An acoustic batt carries an R-value and a thermal batt absorbs some sound, so they look interchangeable on a price list. They are not. Thermal products are specified by R-value. Acoustic products are specified by density, to suit a tested system, and the R-value is a by-product. Order on the wrong number and you get a wall that passes the energy report and fails the acoustic one.
In short
- Density is the acoustic specification. R-value is what falls out of it.
- Two products at the same R-value can be completely different acoustic products.
- Rw belongs to the whole system, not to the batt. The lining and the fixings move it more than the batt does.
- Absorbing and blocking are different jobs. A mass barrier is not insulation.
- Gaps matter more in acoustic work, because sound goes wherever air goes.
This one costs real money on apartment and fitout work, because the acoustic requirement is usually a code requirement with a tested rating attached, and a substitution that ignores density gets rejected at the worst possible time.
Two different jobs
| Thermal insulation | Acoustic insulation | |
|---|---|---|
| What it is fighting | Heat moving through the building fabric | Sound energy moving between spaces |
| How it works | Traps still air in the fibre | Absorbs sound energy in the cavity, and mass blocks it |
| Specified by | R-value, declared to AS/NZS 4859.1 | Density, to suit a tested system rating |
| Where it goes | The building envelope: roof, external walls, floor | Internal walls and floors, between rooms and between dwellings |
| Proof | Product data sheet and the Total R-value calculation | A test report for the system, not the product |
The overlap is real, which is why the confusion is understandable. An acoustic batt does carry a thermal figure and it is useful. It is just not why anyone buys it.
Density is the number that matters
Our Acoustigard partition rolls come in three densities, and putting them side by side makes the point better than any explanation.
| Thickness | 24 kg/m³ | 14 kg/m³ | 11 kg/m³ |
|---|---|---|---|
| 25 mm | R0.65 | ||
| 50 mm | R1.4 | R1.2 | |
| 75 mm | R2.1 | R1.8 | R1.7 |
| 90 mm | R2.5 | ||
| 100 mm | R2.4 | ||
| 110 mm | R2.5 | ||
| 165 mm | R3.5 |
Read the R2.5 row twice. You can reach R2.5 with 90 mm of the 24 kg/m³ product or 110 mm of the 11 kg/m³ product. Identical R-value, 20 mm difference in thickness, and two products that behave very differently in an acoustic system.
If you are shopping on R-value you might take either. If the tested system names a density, only one of them is the right order, and matching the R-value does not make the substitution acceptable.
The dense grade also buys something a thermal product cannot: useful acoustic performance in a thin section. The 24 kg/m³ range starts at 25 mm, thinner than any batt, which is genuinely useful where the cavity is shallow or the floor plan is being fought over.
Rw belongs to the system
This is the point that saves arguments on site. The acoustic rating of a wall or floor is a property of the whole build up, not of the material inside it.
The lining, the framing type, the fixings, whether the studs are staggered, whether there is a resilient mount, and any gap left at an edge all move the result, often by more than changing the batt does. Two walls with identical insulation and different linings are two different acoustic systems.
The practical consequences:
- Ask for the system report that matches your build up, not the product datasheet.
- Do not substitute anything in a rated system, including the lining and the fixings, without checking the report.
- Tell your supplier the whole build up when you ask for advice, because the batt on its own does not answer the question.
Absorbing and blocking
Inside the acoustic category there is a second split that trips people up. Absorbing and blocking are different physics and different products.
| Absorbers | Barriers | |
|---|---|---|
| Examples | Dense glasswool batts and rolls, polyester baffles, absorptive panels | Mass loaded vinyl, damped composite panels |
| How they work | Sound energy is absorbed inside the porous material | Mass reflects and blocks sound rather than absorbing it |
| Thermal role | Real, and declared as an R-value | Essentially none. This is not insulation |
| Where they go | In the cavity, filling it | Layered into the wall, ceiling, floor or around pipework |
Pyrotek Wavebar and Quadzero are mass loaded vinyl barriers, and they work by mass, not absorption. Putting a barrier where an absorber is needed, or filling a cavity with absorber when the system called for added mass, produces a disappointing result and a puzzled installer.
Flanking, the path people forget
You can build a perfect partition and still hear the meeting next door. Sound does not only pass through the wall, it goes around it.
In a commercial fitout the usual path is over the top of the partition, through the continuous ceiling void that runs across both rooms. The wall is doing its job and the void is undoing it. Building the partition full height to the slab is the thorough fix and it is expensive. Dropping baffles into the void above the partition line closes the path for a fraction of that.
That is what a product like the Martini polyester baffle is for: a compression packed block that expands out of the pack to fill a 600 mm void. It is a targeted fix for flanking, not a general insulation product, and it is worth knowing about before anyone prices raising the walls.
Facings can undo the absorber
Here is a failure that looks like nothing is wrong. An absorbent material is installed above an exposed ceiling, then a solid facing is fixed under it, and the acoustic performance quietly disappears.
A solid sarking facing reflects sound. If the material above it was installed to absorb, a non-perforated facing undoes a good part of what it was there for. That is why perforated facings exist: they are acoustically transparent, so sound passes through to the absorbent material behind rather than bouncing off the surface.
Choose perforated where acoustics matter and non-perforated where the barrier matters. It is not an appearance decision.
Gaps
Gaps matter in thermal work. They matter more in acoustic work, and the reason is simple: sound will find any path that air can take.
A batt that does not touch the frame on all four sides leaves a slot. A poorly sealed service penetration, a gap at the head track, a back to back power point in a rated wall, all of these are acoustic leaks in a way that a small thermal gap is not.
So friction fit tight, cut accurately, fill the full cavity depth, and treat penetrations as part of the system rather than as something the electrician sorts out later.
What goes wrong
- Substituting on R-value in a rated system. If the system names a density, the density is the order.
- Expecting a thermal batt to deliver an acoustic rating. Same thickness, lower density, different product.
- Quoting Rw from a product datasheet. Rw belongs to the system, and the report has to match your build up.
- Using a barrier where an absorber is needed. Mass and absorption are different physics.
- Ignoring the ceiling void. A perfect wall with a shared void above it still leaks sound.
- Solid facing over an absorber. Use a perforated facing where the material behind is meant to absorb.
- Leaving edge gaps. Sound goes wherever air goes.
We supply it and we install it
AWBM carries acoustic batts and rolls in the specified densities, mass loaded vinyl barriers, baffles and absorptive panels across Australia, and we install batts, wall wrap and sarking. Send the system your acoustic report names and we will match the product to it rather than to the R-value.
Or browse acoustic insulation, wall insulation and commercial insulation.
Frequently asked questions
What is the difference between acoustic and thermal insulation?
Thermal insulation is specified by R-value and works by trapping still air. Acoustic insulation is specified by density to suit a tested system, and absorbs sound energy in the cavity. Acoustic products carry an R-value too, but it is a by-product rather than the reason to buy them.
Can I use normal thermal batts for soundproofing?
They will absorb some sound, but they are not the same product. Where a system rating has to be achieved, the specification names a density and a lighter thermal batt at the same thickness will not satisfy it.
Does a higher R-value mean better sound insulation?
No. In our Acoustigard range you can reach R2.5 with 90 mm of the 24 kg/m³ product or 110 mm of the 11 kg/m³ product. Same R-value, different densities, different acoustic behaviour.
What Rw will my wall achieve?
That depends on the whole system: the lining, the framing, the fixings, whether the studs are staggered, and how well it is sealed. Ask for the test report that matches your build up rather than reading a number off a product datasheet.
What is mass loaded vinyl for?
Blocking rather than absorbing. It works by mass, adding weight to a wall, ceiling, floor or pipe run so sound is reflected rather than transmitted. It is not insulation and it has essentially no thermal role.
Why can I still hear the next room through a good wall?
Usually flanking. Sound travels over the top of the partition through the continuous ceiling void, so the wall is performing and the void is undoing it. Baffles dropped into the void above the partition line close that path.
Should the facing over acoustic insulation be perforated?
Where the material behind it is meant to absorb, yes. A solid facing reflects sound and undoes much of the absorption. Perforated facings are acoustically transparent so the sound reaches the material.
Do small gaps really matter for sound?
More than they do for heat. Sound will take any path that air can take, so an edge gap, an unsealed penetration or a gap at the head track is an acoustic leak even when the batt itself is correct.
Can AWBM match products to my acoustic report?
Yes. Send the system the report names, including the lining and framing, and we will supply the product at the density the system was tested with. Where it is batt work, we can install it as well.
Sources
- AS/NZS 4859.1, Thermal insulation materials for buildings, for declared material R-values
- Densities, thicknesses, R-values and roll coverage from the CSR Bradford Acoustigard, SoundScreen and Martini product data
- Mass loaded vinyl and acoustic panel data from the Pyrotek product range
- Acoustic system ratings are quoted from manufacturer tested system reports, which apply to the specific build up tested
General information. Acoustic performance is a property of the tested system rather than of any single product, and sound insulation between dwellings is a code requirement in some building classes, so work from your project's acoustic report and confirm any substitution with the acoustic consultant before ordering.