RC Spacing

Eggman · Post by **Eggman** » Thu Oct 21, 2004 9:10 am

One more question as I begin my ceiling:

The ceiling joists are 24 OC. What is the optimum spacing for the RC? Is 12" overkill? I plan to hang two 5/8" layers.

I can't begin say how much I appreciate the info I've gleaned from this forum.

Doug

AndrewMc · Post by **AndrewMc** » Fri Oct 22, 2004 1:38 am

My ceiling is the same 24" joists, 2 layers of 5/8. I spaced the RC at around 2 feet. There maybe some exact distance though you are supposed to use - not sure. 1 foot spacing is probably overkill. Just make sure you have the RC well attached to the joists.

Post by **Aaronw** » Fri Oct 22, 2004 3:06 am

I used 16" spacing for mine. Look at the USG handbook. It should specify exactly what to use. But I think it says to do 16".

cadesignr · Post by **cadesignr** » Fri Oct 22, 2004 4:02 am

I'm not sure either, but I AM sure of this. IF your joists are spaced at 24"oc, and you fastened your RC from the center or each panel outward at 22"(leaving a 2" gap to the edge of each panel of sheetrock as I would NOT attatch adjacent panels of the first layer to ONE RC channel

) this MEANS........There are 3 channels per 48" width if you attach the panels lengthwise to the RC. That means, there are 5 fastening points per channel from RC to joist. That is 15 screws holding the weight of TWO layers(correct?) of sheetrock to the joists. There are more screws fastening the drywall to the RC, but that does NOT support the weight to the framing above.

Sooooooooo, the more the better. At 16"oc, that adds 5 more screws per 4'x8" panel.......hmmm, was this worth typing.

fitZ

Eggman · Post by **Eggman** » Fri Oct 22, 2004 4:19 am

Thanks for your replies.

I always overbuild everything. I have quite a bit of RC so I have enough to put it every foot. I am wondering if spacing it this close together will lessen my STC. I'd be interested in your comments.

Thanks Doug

dymaxian · Post by **dymaxian** » Fri Oct 22, 2004 4:54 am

Interesting question for the older voices on the board...

Does adding more channels (spacing them 12" or 8" or whatever) negatively affect the isolation properties? From a structural standpoint, having more RC means you'd be able to load them up with more drywall or MDF (it'd be nice to be able to hang 5/8" drywall + 3/4" MDF + 5/8" drywall from RC). I wouldn't think this'd be a problem; I'd want to ask the guys at USG about the strength of the stuff, but would the acoustics be affected badly?

I'd think the only real headache would be that you be extra careful with your screw locations...

knightfly · Post by **knightfly** » Fri Oct 22, 2004 9:43 pm

Think of it this way; if you use the exact same type of springs on your car, but use twice as many per wheel, don't you think the ride might get a bit rougher? It's the same physics - the car is your ceiling, the extra springs are the extra RC, and the bumpy road is the other side's wall panel trying to get through to the side your RC is on; if you stiffen the spring rate, you get less de-coupling from side to side in the wall. This doesn't affect the lower frequencies, as RC does almost NOTHING for those anyway; but sound isolation, in order to sound "natural", and therefore NOT be as noticeable, needs to have a smoothly sloping increase from low to high frequency; if you stiffen your RC too much, you lessen the effect it has on higher frequencies, so the perceived isolation becomes LESS.

Generally, most makes of RC should be on 16 x 24 centers for double 5/8 wallboard; if the framing is 24" OC, make the RC on 16" centers. If the framing is on 16" centers, use 24" centers for the RC.

You can usually add one more RC per 4' section of wallboard without TOO much negative effect, but unless you're using 24" centers on the wood framing I wouldn't put RC as close as 12" apart... Steve

dymaxian · Post by **dymaxian** » Sat Oct 23, 2004 12:57 am

Think of it this way; if you use the exact same type of springs on your car, but use twice as many per wheel, don't you think the ride might get a bit rougher?

Following that example: The springs on a passenger car are very different than the ones on my 3/4 ton cargo van. Applied here- if we wish to put more than just 2 layers of 5/8" drywall, we'd overload RC channel that was spaced 16" o.c. If we installed the RC at 12" o.c., and then loaded it up with 5/8" drywall over 3/4" MDF over 5/8" drywall, would the added weight of wall panels counter the stiffer springs?

Using that kind of system for the inner leaf of a wall, I'd think that high-freq isolation would be plenty. It'd all get thru the doors and/or windows, obviously, but just looking at the wall system I'd think that'd provide plenty of isolation. And after heavying up the walls like this, low freq sound and structural vibration would be the remaining problems.

Just thinking out loud. Personally, if I was going to buy that much wall panelling, the cost of having a double wall structure to hold it up wouldn't be all that big of a deal.

knightfly · Post by **knightfly** » Sat Oct 23, 2004 8:25 am

First, yes it's the same principle. heavier mass needs heavier spring rates (or more light ones) to compensate.

Consider an 8' tall wall, 12' long; that would take at least 6 12' lengths of RC, spaced 24" and with the middle two RC's close together, you'd have RC at bottom, middle and top of each 4' tall, 8' wide panel - at $.20 a foot, that adds about $15 to the cost of framing; if instead you were to double frame, using wood 2x4's you'd use (24" centers) at least 7 8' studs and 3 12' plates (two if not load-bearing), so a total of at least 80 feet of 2x4 @ roughly $.50 a foot, comes to $40 - subtract the $15 for RC, leaves $25 difference. If you figure a set air gap of 8" (just for argument sake) and double 5/8 wallboard each side, the STC will be roughly 49 for single studs (2x8's), 55 for staggered studs on same plates/caps, 58 using RC or LIGHT steel SINGLE studs, and 67 for double framed wall construction; so, for $25 difference in cost for a 8 x 12 wall, you get 9 dB better isolation. This would require 6 layers of 5/8 wallboard on one side and 5 layers on the other side of the RC wall using the same air gap; first, you couldn't even PUT that many layers on RC and either support it OR reliably hit the RC with the 4" long screws you'd need; second, even if you COULD the cost (at $7 per sheet) would be an extra 27 sheets of wallboard plus mud, screws, etc; or around $ 200;

Sorta makes double frames look pretty reasonable, in more ways than one; this isn't even taking into account that the RC doesn't make ANY difference in LOW frequency isolation, only mass and air space will help that. So, as long as you can spare an extra 6" for wall space it's pretty much a no-brainer to my mind - if you CAN'T spare the space, then RC (or RSIC clips and hat channel)is the next best thing... Steve