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Service cavity construction techniques

I'm looking into adding service cavities to some walls that have been opened up for insulation upgrade and possibly some ceiling as well.

I've seen the GBA article about constructing wall service cavities but have some questions from basic to exotic. I'm thinking in terms of remodel/energy retrofit not in terms of new construction.

Depth:
The GBA article mentions a few different depths of service cavities. Most walls only have electrical (and perhaps network/phone) in them. Plumbing vents and supply are usually limited to a few walls. Given that context, two depths seem to make sense to me: 1.5" (2x2s) or 1.75" (bisect rip 2x4s). Obtaining electrical boxes with a depth less than 1.5" is difficult. In the GBA article, one comment mentions ripping 2x6s (perhaps 2x3s might be more economical in some markets) for a depth of 2.5". I can't see the point of the extra inch for non-wet walls. Anyone out there have a suggestion as to the value of 2.5"?

Metal:
I'm sure most of the readers here are familiar with resilient channel, a product for standing drywall off of framing for the sake of reduced noise transmission. It has occurred to me that building the service cavity with a type of resilient channel might provide a sound mitigating effect. However, resilient channel is typically a 1/2" standoff depth (and fairly pricey). First, has anyone tried this? Does it provide any benefit beyond what horizontal framing provides? Second, can you think of a common product that might work? A plank orientation metal stud would probably produce the correct size channel but could only be easily fastened in to one of the surfaces since they lack an outer flange. Well, it's an idea.

Insulation:
It seems like leaving the service cavity completely void is a bit of a waste. At a minimum, insulating over window headers, if they aren't insulated headers and don't have significant outsulation, seems like a no brainer. It seems like there could be a pattern for insulating at certain heights and not others. Anyone figured out a 'pattern' for maximizing the insulative value of the service cavity without compromising its function? Open 'channels' at light switch and outlet height with one or two vertical channels as well? Or does that eliminate the electrical retrofit value of the service cavity?

Non-dropped service cavity in trussed ceiling:
Given a trussed insulated ceiling and a low ceiling height (92/96), a dropped ceiling service cavity seems like a waste of head height. Something like 2x2s screwed to the trusses 6" into the truss space with an air or vapor barrier run across the trusses loosely and affixed to the 2x2s? The space would provide room for can lights and other overhead boxes as well as wire pulls. Duct work, if present, would probably require more room in the service cavity; possibly too much room.

Thanks!

Asked by Keith H
Posted Tue, 12/31/2013 - 11:58

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Keith,
Q. "I can't see the point of the extra inch [i.e., going from a 1.5 inch depth to a 2.5 inch depth] for non-wet walls. Anyone out there have a suggestion as to the value of 2.5 inches?"

A. Yes. It allows for deeper electrical boxes, facilitating wiring.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/31/2013 - 14:52

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