Sound Solutions

Let’s step away from the bathroom and tiles. We still have some serious work in the living space that requires our attention. Let’s start with a look at the ceiling.

Our two insulation layers are in place, the open cell foam and the rock wool. Although these are insulation layers, insulating is not necessarily their primary purpose.

The three inches of open cell foam provide us with an airtight ceiling. Yes, this layer as well as the rock wool will slow down heat transfer, but the other function that we are really after is sound control between the first floor and basement.

Both products, the open cell foam and rock wool, have sound reducing properties. Reading through the product specs, I came across the term ‘Sound Transmission Class’ (STC).

I learned that STC measures the transmission of airborne sounds. I could not find any reference to the STC scale range, other than that the higher the STC, the greater the sound transmission reduction. The International Building Code of 2006 requires an STC of 50 between dwelling units.

The open cell foam, according to the product specs, should provide an STC of 37 in a 2 by 4 wall assembly. The rock wool offers an STC of 45 under the same conditions.

We don’t have either product in a 2 by 4 wall assembly, but rather in our ceiling, and I am not sure what their combined STC would be. What we did notice, though, was that we could no longer freely communicate through the ceiling.

“Even though a joist floor has a good IIC rating, footstep sound with a frequency of less than 100 Hz can still be annoying to the people below.”

Source: Controlling the Transmission of Impact Sound through Floors – National Research Council Canada

Yep! I can confirm that footsteps were still audible, because their sound originates through structure borne vibration, not airborne sound, and easily transmit through the floor joists. But what is that IIC rating thing? Well, structure borne vibration is not measured in STC but has its own metric called Impact Insulation Class (IIC).

“The higher the IIC, the better the attenuation of impact sound, with 50 usually considered the minimum rating for occupant satisfaction in residential buildings.”

Source: Controlling the Transmission of Impact Sound through Floors – National Research Council Canada

Even though I came across an interesting IIC ratings catalogue, I could not find a firm rating for our scenario.  I assume, through interpolation of the data, that we should be somewhere around IIC 50. However that would not take into account the three inches of open cell foam, but assume the use of a resilient channel.

resilient-channelThis is where I came across the resilient channel concept. It is basically a metal furring strip that allows us to decouple the drywall from the floor joists, and as such reduces the structure borne vibration transmission (i.e. the sound of footsteps).

We had to improvise a little to get the maximum decoupling effect. The use of a half inch insulation pad between the floor joist and channel and neoprene washer under the screw head should help with the reduction in vibration transmission from the joist to the channel. The closed cell foam tape at the bottom of the channel may further reduce transmission from the channel to the 5/8 inch drywall.

Boy, they were not kidding when they put the science into building science. How about leaving the brain behind and getting the brawn involved?

I don’t know what IIC rating our floor/ceiling assembly would yield. I assume that it will get us above the critical 50. And if we use runners on the first floor in the high traffic areas, we should be in great shape.

PS: Thanks to Drew for his help with the installation!

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

2 thoughts on “Sound Solutions

  1. Nice work Marcus, footsteps from above are likely to be the most annoying, and the metal channel furrings are a great way to reduce that sound transmission through the structural components (joists).

    This construction technique is similar to the assemblies used in music recording studios, so if your future basement tenant is a musician they (and those above) will be very appreciative!

  2. Wow! I wish that more builders took such a thorough approach to sound insulation. I used to live in a building where every footstep carried from the unit above to my apartment. For a while, the upstairs tenants were a family with three young children. To describe it as hell would be a huge understatement. It looks like your tenants will be much more fortunate in that respect.

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