Going through our deep energy retrofit in phases has allowed us to fine tune along the way where needed.
One example is the ventilation closet, which on the 1st and 2nd floor borders along the bedroom. On the 1st floor the access doors to the ventilation closet are on the bedroom side. To better manage the little noise the ERV emits, we shifted the access doors for the 2nd floor ventilation closet to the living room side.
But the sound management doesn’t have to end here. We can further soundproof the wall between the ventilation closet and bedroom with the magic material I alluded to in my last post: rock wool.
I started the process before we installed the ceiling drywall by blocking the small attic space. That should prevent most of the ERV noise from travelling across the bedroom ceiling. The next step was to fill the wall framing cavities.
The beauty of rock wool is that it doesn’t only provide sound insulation, but also excellent fire protection and thermal insulation. We used rock wool for these latter two properties in the ceilings and all exterior walls. But for the wall between the bedroom and ventilation closet, we were mostly after its sound insulation properties.
Our careful planning of the ventilation system begins to pay off. We sacrificed nine to 12 inches of the ten-foot room height in the less-public rooms. That gave us the space for drop ceilings, which accommodate the ERV ductwork.
Although we would like to preserve the full ten-foot ceiling height in the more public rooms, we have to accommodate the low profile utilities such as the insulated PEX runs and forthcoming electrical conduits.
Because the depth of the insulated PEX is only one and three quarter inches, we can provide enough space with a simple furring channel application.
To prevent the transmission of impact borne noises, such as footsteps, I have to decouple the furring channels from the floor joists. We accomplish that with a device called the IsoMax Sound Clip. It is basically a rubber block with a profile cut-out for the furring channel and a metal bracket on either end through which the piece is fastened to the floor joist.
We added a half inch plywood piece between the floor joist and IsoMax to give us the required one and three quarter inch clearance space. As you can see, the furring channel is decoupled from the floor joist and fastening system through the rubber block.
We hope that this little trick will provide additional peace and quiet. Actually, we really, really hope so, because those IsoMax Sound Clips cost an arm and a leg – between $5.00 and $6.00 apiece. Do the math and you know we have an expensive ceiling.
Based on my research, there is only one company that offers this kind of sound control solution – Kinetics Noise Control. It looks like they pretty much can charge what they want with only one player in the market. I tell you, that is an expensive piece of rubber!
While we are on the subject of framing, there are a few more carpentry tasks competing for my attention. One of them is the installation of drop ceilings.
The drop ceiling is needed to accommodate all the new utilities we installed on the first floor. These include the ductwork for the Energy Recovery Ventilator (ERV) and the PEX tubing for the radiators. We still have to add the electrical conduits.
The insulated PEX runs and forthcoming electrical conduits have a low profile and only require a clearance up to one and three-quarter inches. The ductwork for the ERV, six and eight inches in diameter, are a different story.
The framing
Only the master bedroom has an eight-inch diameter duct and thus requires a drop ceiling with a clearance of 12 inches. On each short end of the bedroom we mounted a 2 by 4 with joist hangers 16 inch on center, into which we placed the drop ceiling studs.
The other rooms with ERV ductwork have the six inch diameter kind, which requires a clearance of 9 inches. I had it easy and could attach the 2 by 4’s for the drop ceiling directly to the wall studs because they lined up with each other across the room.
Span support
The span of the 2 by 4’s for the drop ceiling is anywhere between eight to 12 feet. That is too much considering that I will hang heavy 5/8 inch drywall onto the ceiling. To prevent sagging, I need to support the 2 by 4’s every four to five feet, by attaching them to the floor joists of the second floor.
That gets me back into the issue of noise, or to be more precise, impact transmission – dampening down the noise of footsteps. We tackled the same issue while finishing the basement.
To decouple the support for the drop ceiling from the floor joists, and thus reduce the impact transmission, I used a neoprene bushing with a neoprene washer followed by regular steel washers.
As you can see, the long screw, which is the transmission rod for any vibration from the floor joists, is not in direct contact with the drop ceiling framing. The neoprene bushing and washer keep it from direct contact with the 2 by 4, and so does the over-sized hole above the washers (3/8 inch hole for a 1/4 inch screw).
The soffit
I have one short section of six inch duct that is running along a short wall in the living room. Rather than building another drop ceiling for the entire living room, we opted for a short and small soffit instead.
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.
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.”
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.”
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.
This 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!
