Thickness or density, which should I buy?

Thickness, almost always. They cost the same per pound and the same doubling of either doubles the bill, but thickness moves useful absorption about an octave lower while density buys a much smaller improvement and then stops. Density earns its keep for rigidity, self-support in a vertical cavity, fire rating, or resistance to airflow erosion in a duct: real reasons, none of them absorption.

Why does absorption stop improving above about 8 pcf?

Because a porous absorber works by letting air move through it and dissipating that motion. Flow resistivity is what governs it, and there is an optimum: too low and the sound passes through without much loss, too high and it reflects off the surface without entering. Denser fibre pushes past the optimum, so the face becomes progressively more like a hard wall. The exact turning point depends on fibre diameter and thickness, which is why 8 pcf is a guide rather than a threshold.

How do I convert a per-batt price?

Work out the batt's weight and divide. A 24 by 48 inch batt at 3 inches thick is 2 cubic feet, so at 3 pcf it weighs 6 pounds; a batt priced at $7 is therefore about $1.17 per pound. Doing this for two competing quotes is usually the fastest way to discover that the cheaper-looking one is a lower density and not the same product.

Does compressing the batt into a cavity change anything?

Yes, in both directions. Compression raises the installed density, which raises flow resistivity, and reduces the thickness, which is the more important loss. Squeezing a 4-inch batt into a 3-inch cavity gives you a 3-inch treatment at higher density. Generally worse acoustically than a 3-inch batt at the right density, and you paid for 4 inches. Enter the installed thickness and the resulting density if you must model it.

Faced anywhere fibre can reach air people breathe or an airstream that will erode it: plenums, ducts, and any surface within reach. A scrim or perforated facing costs more per pound and barely affects absorption if it is acoustically transparent, which most are. Unfaced is fine behind a sealed panel or inside a closed cavity. The safety case usually decides it before the cost case does.

Why is areal density on this page?

Because fibre in an overhead or hung installation is added dead load, and because it feeds the barrier mass loading calculation if the treatment is doing double duty as mass. A three-inch, three-pound batt adds 0.75 lb/ft², which is modest but not nothing over a large ceiling, and it is the number the structural engineer will ask for.

Acoustic, Noise, Vibration & NVH Products calculator

Acoustic Fiber Cost Calculator (Density and Thickness)

Price mineral wool or glass fibre the way it is actually sold, by weight. Enter the area to treat, the batt thickness, the fibre density and the price per pound. The calculator returns the total cost, the weight to buy, the cost per square foot, the areal density the treatment adds, and a thickness-normalised unit cost for comparing products. Density and thickness both drive weight and therefore both drive cost, but they buy very different amounts of absorption, and a per-square-foot price makes those two very different purchases look the same.

What this calculator does

Price mineral wool or glass fibre the way it is sold. By weight, so the cost of density and the cost of thickness are separately visible.
Use it for choosing between a thicker low-density batt and a thinner high-density one at equal cost, converting a per-batt supplier price into a comparable per-pound rate, pricing the fibre line of an acoustic enclosure or plenum, checking the dead load a hung fibre treatment adds, comparing fibre against foam on delivered cost per square foot.
Price mineral wool or glass fibre the way it is sold. By weight, so the cost of density and the cost of thickness are separately visible.

Formula used

Volume = area × thickness in inches ÷ 12
Fiber weight = volume × density
Total fiber cost = fiber weight × price per pound
Cost per ft² = total cost ÷ area treated
Areal density = density × thickness ÷ 12
Cost per ft² per inch = total cost ÷ (area × thickness)

Inputs explained

Area to treat: Surface area the fibre covers.
Batt thickness: Installed thickness. The strongest single lever on low-frequency absorption, and it costs money linearly.
Fiber density: Nominal density. Acoustic mineral wool commonly runs 3–8 pcf; building insulation batts are nearer 0.5–1.5 and behave differently.
Fiber price per pound: Delivered price per pound. Converting from a per-batt price: divide the batt price by its weight, which is its density times its volume.

How to use the result

Best suited to choosing between a thicker low-density batt and a thinner high-density one at equal cost, converting a per-batt supplier price into a comparable per-pound rate, pricing the fibre line of an acoustic enclosure or plenum, checking the dead load a hung fibre treatment adds, comparing fibre against foam on delivered cost per square foot.
Prices material only: no facing labor, framing, retaining mesh, fixings or freight. Predicts no absorption. Density and thickness govern performance through flow resistivity, which depends on fibre diameter and binder as well and is not an input here. Ignores batt sizes and minimum order quantities, which on small jobs matter more than the per-pound price. Does not model compression, which raises the installed density above nominal and changes both cost per volume and performance. Says nothing about handling, containment or the respirable-fibre precautions an unfaced product requires.

Common questions

Thickness or density, which should I buy? Thickness, almost always. They cost the same per pound and the same doubling of either doubles the bill, but thickness moves useful absorption about an octave lower while density buys a much smaller improvement and then stops. Density earns its keep for rigidity, self-support in a vertical cavity, fire rating, or resistance to airflow erosion in a duct: real reasons, none of them absorption.
Why does absorption stop improving above about 8 pcf? Because a porous absorber works by letting air move through it and dissipating that motion. Flow resistivity is what governs it, and there is an optimum: too low and the sound passes through without much loss, too high and it reflects off the surface without entering. Denser fibre pushes past the optimum, so the face becomes progressively more like a hard wall. The exact turning point depends on fibre diameter and thickness, which is why 8 pcf is a guide rather than a threshold.
How do I convert a per-batt price? Work out the batt's weight and divide. A 24 by 48 inch batt at 3 inches thick is 2 cubic feet, so at 3 pcf it weighs 6 pounds; a batt priced at $7 is therefore about $1.17 per pound. Doing this for two competing quotes is usually the fastest way to discover that the cheaper-looking one is a lower density and not the same product.
Does compressing the batt into a cavity change anything? Yes, in both directions. Compression raises the installed density, which raises flow resistivity, and reduces the thickness, which is the more important loss. Squeezing a 4-inch batt into a 3-inch cavity gives you a 3-inch treatment at higher density. Generally worse acoustically than a 3-inch batt at the right density, and you paid for 4 inches. Enter the installed thickness and the resulting density if you must model it.
Faced or unfaced? Faced anywhere fibre can reach air people breathe or an airstream that will erode it: plenums, ducts, and any surface within reach. A scrim or perforated facing costs more per pound and barely affects absorption if it is acoustically transparent, which most are. Unfaced is fine behind a sealed panel or inside a closed cavity. The safety case usually decides it before the cost case does.

Last reviewed 2026-08-25.