Vibration transmission through ceiling suspension

[noxix]

Hi all, I am now constructing the ceiling for my basement studio.

It will be 2 layers of 5/8" drywall, I am putting thermafiber (glassfiber batts) in the airgap between the joists above.

Thanks to some helpful posts, I realized no way can I span 10.5' with 2x4s.

I was going to use special isoaltion hangers but they seem to be expensive and eat up ceiling space.

So I am trying to decide whether to use RC mounted to the wood joists or ...

frame the ceiling independently on lightweight steel studs which would be supported by the stud wall.

It's alot easier to go with the lightweight studs if I tie them to the joists above in 1 spot in the center of each stud to prevent deflection. Obviously I will transmit vibration though this connection, but if I were to use say, monofilament, would I really transmit more that way than with RC?

Thanks in advance


Noxix

[AndrewMc]

I would just attach your RC1 into the ceiling joists and then attach your drywall to that - taking care not to put any screws into the joists.

If you add a 2nd frame below the joists - not sure what the level of improvement will be, but probably not a lot - plus you will eat into your ceiling height a lot. Basements tend to have fairly low ceilings already.

http://www.the-backhouse.com/imagepages/studio041.html

http://www.the-backhouse.com/imagepages/studio046.html

[noxix]

Thanks for the reply.

Actualy, I would gain an inch or 2 of ceiling height.

I guess I ned to compare the rating of equivalent partition systems, and find out what I can about the properties of monofilament.

[knightfly]

2x4's, if placed on 12" centers, will span up to 12'5" for 10 live,/5 dead load - that's assuming #2 and better grade Douglas fir. That's enough for two layers of 1/2" drywall, or one layer of 1/2 and one of 5/8. (nobody walking on top) - If they are further apart, spans shorten (for the same layers) to 11'3" for 16" centers, and 9'10" for 24" centers.

All these figures are for 10/5 loading, ceiling joist only (no living space above) - 10/5 (don't mean to "talk down" to you, so if you already know this just ignore it) means 10 pounds per square foot live load (temporary things, like people) and 5 pounds per square foot dead load (ALL materials used in construction) - framing typically uses up about 1 PSF of this for straight ceiling joists, which leaves 4 PSF for wallboard - 1/2" wallboard weighs about 1.7 PSF, 5/8 weighs about 2.3 PSF - so one layer of each would weigh 4 PSF, plus framing puts you right at max load of 5 PSF dead load. Any shorter spans from the above numbers would just make things a bit stronger.

I'm not sure what you're calling "monofilament", but if it's the nylon used in fishing line I don't think that would work.

If you can run your inner wallboard layers to within 1/2" of the existing ceiling joists (and they can support the extra weight) you can eat the least amount of headroom by using RC on the existing joists. Floating the inside layers (walls and ceiling) on RC would give the best isolation short of using separate ceiling joists, and would gain (as you said) a couple inches of headroom over separate joists.

Hope that helped... Steve

[noxix]

Hey steve, thanks for the help.

Don't worry about talking down to me, this is all pretty new to me, I've just been doing alot of digging around. Any advice is helpful.

OK, forget the monofilament. (yeah, I meant fishing line, that stuff is STRONG, hey?)

The span tables I had access to wouldn't show any info for 2x4s at that span, so I'd ruled that option out. Is that on edge or flat? I was planning to do 2 5/8" layers of drywall so it's touch and go ...

Call me crazy, but I've ascertained I can get identical ceiling height by either method by staggering the new joists between the existing joists. Takes some tweaking, but it *should* be ok, the only snag being some blocking underneath an old fireplace ...

I forgot to ask for the price on RC, and as I said, I'd ruled out 2x4s, but the price on 18 gauge steel studs 3 5/8" x 1 5/8" is 165/10 peice bundle, I'd need 2 bundles for a 10X18 area so I'm looking at $320.

Cold formed steel studs of the above dimension by dale/incor and deitrich are rated to span 12ft with 39 PLF dead load. At even 24" center that should give me 17lb/sq ft which seems like plenty of tolerance.

http://www.daleincor.com/Catalog/Structural_final.pdf

Anyone want to check the way I'm doing that math, I'm no structural engineer ...

Problem being I can only get studs made by marino, identical sizing as the dietrich or dale/incor, but the marino span tables don't have info for that sizing in that application. I'm thinking it's ok to use the data from one manufacturer to another for such a standard item?

[knightfly]

Spans are always calculated on edge -

RC at my west coast area runs around $2.20 per 12 foot length - on the internet, you can pay up to $1 per foot and more.

Wood 2x4-s here run around $0.50 a lineal foot, last time I checked. If you bought 15 12-footers (so you can trim knots and splits from the ends if necessary) you'd be looking at 180 lineal feet or around $90 west coast price. I have no idea what they'd cost in Brooklyn. You also should check local fire regs to make sure you don't HAVE to use steel.

Actually steel isn't as "standard" as you might think - different manufacturers roll their edges differently (or not at all), so it might not be safe to assume. Better to call their customer service and ASK than to end up with a ceiling on your head.

Your math is fine, too bad you can't get the Dietrich stuff. I would definitely ask about spans, it may be that the marino stuff is a totally different application... Steve

[noxix]

Hey Steve,

Thanks for the info, I didn't know that about possible variance in stud manufacturing.

I will make more effort to find dietrich, or dale/incor, the marino rep was the one thinking maybe we needed just one *little* tie in the center of the ceiling to prevent sag, which provoked the monofilament line of questioning ...

[knightfly]

I wouldn't worry about putting any ties in the middle; the simplest way to do your ceiling is to stop the walls about 1/2" from the existing joists, then after the wallboard layers are on the walls, hang RC on your existing joists and hang your two layers of wallboard on the RC (make sure the insulation is up BEFORE you fasten the RC to the joists)

Use screws made for wood when mounting the RC, then use screws made for steel when mounting the wallboard layers to the RC. mark where all studs and RC's are using masking tape along the edges to write on - you do NOT want any screws going thru the wallboard/RC and into a joists.

Hang the first layer on the RC, mud and tape, caulk the edges with acoustic rated caulk (local commercial drywall places are best, call around til you find someone who will sell it to you - the Home Depot stuff is NOT what you want) - then, mark where all of your first layer's screws are, and put up the second layer at 90 degrees to the first layer - be sure to miss the marks you made where the first layer's screws went so you don't drive a screw on top of another one - mud and tape, and caulk the perimeter like before. You want 1/4" gaps around the perimeter before caulking, so your ceiling doesn't have hard contact with the walls.

IF you decide to go with steel joists, just spec them for the span you have based on 10 pound dead load, and you won't NEED any support in the center -

Main thing is, your inner mass leaf should be isolated everywhere from your outer leaf - this is what the RC does for you, or in some cases double wall frames instead of RC.

Hope that helped... Steve