Originally Posted by monsoon
I've read in several posts that a "poor man's" alternative to RSIC clipping the ceiling is to run furring strips 90 degrees to the floor joists for attaching two layers of drywall (GG in between) to. I'd like to understand this alternative a little better and what the satisfaction level is for those that have done this.
What size furring strip was used?
Was anything used between the furring strip and the joist?
I think this suggestion stems from Audio Alloy's labs. The strips we have used fall into two basic categories: 1x3 / 1x4's and plywood laminated with Green Glue (typically 1/2 ply / gg / 1/2 ply)
|What thickness drywall was used?
same options as for any other type of assembly
|What percentage coverage was used for Green Glue?
same basic tradeoffs with coverage as exist for any other Green Glue wall. 50% coverage is only somewhat of a compromise relative to 100%, and Audio Alloy hasn't thoroughly explored the use of even less Green Glue than that (than 50% coverage).
The primary consideration is cost, if you are on a budget, use 50% coverage.
|What technique was used for transitioning from ceiling drywall to wall drywall?
Any fear of sagging the furring strips over time?
i always refer people to their local building offices with any question of a structural nature.
|I'm already (at least I think I am) aware of the other things I'll need to do at the ceiling, such as replacing A/C runs with insulated flex duct, deadening the backs of registers that feed the upper floor, treating recessed light cans, etc.
Any info, including how it compares to the more professional methods, is greatly appreciated.
TIA - monsoon
what do you mean more professional methods? not every acoustical consultant recommend flex ducts, although the ducts will have to be dealt with. if you have a designer, confer with them, otherwise search this forum.
I would like to add some thoughts on furring strips. Audio Alloy has spent alot of time/resources looking into the construction of walls, and one of the priorities, obviously, has been to outline cost effective methods of sound isolation construction. much/most of our work has focused on Green Glue.
I suspect that the furring strip concept originates here, and it is effective, particularily in combination with Green Glue. This does not represent an RSIC clip, this does not represent a true decoupling scheme. In essence, what you accomplish is (assuming 16" OC joist or stud spacing) expanded "stud" spacing. In combination with damping, wider spacing is effective because part of the function of Green Glue in a wall is the dissipation of energy over distance. using furring strips 24" OC provides more distance over which the GG can dissipate energy.
To shed some perspective on this situation, we tested GG + furring, GG + RC, GG + RSIC , on 2x4 studs, 16" OC. RSIC hat channel, RC, and furring strips were all 24" OC. drywall was normal 1/2" drywall.
The performances for RC and RSIC were about as one would expect, showing the decoupling behavior typical of these products. As one would expect, the RSIC wall offered a lower decoupling point (by virtue of lower resonance), and superior performance as a result.
The RSIC assembly performed best in the group from 80hz and up. the RC assembly performed better in the STC range than the assembly with furring strips (STC=57 -vs- STC=54), but worse in the subwoofer region (4-5 dB at 50/63hz).
Short-circuiting the RC assembly heavily (adding 16 short circuits on a 64 square foot wall section, or 8 short circuits per sheet) raised low frequency performance of the RC assembly to be in line with the furring strips, and lowered stc a little (the 16 short circuits dropped STC from 57 to 55).
The overall study could be discussed in great length (and will be on our site someday soon, although the RSIC data wouldn't ever be released w/o pac's permission), but the basic lessons are just that in combination with Green Glue the furring strips work better at low frequencies in combination with Green than RC because they create a more mechanical system, which is better controlled by the GG. Short circuiting the RC moves performance to very similar to that of the furring strips because the short circuits make the flexible RC assembly more mechanical.
It's worth mentioning that one benefit of GG is a virtual elimination of performance loss resulting from short-circuiting RC.
Do not tell people that short circuiting their RC walls will improve performance. These walls had GG and should not be equated to normal walls. With normal drywall, short circuits in RC can cause catastrophic failure.
do not tell people that furring strips work as well as RSIC clips, these walls had GG and should not be equated to normal walls. With conventional drywall the RSIC would enormously outperform the furring strips.
But what can be said, is that in combinatoin with GG, furring strips offer a nice way to improve performance, to yield performance approaching that of more exotic assemblies at basically zero cost/installation trauma.
On a ceiling, with a vastly deeper air cavity, the advantage will tip more heavily towards the clips (though not really towards RC). As i've said many times before, ceilings and their deep air spaces are a match made in heaven for clips. To summarize the results for these assemblies, in essence for theaters we ahve GG + RSIC > GG plus furring or GG +RC with short circuits, > GG + RC
i hope that clarified alot of the relevant points.