I am installing a subfloor in an 12X20 ft. attic space. The celing joists (attic floor) are 2X8, 16 oc running parallel between two paralams. The ceiling below (under joists) has sheet rock flush with the top of the ceiling joists; the room underneath is a music room with a waffle pattern recessed sheet rock panels that are flush with the top of the 2X8 joists. The owner wants to put a floor in the attic but wants it insulated for sound. So basically I’m going to fir up the 2X8s with 2X4s to allow room for insulation, then put down subfloor. My question is what is the best way to run the 2X4s? Should they run right on top of the 2X8 joists on edge, or should I run them perpendicular to the 2X8s, with hangers on the paralams? Is there any structural difference?
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If I understand you, you have 2x8s, 16OC, spanning 12'. I hope 2 G they aren't spanning the 20' dimension.
And you want noise reduction.
First, in order to support a floor over 12' I would add 2x12 joists, centered between the 2x8s. For noise reduction, I would add a 1x4 shim to the top of the plates bearing the joists so that the floor joists never contact the ceiling below.
Because you're spanning 12' you will need center span blocking, but you don't want the floor and ceiling structure touching anywhere... So, place a 2x4 block flat flush to the top of the 2x12s, cut a 2x6 into 9 1/2" lengths and set them verticle in the corners of the 2x4 flat block and the sides of the 2x12 joist. I'd attach the 9 1/2ers to the joists before setting the joists. If material is more important than labor, you can cut them into 5 1/2" x 9 1/2" triangles. That will also give you 1/10% more noise reduction.
For maximum sound reduction, before you ever start work on the floor joists, use acoustic sealant to seal from the top every DW joint, every edge, and every place else it may be possible for air to pass in the ceiling structure. After you've laid the floor joists, fill the area with FG or cellulose insulation.
If this attic room is only going to be used for very light duty, a real structural engineer may sign off on using 2x10 floor joists.
SamT
For stiffness it would be best to glue and screw the 2x4s on top of the joists. If you ran them the other way there might be some sort of resiliant buffer you could put at the crossings, though.
Are we trying to keep sound from coming up or going down?
I'm having trouble parsing your description, though. Are there two sets of joists (since the drywall is "flush with the top of the ceiling joists")? If so, how much space is there between the two sets of joists (or between the bottom of the floor joists and the drywall below)? If you, eg, blow insulation in between the two layers you may actually increase sound coupling between the two sets of joists.
I am confused how the drywall is flushed at top of the 2x8s and wondering what it is attached to.
You don't say whether those 2x8s are spanning the 12' or the 20' direction. 2x8 will span 12' OK with blocking midspan but not 20'
For structural your best gain is aligned with the 2x8, glued and screwed, but for sound reduction you would want to go the other way.
I would want to know more about what kind of sound insulation goal he is seeking before commenting on that. Simply adding dense FG batts won't do much good
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> For structural your best gain is aligned with the 2x8, glued and screwed, but for sound reduction you would want to go the other way.I was thinking that crosswise would be better for sound reduction, but changed my mind. You really wouldn't gain that much additional resiliance over the natural flex in the sheathing, unless you used resiliant pads at the crossing points.Probably better to gain the extra stiffness of parallel alignment.Though maybe a better approach with the same (or less) height increment would be to put down sheathing, then foam, then more sheathing. Adding some mass (layers of drywall, eg) would also help, but you'd have to consider the strength limitations of the joists.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Keep on changing your mind. I state what I know from experience.Much of the sound transition - especially in the lower frequency base sounds, is transferred through the more solid framing, so the less contact surface to surface of that framing material there is, the less sound travels to resonate again on the opposite side of the assembly.As you know, we use strapping on out ceilings here in New England. There are a lot of reasons, but one of the advantages is that there is far less sound transfer from floor to floor with strapping than without, in otherwise identical situations
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But a 2x4 edge on is a lot more than "stapping". Not that much flex.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
makes no difference comparing 2x4 to strapping. The point is that you are isolating solid contact to a minimum
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You're not, but it's not worth arguing about.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Piffin, I wish I had a photo...anyway, when you are in the downstairs music room looking up at the ceiling the sheet rock is flush to the top of the joists, recessed from the bottom of joists setting on top of a crown moulding detail that comes down almost to the bottom of the joists. The 2X8 joists are blocked to form a grid of 16" squares that each have the crown moulding detail around the 16" perimeter. The whole music room ceiling looks like an enlarged waffle, with crown moulding inside each square. So when you go into the attic, you see the 2X8s top edge, in a 16" grid pattern, with the back of drywall almost flush (less than 1/2 inch) with the top of the 2X8s. Clear as mud? The owner wants to dampen sound going both directions, but also realizes it won't be totally sound proof. Thanks for the advice.
Edited 8/26/2007 5:23 pm ET by cjgriz
That is a very clear description.
He has a coffered ceiling.too bad the SR is flush at top of the joists or you could drop some absorbant material in there.For low budget, here is what I would do( if the span is 12'):Use sound reducing caulk and fasten Homasote ( or other sound absorbing sheet product) to top of that frame grid.Then run 2x4s at 16" OC perpendicular to joists across the tops of them treying NOT to be in direct contact with the blocking as you lay it out.Fill between those with FG batts or mat.Then the subfloor and finish floor.To help protect against sound from attic going to the room below, use carpet with wool pad
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The odds are that is a pretty stiff floor. I imagine they used a good grade of joist and blocking, and they cut the blocks for a cabinet grade fit.
SamT
yah - sure won't be much deflection in those joists!
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I'm just worried about the level of noise reduction. I'm thinking, (Hey, I heard that!) that a 1x3 grid on top of the 2x4s would give room for more sound insul and much better structural isolation. Maybe with another layer of Homesote on top of the 2x4s.Don't have a clue about how much he can raise this floor.Cost shouldn't be that much of a concern, if this house already has coffered ceiling$.SamT
Piffin, Yeah, that is the plan I was leaning towards, running 2X4s perpendicular to 2X8s, that way they are spanning 12 ft. How much (or little) of a space would you put between the 2X8 existing ceiling joists and the new 2X4 attic floor joists? SamT, I can raise the floor in attic up to 6 inches, by hanging the 2X4s or 2X6s flush to the top of the paralams running down the 20ft sides. There are walls on each paralam supporting tgi rafters mid span. The roof is pitched 7/12 and the ridge is running down the middle of the room 20ft to a gable end. The new room will be used for storage, but maybe down the road a sleeping loft. There will be a step up from the rest of the attic floor. DanH, Yes, when you're up there the edges of the 2X8s are visible, with 16" squares of sheet rock almost flush with the tops of 2X8s. Makes for some tricky walking without cat walks.Thanks for all your input, I will have photos in a couple of weeks before I start the job that maybe will help clarify.
"running 2X4s perpendicular to 2X8s, that way they are spanning 12 ft."Does that mean the 2x8's are spanning the 20' dimension?Whoa! Rethink!.
.
."How much (or little) of a space would you put between the 2X8 existing ceiling joists and the new 2X4 attic floor joists?"That question makes me wonder just how much framing experience you have. It implies a very dangerous concept. It may be that you are failing to correctly express a good idea, but...?????SamT
SamT, This attic floor project is not for sure a go, it might be over my head, and there needs to be more investigation of how the joists are framed (and perhaps a better explanation) before posting the question, I agree. I am experienced enough in framing to know that I shouldn't take leaps of faith in structural elements, and I appreciate your warning. The client is a friend of mine, and I was just there finishing up some trim and exterior siding the original builder never finished, and the attic floor question came up. So yeah, I may decline that attic floor job (was sort of my hunch), and your input is appreciated.
Take some pictures, and find out how the coffered ceiling is supported. Are the 2x8 ceiling joists 12' long or 20'long? I strongly suspect that they are only 12' long, but... make sure.Find out what those paralams are supporting and what is supporting them.Look closely at the top and bottom edges of the 2x8 joists and blocks and see if there are any knots and what size and type they are. Knots can be big or small, tight or loose. Take some good pictures of those edges, top and bottom, knots or not.If the builder used premium grade wood and made up good tight fitting joints between the blocking and joists, that could be a really strong structure and it would be an easy job. A lotta IFs.SamT
SamT, A lotta ifs is right. I will take pictures and post in a couple of weeks. I found out that I will also get to meet with the original builder and see what he has to say about it. I'm finding out that remodeling framing is much more of a head scratcher than new framing. I will prepare my posted question a little better next time. Cheers y'all,
CJ
OK, get your actual info and come back with it.Good luck
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If you really want decent sound isolation the only viable option would be to install steel joists between the beams, so that the floor is physically isolated from the ceiling. You can't get enough wood in there to support a floor.In addition to the sound isolation problem, you have a major risk of damaging the ceiling below or causing visible sag if you install a floor on the joists. Especially if the area is to be used as storage.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
PS: I think the suggestion to walk away from this project was probably the best one.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
"running 2X4s perpendicular to 2X8s, that way they are spanning 12 ft. How much (or little) of a space would you put between the 2X8 existing ceiling joists and the new 2X4 attic floor joists?"OK, let's tear up the paper we were drawing on and throw it on th e floor here! We have asked so far five or six times which way the 2x8s were spanning, the 12' or the 20'. if these 2x8s are currently 20' lonmg, then they are barely adequate to support them selves and the cieling. Gotta know more to add a floor above that. If this is as now indicated, you need 2x8 floor system above the cieling joists, independent of them, to span 12'So how much headroom you got?
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Piffin, Yeah, I know I need more information for you guys, ( write it off to new user premature question asking on my part), sorry if I wasted your time. I will update a post with photos and consultation notes with the original builder in a couple of weeks.
CJ
You're saying that there's no additional framing above this coffer framing? If you walked around up there you'd be walking on the sheetrock?
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
My advice? Run from this job like your feet were on fire. There is no way the customer will ever be happy. You can't make a silk purse from a sows' ear.
Now ... for the specifics ...
Your typical room, nice and boxy, with walls that reflect sound, is an acoustical nightmare. Any music played in such a room will sound like mush, as battling sound reflections cancel and reinforce each other. So, your first approach is to work on the inside of the room.
The first thing is to get rid of those right angles where walls, floor, and ceiling meet. Then, do what you can to eliminate any two surfaces from being opposite from, and parallel to, each other.
That brings up acoustical ceiling tiles. Most don't realize it, but those tiles dampen sound by NOT being flat. Rather, they are filled with pores. Sound is reflected into the pores, and absorbed within.
If I were making a music room out of a conventional room, I'd remove the drywall from inside the room. I'd put dense fiberglass sheets between the studs, and face the walls with ceiling tiles. I'd do the same with the ceiling.
Above the rafters, I'd first lay drywall. Atop the drywall, I'd first lay a very thin sheet of foam rubber, then cover that with 3/8 plywood. (I want to dampen vibrations between the gypsum and the wood).
What I'm doing with this design is two-fold. By opening up the stud and rafter cavities, I am providing an irregular surface. Opposing sheets of drywall (as in your usual room) act like sound mirrors. You don't want that.
The ceiling tiles, and the fiberglass behind them, will act to absorb sound. Using two different materials - fiberglass insulation and cellulose tiles - is a plus, as different materials absorb different sounds.
Finally, the drywall on the 'back side' will reflect any sound that gets through the fiberglass back into the fiberglass.
In the attic, the only reason for the plywood is to prevent you from putting your foot through the drywall.
You can 'fine tune' acoustics by making 'sound mirrors' and 'baffles.'
For example ... let's assume that speakers next to a wall result in too much sound passing through that wall. A piece of plywood, hanging freely in the air, placed behind the speakers will reflect sound away from the wall - meaning that less will pass through. You've made a 'mirror.'
Or, you have the opposite problem ... too much sound is passing through the wall opposite the speakers. You've already placed a 'mirror' behind the speakers, so you don't want to make another. Instead, heavy drapes will act to muffle the sound. You've made a baffle.