Replies

1. GBA Editor
   Martin Holladay | Aug 03, 2015 12:41pm | #1

   Sam,
   To reduce the transfer of smells from on unit to another, insulation is irrelevant. What you need to do is to seal air leaks between units. If that isn't enough, you can install a supply air ventilation system that pressurizes your unit; that will reduce the chance that smells will migrate inward.

   The fact that you have gas appliances doesn't mean that you have to live in a leaky home. Most gas appliances should be properly vented. If you have a gas stove for cooking, make sure that you have a range hood equipped with an exhaust fan, and use it when you cook.

   The techniques used for reducing sound transmission are different from those used to reduce smell transmission. Among the possible details to consider: installing mineral wool insulation between the studs (the denser the better); installing drywall on resilient channel; and installing two layers of drywall.

2. 
   samyork | Aug 03, 2015 01:47pm | #2

   Thanks for the response. Doesn't the supply air ventilation system require the supply air flow to be higher than the leakage airflow in order to create positive pressure? If so, will it work without insulation in the walls? Tearing out walls to install insulation between the studs is not an option which is why I asked about the blown-in insulation.

3. Expert Member
   Dana Dorsett | Aug 03, 2015 02:31pm | #3

   Air sealing is primary for sound reduction.

   Blown in low-density cellulose insulation (not "dense packed") will improve the air tightness and reduce sound transfer, and dramatically reduce the amount of air that will move through those cavities via convective forces. If you dense-pack it the mechanical coupling from one side to the other goes up and more sound passes through.

   If you get to the point of ripping the walls open, mid-density rock wool batts that aren't thick enough to fully span the depth of the cavity will do more for sound reduction than full-depth batts, but unless it's air-tight you'd still have the smells issue.

   A second layer of wallboard attached with Green Glue (tm) also has a measurable benefit on the sound reduction.

   The Canadian National Research Council has spent a lot of time researching soundproofing, and is something of the Gold Standard on the issue. They have developed lots of tools & methods, and published tons of research, much of which is available online. Try surfing through some of their mateirial to find what's most relevant to your situation:

   http://www.nrc-cnrc.gc.ca/ctu-sc/category/st

   I haven't played with their acoustic design web-tool, so I'm not sure whether it would be of use to you, but I believe use is gratis if you want to register:

   http://www.nrc-cnrc.gc.ca/eng/solutions/advisory/soundpaths/index.html

4. 
   samyork | Aug 03, 2015 05:33pm | #4

   I should also mention that sounds and smells tend to travel up and down rather than side to side. Maybe firewalls help prevent side to side transfer. Not sure. Thanks.

5. Expert Member
   MALCOLM TAYLOR | Aug 03, 2015 08:04pm | #5

   Sam,
   I second Dana's recommendation to look at the NRC's work on sound attenuation. It is very helpful and practical.

6. GBA Editor
   Martin Holladay | Aug 04, 2015 06:09am | #6

   Sam,
   Q. "Doesn't the supply air ventilation system require the supply air flow to be higher than the leakage airflow in order to create positive pressure?"

   A. Yes, more or less -- although any supply air flow should reduce the chance that migrating smells will easily enter your home. Obviously, the more air tightening work you can do, the more effective a supply air system would be -- and the more likely this solution would work with a smaller fan.

   Q. "If so, will it work without insulation in the walls?"

   A. Yes. You can make a wall airtight even if the wall is uninsulated. But the insulation is definitely useful to reduce sound transmission.