedroom stops smelling like a gas station.
If you have access to the back side of the electrical boxes, you can seal them using a moldable putty in sheet form. 3M™ manufacturers 3M™ Fire Barrier Moldable Putty Pads, which are intended to be used as a firestop but will also block air flow.
This is probably a good thing to do anyway, since garage walls/ceilings should be a firestop anyway.
If you don't have access to the back of the boxes, you'll have to seal them from the inside. To do this, you'll have to use a couple different products. Seal the openings where the cables enter the box using Duct Seal, making sure to get a good seal all the way around the cables. Duct Seal is a permanently soft, non-toxic compound, that can be molded to seal around cable penetrations.
Then install foam gaskets under the cover plates.
I am guessing that both walls have drywall right? If they don't that is the problem. If you do have drywall you need to probably open up a section behind each outlet. Then stuff in your insulation on all sides of outlet. Then redrywall and mud. On the bedroom side you can use Great Stuff in between the outlet box and open space/insulation. And DA01 mentioned a gasket. I haven't used these but that would be one more barrier.
The most important thing is making sure your walls are air tight from the garage side though. Example... some people are lazy when drywalling garage and don't mud everything. Cutting out a couple squares of drywall, adding a bit of insulation, maybe a gasket, bottle of Great Stuff, doing a little mudding in your garage. Maybe a one hour job and $30-50. Good practice too since garage doesn't have to look good - just needs to be sealed.
Caulks to use:
Silicone could be good for the wire holes. It is flexible, and difficult to paint. In my experience, it's difficult, and often has a short shelf life.
For the box's perimeter, and possibly the wire holes, I would use something flexible and paintable. Acoustic sealant, like Green Glue sealant, or Sashco Big Stretch might be good.
With no access to behind the wall:
Here are ways to seal the wire holes. I don't know which is best. Which is most flexible and crack resistant?
Option 1: Spray foam. Fire block foam could be good. Window foam might be more flexible, but is it safe?
Option 2: Caulk. To get caulk in, tape a tube (like the one from a pen) to the caulk nozzle.
Option 3: duct seal putty
Seal the box's perimeter. If there's plaster around it, it might not be attached well. If so, remove it. Make a big enough gap around the box for caulk. Then, caulk.
With access behind the wall:
Use putty pads. I would still caulk the perimeter and holes first.
Ideally use a box that's better for sealing.
For both ways, use outlet gaskets.
It's also for soundproofing and draft.
