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Cellulose insulation ?

kevinkeegan | Posted in GBA Pro Help on

I would like to use Cellulose insulation for sound purposes. I have an old house 1800s, and I was thinking of using cellulose, and my thought was to blow it into the finished ceiling / floor joist bay cavities. Currently there is no finished ceiling, and I was originally going to use high density fiberglass but because the framing is irregular not 16″ on center and there are many wires and mechanicals in the joist bays, I thought blowing in the cellulose once the finished sheetrock is installed, I thought the cellulose would more evenly fill around the obstructions in the joist bays. The longest run is about 16 feet, and the depth of the joist bays are about 8 inches.

I have a couple of concerns which I’m hoping you can help me with.  Is this a good idea for sound purposes? Can I do this with the blowing machine that is offered at the big box stores, Will I get to much settlement to make it an effective sound insulation? Any advice will be greatly appreciated.
Thank you!

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  1. BPontolilo | | #1

    Hey Kevin,

    There are some insulations that are made specifically to help reduce sound transmission including some fiberglass and mineral wool batts, and I believe Green Fiber has a blown in cellulose product that is marketed that way. I'm not sure how well these products work on their own or how ordinary cellulose compares.

    Because you don't have a finished ceiling in place yet, you have a good opportunity to create separation between the finished ceiling and the framing, which may help even more than the insulation because in a building, sound is transmitted through the framing quite well. There are metal resilient channels sold for this purpose that are fastened to the framing and then the drywall is screwed to the channel.

    I've also seen projects where the framing is lined with mass-loaded vinyl to deaden transmission, or double layers of drywall are installed (there is sound deadening drywall available too, but again, I'm not sure how effective it is). You'll also want to give some attention to penetrations and joints and seal them with acoustical sealant and/or putty.

    I'm sure you'll get some thoughts on these products, but I know that cutting down on sound transmission is easier said than done. If you need quiet, I'd consider a more thorough approach than just insulating the floor.

  2. Expert Member


    I'd imagine you would end up with as many voids in the cellulose as you would installing batts. Cellulose in ceilings within the conditioned house also makes future renovations a lot more messy.

  3. DennisWood | | #3

    We did exactly this on a commercial project. It was a sound studio on the 2nd floor, so underneath it the floor trusses (about 16" high) were "closed" in with mesh, cellulose blown in, then resilient channel and two layers of 5/8" type X were applied. It was quite effective.

    And yes, further renovations will be messy. If it's blow in carefully, using mesh, there should be very few voids. You would do each stud bay at a time, pop a hole for the hose every 8-10 ft, insert hose..then back it off as you blow it in. Resilient channel installed after the mesh and before blowing cellulose will help support the mesh during blowing. Cellulose works very well for sound attenuation. The STC ratings of cellulose vs roxul are very similar, but chances are you can pack the cellulose more densely and with fewer voids. It will likely price out cheaper as well.

    If you do two layers of type X with resilient channel and green glue between the layers, you will get an excellent result if you also address other sources of flanking transmission.

    If you are an a cold climate, be careful adding insulation between interior floor/ceilings in the areas immediately adjacent to outside walls. You will find the floor above may get a lot colder at the walls if you don't allow heat from below to temper thermal transfer. I made this mistake in the home theatre installation in my own home! The NRC has a warning to the same effect when retrofitting insulation at the rim joist.

    1. kevinkeegan | | #4

      Hi Dennis,
      Thank you. I did have the entire band joist spray foamed. But I would like to do this work myself so I need to know if the machine that is available from the big box stores like Home depot would do the job? if I use this method I would use Sanctuary Green Fiber. For impact sound I was planning to use Rugapol which is a rubber under the finished floor. I understand there are other systems using wave hangers and channel but I have can not lower the ceiling any further and I have yo keep an eye on the costs. Do you know someone that can tell me if the blowing machine available ay the big box store will work and the best way to do it?
      Thank you, Kevin

  4. DennisWood | | #5

    The big box blower will work. Get a helper to load the hopper and stop right away if flow stops. I’ve used the HD blower a few times with varying results due more to the condition of the equipment vs design flaws. Just make sure you don’t try and pack too dense (it will jam up) and you’ll be fine. If HD does not have insulation fabric in stock, you can try Brock-White.

    If you don’t mind a mess, mesh in 6-8 feet, then run the hose to the back of that cavity, pulling it back as you fill. You can also mesh in fully and cut small holes every 6-8 feet to run the hose in, then staple pieces over to close those holes as you work your way back.

    Your joist rim band is spray foamed which is great. However if you fully insulate that floor cavity adjacent to the foam (particularly for a basement ceiling), thermal wall bridging (as in cold) will create a cold zone at the upstairs wall/floor interface. If you have exterior foam insulation, this will be less of an issue, but if you have a typically framed house with full or partially cast in floor joists, the lower heat transfer in this zone (which you create by insulating) in that specific area will be a problem. This is one of the subtle detail areas in a retrofit where adding too much insulation can actually cause a problem.

    The local HD blower rental requires 2 x 15 amp circuits @120V to operate.

    1. kevinkeegan | | #6

      Dennis, I really appreciate your help, thank you. The area of the house where I'm working is the second floor ceiling with a conditioned floor above. From reading your advice it seems you are suggesting I use a fabric attached to the bottom of the ceiling / floor joists and then blow the cellulose into the framing bay cavities. Can you tell me how I should attach the fabric so I can be sure the cellulose will be properly supported, until the sheet rock is installed. I like your idea because I believe I will be able to see if the cellulose is filling the framing cavity. Which I would not be able to do if I install the sheet rock first. You recommended Brock White, I assume that is a fabric, I searched for that and can't find it. Can you provide more detail?
      Thank you, Kevin

  5. DennisWood | | #7

    Kevin, the fabric is normally just stapled on, but if you use resilient channel every 16", you'll also have about 1/4" of "belly" room in the fabric. Brock White is a supplier we use here in Canada for commercial insulation products. They carry products that the big box stores do not. Your local HD or similar may stock the insulation fabric, but commercial insulation suppliers will likely all sell it in your area. This is what you're looking for:

    If you're treating a 2nd floor, there should be fewer issues with thermal bridging/cooling...just something to be aware of as you add insulation to a normally un-insulated area. I'd take a few reference temps at the floor above, if adjacent to an outside wall before and after you blow in the cellulose. You can always remove some cellulose from the outboard stud bay if you see low temps...and easy to do if you just have mesh/fabric up to start.

  6. kevinkeegan | | #8

    Hi Dennis
    That's great, Yes I found the fabric, thank you. I can't use the resilient channel. Do yo think the 1/4 inch belly will cause a problem for my sheet rocker? I plan on using 5/8" sheetrock on the ceiling. I'm trying to find out how tight the fabric has to be and how often a staple has to be installed. I would really like to see a video on the procedure.
    Thank you, Thank you, Thank you! AND Happy New Year!!! Kevin

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