TM Soundproofing Effect of Insulation in the Common Wall

Effect of Insulation in the Common Wall

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

Effect of insulation in a conventional wall

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.

Effect of adding layers to a normal single wood stud wall

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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Customers Questions and Answers

1) steve: I am remodeling a guest room and bath adjacent to my room, and want to the sound transition as low as I can, and afford. I have opened the walls, so have an opportunity to make a good effort. I think I will use your putty pads around existing elect boxes, and on a cold air return duct which are in the shared wall. then build another decoupled wood framed wall with double 1/2' sheetrock with green glue compound between layers, and green glue caulking all around. Should I use the G.G compound on the studs prior to the wall board installation? And will the putty pads be an effective product for the cold air return duct? We have a lot of mechanical noise coming thru the under slab C.A. return ducts as well! Do you have any suggestions for mitigating this noise? Baffles with little air restriction? Thanks

Trademark Soundproofing Reply:

Hi Steve, The Green Glue on the studs will not offer much benefit so save your money and skip that. I would not use drywall between the new wall and the existing wall rather only on the outer side of the new wall in order to avoid the Triple Leaf Effect. Putty Pads would help slightly however they are not a damping compound and will not do much to damp the ducts. We will have a new product in 2016 for this application, stay tuned. Thank You

2) Doug A: I purchased a 22 unit apartment complex that is noisy between the wall units. I discovered that there is no insulation in the walls. What do you recommend I do to damping the sound in the walls. Would blowing insulation in the walls help Doug Anderson

Trademark Soundproofing Reply:

Hi Doug. It would help however not enough to be worth the effort. I would suggest you blow insulation into the wall and then cover with a 2nd layer of drywall and Green Glue Damping Compound.

3) E. P: Have noise problem between walls in a brand new reconstructed farmhouse. We took it down to the studs. Walls and ceilings are butt joined painted wood and floor is also wood. Builder sprayed all walls with icynene telling us everything that is spray foamed will be soundproofed! My husband and I can carry on a conversation with each other when one is in bed in the the master bedroom and the other in the guest bathroom directly behind the bed. Help!!

Trademark Soundproofing Reply: Hi, That is unfortunate. We hear that from many customers and have mentioned it in our articles, spray foam can actually sound worse many times because it glues all the building components together. At this point your best bet would be to add another layer of drywall with Green Glue to your existing wall.

4) Paul : I am putting In a new subfloor In about half of my main floor and want to add green glue between my 2 plywood sheets to reduce noise between the main floor and basement unit. Does it make sense to use green glue only on half the floor? Will the sound from the basement simply come through the part of the floor that doesn't have green glue installed ? I can see that it would only reduce structural vibrations from the main floor from transferring to basement on the areas it's installed. Thanks for your insight! I will be filling 100% of the floor with blown fibreglass.

Trademark Soundproofing Reply: Hi Paul, I would not recommend that as the sound will come through the untreated part. You want to cover the entire envelope/floor.

5) franco: i just bought a home next to a 4 lane road. the noise from road is crazy. what is the best solution. what insulation product works the best for road noise..spray or batts? please help. we are considering insulation and adding another layer of drywall

Trademark Soundproofing Reply:

Hi Franco, Fiberglass is better than spray foam and adding a 2nd layer of drywall with Green Glue will help a lot. For the windows you can use our window panel barriers. Thank you

6) Joshua E: I am building a soundproof booth to record music. I was going to put joint gasket tape on the studs. Use roxul rockboard as insulation and double sheetrock and green sealat between the seems. I am also using professional soundproof padding on the sheetrock. Am i heading in the right direction. What would you suggest?

Trademark Soundproofing Reply:

Hi Joshua, You should use the Green Glue damping compound in between the 2 layers of drywall and the Green Glue sealant at the perimeter.

7) Mike: We just recently moved into a house with our bedroom facing a road. It is fine most of the time during night. However, in daytime, whenever there're pick-up trucks accelerate on the road, we can hear the rambling engine sound pretty clearly. After reading into your website, I'm considering to approaches. Option #1. Blow in cellulose insulation into the wall (currently no insulation in exterior wall), and add another layer of drywall with green glue. Option #2. Take the existing wall out, add fiber glass bat, put in clip and channel for decoupling, and add two layers of drywall with green glue. Of course, the option#2 would be much more costly. Do you think it is worth the effort and money to go the extra-length? What would be the low frequency performance difference between the two options? I'm looking to reduce the truck rambling noise by half if possible. Thanks. Mike

Trademark Soundproofing Reply:

Hi Mike, If you will be happy with reducing the noise by half then Option 1 would work. If you are looking to do the most possible then option 2 would be the way to go. What are you planning to do for the windows? Take a look at our sound control window panels.