Two jobs, one word
Insulation means two different things on a commercial job. On an exterior wall, it controls heat flow and sits under an energy code requirement. Inside the building, between a tenant suite and a corridor or between two offices, nobody is trying to hold heat. The batt is there to absorb sound energy in the cavity.
The two uses are specified, priced and inspected differently, yet the drawings often just say insulation. A tender that does not say which one it means leaves the drywall subcontractor to guess, and the guess is usually the cheapest batt that fits.
What the insulation does inside a partition
A drywall partition is two membranes with an air space between them. Sound hits one face, sets the air in the cavity moving and drives the other face. Fibrous material in the cavity damps that air movement, which is why a wall tested with a batt rates higher than the same wall left empty.
Acoustic insulation in partitions gives a real gain, but it is not the biggest lever. Board weight, stud depth and decoupling usually move the rating further. Published manufacturer data points the same way: the first inch of fibrous fill gives most of the benefit, and a cavity filled across its full area performs better than one that is partly filled. See our note on STC ratings in commercial partitions for how the whole recipe fits together.
What the tested assembly assumes
A listed acoustical assembly names the insulation as part of the wall. The listing normally describes the material, such as glass fibre or mineral wool, and a thickness, and the test was run with that product in that cavity. The rating belongs to that combination.
- Material. Glass fibre and mineral wool are both common. Some published research found low-density glass fibre close to high-density mineral wool, with small differences in parts of the frequency range. Manufacturers of each product claim an edge. Do not treat them as interchangeable unless the listing says so.
- Thickness. A batt sized for a 3 5/8 inch stud is not the same product as one for a 6 inch stud. The listing ties the batt to the stud depth.
- Coverage. The tested wall had insulation across the whole cavity, top to bottom. A batt that stops short or slumps leaves a void at the head of the wall.
- Foam is not a batt. Spray foam does not do the same job in the cavity. Do not swap it in without the designer's approval.
Why a fuller cavity is not always better
The usual instinct is that more insulation means more sound control. Past a full, gap-free fill, the evidence does not support that. Insulation manufacturers describe the goal as a cavity filled stud to stud and top to bottom, and the installation guidance says to match the batt to the actual cavity depth rather than force a thicker one in.
Compressing a batt changes the product that was tested. It also tends to bow the batt and leave pockets behind it. We have not seen published test data that shows a stuffed cavity beating a properly filled one, so we do not recommend it. If a higher rating is needed, ask the designer for a different assembly, such as heavier board or a decoupled frame.
Where thermal and acoustic insulation meet
Some interior walls carry both requirements, such as a partition between a heated suite and an unheated loading area. Others carry a fire requirement as well, where the listing may name a particular product. The note on the drawings has to say which duty governs.
| Question | Why it matters |
|---|---|
| Is the wall acoustic, thermal, fire rated, or a mix? | Each duty can name a different product or thickness |
| What does the listed assembly name for the cavity? | The rating applies only to that material and thickness |
| What is the stud depth? | The batt is sized to the cavity, and compression is discouraged |
| Does the wall run to the deck? | A batt stops where the wall stops, so the head detail has to be known |
| Is a substitution acceptable? | Only the designer can approve a change to a listed assembly |
What we do on ours
We self perform steel stud framing, cavity insulation and boarding on partitions, so the batt is checked against the wall type before the second face goes on. We install to the assembly the drawings show and ask the designer before any substitution. Send the wall schedule and the assembly references with the tender and we will price them as drawn.