Lets take a look at the effect of insulation in the common wall based on tests conducted at Orfield Labs (an NVLAP accredited lab) and at the labs of the National Research Council in Canada.

Insulation is a valuable, even invaluable, tool in the battle to isolate sound, but in the conventional wall the performance that it offers is limited. The reason for this is as follows

Above we see two walls, one a conventional wall with drywall screwed directly to a single row of studs, and the other a staggered stud wall. Sound can pass through these walls via the air in the cavity (red lines) or via structural paths (blue lines). The staggered stud wall has less mechanical connections than the conventional wall, and sound cannot easily pass through the structure. The same applies as when we decouple the wall with Whisper Clips. In the conventional wall, vibration/sound can easily pass directly through the studs, and does not need to go through the air cavity to be transmitted to the other side.

Adding insulation absorbs sound that is trying to pass through the air in the cavity, but has no effect on structural noise. In the staggered wall, with less structural noise, insulation has a distinct positive effect at middle and high frequencies, but in the conventional wall, where structure-borne noise is strong, the effect is far less.

EXAMPLE 1 - Data from Orfield Labs

B oth walls are constructed with a single layer of 5/8” drywall screwed directly to 2x4 studs spaced 24” on center, the difference being the inclusion of R13 fiberglass in the “with insulation” example.

Its worth noting that at both low and high frequencies, the effect of the insulation is minimal – the primary gains coming in the lower midrange.

The overall improvement in STC is just 3 points, from 37 to 40, and gains at low frequencies are non-existent.

Now lets take a look at another example, this time from the NRC labs in Canada, one of the finest labs in the world. This data is taken from IR-693 and presented here with the NRC’s permission:

EXAMPLE 2 - NRC’s lab, taken from IR-693

Effect of insulation in a conventional wall

The comparison shown here is as above, but with studs at 16” on-center. Both data curves can be found in IR-693, available at www.nrc.ca.

The effect is very similar – the largest gains are just above the primary low frequency resonance, low frequency gains are non-existent, and high frequency gains are very minimal. In this case, STC rises again by just 3 points.

Often it is thought that using denser insulation is the cure-all to sound problems. But this simply isn’t the case. In general, it is preferable to use standard building type insulations (i.e., normal fiberglass) and it is never desirable to utilize expensive, very dense insulations, as in addition to the expense, they tend to make low frequency performance worse by raising resonance frequencies.

This comparison is taken from IR-761 and IR-693, both available at www.nrc.ca, and courtesy of the National Research Council, and shows the comparison of no insulation to mineral fiber that is considerably denser than the fiberglass used in the comparisons above.

This data is taken from IR-693 and presented here with the NRC’s permission:

EXAMPLE 3 – Comparison with Dense Insulation.

In this case, the gains in the midrange are higher than with conventional fiberglass (denser insulations do have proven advantages at middle and high frequencies), but low frequency performance isn’t as good as with the lighter insulation, and STC rises by just 2 points, from 32 to 34.

Where insulation has much more value – in decoupled walls.

In walls in which the mechanical connection between the two sides, insulation has proven, and considerable, benefits at middle and high frequencies. However, at low frequencies the capacity of insulating materials to absorb sound falls dramatically, and the benefit of insulation at low frequencies is to lower resonance points. Still, lowering resonance is valuable, and in walls with damping or decoupling, insulation has considerable value at middle and high frequencies, and should be considered mandatory for any sound isolation application.

To imagine how insulation becomes less effective as frequency falls, think of this experiment – place fiberglass in front of the mid/high frequency speakers in your room, and listen to what happens. The sound will probably be very muffled and muted. Now put the same fiberglass in front of the subwoofer cones… nothing happens. So it goes with walls.

As we mentioned above, the use of insulation is a critical, and valuable, part of any sound isolation project, but insulation alone can’t win the war, especially in the conventional wall where direct structural connections exist between the sides. The main winner will still be a damping compound like Green Glue that stops the structure as a whole from vibrating.

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