# Wall boards right up against ceiling or leave gap for decoupling



## barry2020 (Nov 26, 2020)

I am doing a basement suite and a putting up the drywall. I put up resilient channel and 1 layer of 5/8 drywall. I wasn't crazy about the sound proofing so I decided to add second layer of 5/8" along with green glue.
My question concerns putting up the wall boards. Should they be pushed right up against the ceiling boards or should I leave a small gap to aid in the 'decoupling' of things for sound proofing.

Over the years I have pushed the wall boards right up to the ceiling but I was not not using resilient channel and not concerned about sound proofing...but now I am.

I think if I leave the gap then I have to fill the gap with acoustical caulk/sealant. Not sure if the extra cost and time will really make that big of a difference.

Any thoughts?

Thanks


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## Magnum Drywall (Oct 12, 2014)

I hope the RC channel can hold 2 layers of 5/8th's drywall or did you actually use hat track.


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## iluvecs (Dec 4, 2019)

barry2020 said:


> I am doing a basement suite and a putting up the drywall. I put up resilient channel and 1 layer of 5/8 drywall. I wasn't crazy about the sound proofing so I decided to add second layer of 5/8" along with green glue.
> My question concerns putting up the wall boards. Should they be pushed right up against the ceiling boards or should I leave a small gap to aid in the 'decoupling' of things for sound proofing.
> 
> Over the years I have pushed the wall boards right up to the ceiling but I was not not using resilient channel and not concerned about sound proofing...but now I am.
> ...


Barry,

Most RC Channel (25 gauge) will bear the weight of 2 layers of 5/8 drywall. For wood framing, use 1 1/4" Type "W" screws to attach RC to studs and joists.For single layer gyp board applications, the fasteners should be 1-inch type S bugle head screws, and 1 5/8-inch type S bugle head screw for double layer applications. 

You should be careful not to run fasteners through the gypsum panels, through the channel and into the supporting framing. This will “short circuit” the channel and cause acoustical performance degradation. The gypsum panels should be installed with their long dimension oriented perpendicular to the channel. 

A 1/4-inch gap should be maintained around the perimeter of the panels. Shim the drywall at the edges by 1/4" away from intersecting plane (ceiling or floor). This gap must be filled with backer rod and a thin layer of acoustical sealant or the assembly will NEVER achieve its specified Sound Transmission Classification. Sealant must be used to fill any gaps caused by penetrations such as electrical boxes, cables and pipes. Sealing all potential air leaks is required to achieve laboratory STC ratings.

It's important to hold back both top and bottom and all other edges of drywall to insure Drywall does NOT TOUCH ceiling or floor. Also keep the baseboard from touching the floor.

The diagram below shows the steps to be taken: 

First fill the 1/4" gap with closed cell backer rod (cheap) and then a thin bead of resilient USG Acoustic Sealant (more expensive) or equivalent on top of the backer rod. Each layer of drywall should be sealed separately at the edge. The joint must remain airtight as the wood framing swells, shrinks and twists.

Sealing at edges of drywall where "flanking noise" occurs can improve overall acoustic performance by 5 dB to 10 dB. With no sealing, performance is degraded an equal amount.

Green Glue plus Resilient Channels properly installed and drywall sealed at edges will provide the maximum potential for this configuration. 

One step better -- and more expensive -- is to use Isolation Clips and hat channels. In all cases, drywall has to be sealed at the intersecting edges.


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