Filling gap in vent created by RISC clips

Drywall got put up today on my new project studio construction (yay!). But, a previously unrecognized issue came up with regards to the place where the air vents comes through the drywall. This is in the ceiling (see attached amateur diagram), which is 3 sheets of 5/8" drywall on RISC clips attached to the 2X4" rafters / roof trusses. There's yellow fluffy insulation between the rafters. Because the RISC clips add ~1" between the rafters and the drywall, there is now an exposed gap in the air vent. Also, the rafters themselves are exposed via the vent opening. As with everything else, I could stuff rockwool in the gaps. But, I think this will be problematic since these are air vents and I don't want rockwool getting blown into the studio or sucked back into the furnace. Any suggestions on what I might do to insulate / isolate at this vent opening?

What about using some type of foam that isn't rigid enough to cause any interference with the RSIC clips. Not expandable, but an open cell very compressable type??? You could fit it inbetween and hot glue it in place?

Hmmm I think (and I am not an expert, but have a little knowledge) that you should extend or push that duct to meet the bottom of drywall flush. Options (without actually seeing it) 1. Detach and push the end of duct down, but this could be problematic. 2. Cut off the last part of the duct just before where I'll guess it turns 90 degrees. Replace with insulated duct board, custom cut. Insulation on inside of duct board helps with sound transmission through duct, but needs to be covered so fiberglass does not fly off.... hmmm I've seen the insulated on inside, duct board and I do not know how it's suppose to work without fiberglass flying off... there must be a safe way to install this stuff. 3. Maybe the easiest option, just extend duct opening with custom cut duct board or correct size duct. Use real duct tape to join ducts. This is available in Home improvement store in same section as ducts. As you do this, as you said stuff rock-wool into the gaps from RC. Backer Rod and caulk drywall gaps. Now a question: Is pic attached the "end" design or plan for your room? Are you not going for a Mass Air Mass, decoupled ceiling? If you are then you'll need to extend the duct again which requires a slightly different approach (rubber to decouple duct). If this is the end design then you're not going to have great iso in the ceiling. The roof counts as another mass, but not a great one cause attic has to be vented. Marc

Thanks for the suggestions. The HVAC guys are going to be out again tomorrow, so I'll talk with them about the different duct extension options they have available. I'd be great if I could come up with a good way to get them to extend the ducts. I can stuff rockwool / backer rod / acoutstic caulk in the gaps if they'll be covered by duct. My diagram represents the final construction: the rafters shown are part of the trusses that hold up the outer ceiling / roof (the outer leaf), and then the 3 layers of drywall (the inner leaf) are decoupled from the rafters via the RISC clips. (I understand how the outer ceiling / roof isn't super isolating.) So, whatever way the duct gets extended, I'd like it to NOT form a rigid coupling between the sheetrock and the rafters. Can you tell me more about using rubber in the duct or other extension options that will decouple the outer duct attached to the rafters from the inner duct / vent attached to the sheetrock.

OK,

the rafters shown are part of the trusses that hold up the outer ceiling / roof (the outer leaf),

You can also call these "bottom chord of truss", the other two that make the large triangle part of truss are also chords. The in between parts are web members.

So, whatever way the duct gets extended, I'd like it to NOT form a rigid coupling between the Sheetrock and the rafters. Can you tell me more about using rubber in the duct or other extension options that will decouple the outer duct attached to the rafters from the inner duct / vent attached to the Sheetrock.

So, borrowing from several other threads & Rods Book (from memory) it is not going to make a huge difference (if any at all) to try and decouple the duct attached to drywall from the duct attached to rafters. Why... RC only benefits isolation in single frame construction. No benefit in staggered stud or double frame walls (other than small increase in air of MAM but would have been better to add this distance to space between frames). This has been written about on many, many threads... folks have taken RC down after installing where there is no need. The truss system compares to staggered stud (kind of) so using RC there I can not imagine much benefit. You may as well rigidly seal duct work. The rubber duct... I tried to find where I saw this previously... can't find it. Basically duct runs to and through outer MASS rigidly attached to that frame, then a short section of the duct is made from rubber to decrease flanking and installed in the AIR, then it attaches to other duct which is rigidly attached to the inner MASS frame. I am using my bottom chord of truss for my outer MASS of ceiling Using new ceiling rafters mounted on new inner frame for Inner Mass of ceiling. I am having a hard time thinking through how you could build your walls keeping things decoupled... though I'll guess you're planning on your walls having higher STC and TL then you ceiling. Marc

Thanks again, Marc.

guitardad72 wrote:

I am having a hard time thinking through how you could build your walls keeping things decoupled... though I'll guess you're planning on your walls having higher STC and TL then you ceiling.

Correct. The decoupling technique / isolation value is not uniform across all the walls or ceiling. The studio walls are double framed. The ceiling doesn't touch any of the walls (drywall or framing). So the major flanking paths to the trusses should be limited to the sound that can pass through the RISC clips. (Or, through my vent gaps if I don't eliminate them!)

Sorry, not much time, can't read all of what's been said already... :oops: Anyway... IMHO, perhaps you should extend the duct to the surface of the drywall, use RC between the ducting and the drywall so that the ducting itself is decoupled from the framing, and then do a firm seal where the edge of the duct meets the drywall opening. Of course, a silencer box will help a lot, which I think is covered in Rod's book somewhat. --Keith :mrgreen: