Acceptable joist depth for a second set of ceiling-only joists

Question

Background

I'm installing a new ceiling in the kitchen of the bottom unit of a multi-family home. The house is quite old and uses 2x8 (actually 2"x7.5", not 1.5"x7.5", due to era of lumber) over a 12 foot span, which experience more deflection than is ideal but is certainly acceptable. This does create more transfer of impact noise (foot steps, dropping things, etc). I have isolation clips for the ceiling furring (http://www.kineticsnoise.com/arch/isomax.html) but, now that we're not installing recessed lighting, I'm considering adding a second, separate set of joists to hang the ceiling from.

Problem

What is the minimum joist depth and maximum spacing needed for new joists that support only a ceiling (since there are separate existing joists carrying the load of the floor)?

My calculations for two layers of 5/8" drywall/blueboard and some mass-loaded vinyl come to just over 5 lbs/sqft. There will be additional weight in veneer plaster and a trivial amount in light fixtures. All span calculators I can find accept a minimum of 10 lbs/sqft of live load and an additional 10 lbs/sqft of dead load. This situation seems to me to be around 6-7 lbs/sqft of dead load and essentially no live load over a 12 ft span. Trying to figure out if I can get away with 2x4s or need 2x6s or can cut 2x6s down to 2x5s, etc and whether I can space them every 24" or need to stick with 16". Not so much a matter of cost as of space for other sound proofing materials and considerations. Deflection concern in this case is to avoid veneer plaster cracking.

To clarify, this image is roughly what I'm planning for, where the red joists are to be added and whose dimensions I'm trying to determine.

Answer 1

You’re dealing with “impact sound control “ not “airborne sound control “. Stacking 2 layers of 5/8” gypsum board on “hat channels “ is not the best way to control impact sound.

First

We control impact sound (walking in high heels, dropping objects, etc.) at the source by using

1. soft materials to walk on, (carpet, cushioned vinyl flooring, etc.), and
2. isolate the underlayment from the subfloor and joists with sound absorbing fiber board. If you don’t have access to the floor above your unit, then adding sound control below is difficult...but not impossible.

Second

Isolating your ceiling from the floor above is correct, but using Resilient Channels, not hat channels. The transfer of impact noises down to your space needs to be isolated....that’s why adding ceiling material to the bottom of the floor joists does not work AND WHY using hat channels does not work as well as Resilient Channels.

Yes, the new ceiling joists (isolated from the floor joist above) helps isolate the ceiling from the floor above, but impact noises travels through the framing. Hat channels do not isolate the impact noise.

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By the way, your loading seems a little light:

1. two layers of 5/8” gypsum board is 6.25 psf,
2. hat channels is about .5 psf,
3. new 2x6 joists at 16” oc is 1.5 psf,

PLUS, I would add:

4. rock wool insulation at 4” at 2 psf,
5. 1/2” fiber sound board at 2 psf and misc. (lights, etc.) at 1 psf...

for a total dead load of 13.25 psf. (Even the crappiest grade of 2x6 at 16” oc will support that for 12’)

I’d recommend:

1. isolating the ceiling joists from the wall by installing sound board between the new ceiling joists and the wall,
2. installing 4” of rock wool in the existing floor joists, in the stud spaces and on top of the new ceiling,
3. don’t install your light in the ceiling (cutting s hole for a wire allows sound into your space,
4. install 1/2” sound deadening board on top of your new ceiling joists (if possible,
5. install 5/8” gypsum board on RC -1 channels on gypsum board on RC-1 channels on the 2x6 ceiling joists,
6. caulk the perimeter with sound control caulk...leave the 1/4” spacing at the wall as per your insert you referenced.

Yes, two layers of gypsum board is better than one, and separating the layers with RC channels is best.

Answer 2

Old I know but worth it: Lee Stan has the right idea but, To answer your question more directly - ****The joists should be AT LEAST 2x6 and can be mounted or hung lower than the rafters using joist hangers.

Note the joist width is entirely dependent on your local codes for your span. California is different than others for example

Also for 5/8 double layer drywall with essentially staggered ceiling Green Glue compound performs better than MLV and saves weight and money** **

Answer 3

You can get a very decent sound reduction by using Roxul insulation and being careful about how it is cut and installed so that there are no holes or channels for sound to travel through. Just stuffing the ceiling with Roxul will give you a pretty good idea how much noise reduction you can expect.

I think that two sheets of 5/8" drywall are overkill and not as effective as separate sheets or a single sheet on resilient channel (even the cheaper variety of channel, but you have to be careful with your screws so as not to accidentally provide a solid vibration path)

Having separate joists will help. A lot of inter-party walls in condos are double walled, so two separate sets of 2x4" studs with insulation and 5/8" drywall on the occupied-space side of the walls. These are very good as long as you don't have outlets positioned back to back on the inter-party walls.

I've seen a 2x4 drop ceiling with a 14' span and 1/2" drywall below a bedroom, and that seemed fine, but the deflection calculators online tend to indicate that 2x4's might be marginal, and 2x6 should be used, especially if they are only hung at the ends and no support along the span, as might be needed to reduce sound transfer. Either 16"or 24" centers, whichever is more convenient.

If it were my project would probably do Roxul against the subfloor above, hang new 2"x6" on 24" with joist hangers and bracing between the new joists, then resilient channel and 5/8" or 1/2" drywall. Electrical boxes can be covered by Roxul as well, or more sophisticated sound deadening membrane. Also, any penetrations in the floor above for pipes should be sealed around with something like a latex caulk.

If I was going to make it cheaper, I would just put the channel on the bottom of the existing joists, and stuff the cavity with Roxul.

(Roxul is way better than fibreglass at sound dampening)

Answer 4

After reading more of what you want I would use 2x6" for the span and 16" centers with hangers this would be enough to support the ceiling and even recessed lighting like this. hung at the bottom this will provide the isolation you are looking for. We use these hangers for supporting overhangs with sheeting and roofing all the time and for decks. I would hand select the boards to get the best quality #2 or better but if you can afford #1's because today's lumber is not of the quality it was 20 years ago because of the smaller trees.