Solving the Sound and Odor Problem with Townhouse Walls

A new two-hour firewall assembly using cellulose insulation solves several of the pervasive complaints about townhouse living.

The American Apartment Owners Association recently published a list of the top ten renter complaints. The list of negatives is similar to those encountered by townhouse owners and renters. Coming in right after deposit disputes, lack of repairs, and (weirdly) bed bugs, was noise: “This problem includes both complaints about loud neighbors, music, equipment and complaints against occupants causing excessive noise,” the association said. For townhouse owners or leasers, the problem goes both ways. An adjoining neighbor who thinks you are too loud will often file a “nuisance” complaint that can cost you thousands in legal fees. It’s best to create a wall that keeps your lives separate and private.

A surprising number of new and existing townhomes lack adequate soundproofing between units. While they’re required by code to have a firewall that reduces the risk of rapid flame spread, there’s no code mandate about sound. Adjoining units often hear way too much of their neighbor’s daily activities, from conversation to loud tvs to plumbing and appliance vibrations.

Cellulose has significant sound-deadening capabilities. For example Greenfiber’s ability to achieve an STC 52 rating also makes it one of the most effective sound blocking firewalls available to the building industry, exceeding most industry standard assemblies by up to 9 decibels. Its firewall assembly goes much further in terms of sound dampening, because it includes an airspace gap between framed walls. That gap is filled with the company’s FRM spray-applied insulation. It’s part of their SANCTUARY line, and is designed to completely fill hard-to-reach cavities. This greatly reduces unwanted air infiltration and air changes per hour. 

But it’s not just sound. Many townhouse owners also complain of odors drifting through walls into their unit. I know from my own experience as a city dweller that this can be frustrating. As a vegetarian, for example, I don’t appreciate the smell of cooking chicken drifting into my bedroom at midnight when a neighbor gets home after a late night out.

More than just a nuisance, however, research shows that no cooking smells are good smells when it comes to indoor air quality. They may also be carcinogenic. Most people don’t ventilate their homes adequately, and even those with range hoods do not use them enough.


Layer approach. To achieve the two-hour, Class 1 fire rating, applicable to townhouse walls and stairwells up to three stories tall, you simply frame two staggered walls, fill with the special Greenfiber mix, and cover with sheetrock. To achieve 47 STC, the assembly is as follows: Double frame and add 1" sound insulation within the partition cavity. Boost to 52 STC by adding a single-layer 3/4" USG Sheetrock® Brand Ultracode® Core Panels and 3" sound insulation.  

A firewall that doubles as an odor shield is a win-win, in my opinion. The reason cellulose has an advantage in this application is its ability to pack densely into the double-framed wall cavity between units. This particular line of Greenfiber’s Sanctuary product is treated with a low-toxicity fire retardant, specifically designed for firewalls. The other benefit, of course, is Sanctuary’s lower embodied carbon footprint. You end up with a “greener” wall system that offers better performance than standard one-hour assemblies created with other design standards.

At present, cellulose insulation represents only about 5 percent of the overall insulation market in the United States. By combining good building science like this with a highly responsible sustainability story (85 percent recycled post-consumer content; low embodied energy; thoughtful leadership), perhaps this product’s reach and impact can grow.


Greenfiber FRM Insulation may be used as a fireblocking material in the SANCTUARY Two-HourFirewall, UL-U370 assembly. This product can be identified by its terracotta color.