Sound Rooms.
Spray foam is an excellent air seal and a poor sound barrier. Canadian NRC research measured direct sound transmission class falling by 4 to 6 points when spray foam replaced glass fibre in a wood-framed wall. If STC is your goal, that matters.
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The insulation industry oversells foam on sound. The measured data is public, it is Canadian, and it does not say what the brochures say — so the useful version of this page is the one that tells you where foam helps and where a fibre batt beats it.
Where the goal is transmission loss through a partition rather than leak-borne noise, batt insulation is usually the better material — and it is a product line we work with as well so the recommendation costs us nothing to make honestly.
What Foam Does and Does Not Do for Sound.
Two different things get called soundproofing. One is transmission loss through an assembly, measured as STC. The other is noise arriving through gaps, penetrations and flanking paths. Foam is good at the second and not at the first.
- Air sealing the room: gaps around outlets, switch boxes, door frames, duct boots, pot lights and the top plate are direct acoustic paths, and closing them is the part where foam genuinely helps.
- The measured STC picture on wood studs, from NAIMA and Intertek testing in 2019: open-cell foam filling the cavity at STC 33, closed-cell at STC 31, fibreglass in the same series at about STC 34.
- Johns Manville's published comparison: 2x6 R-19 batts at STC 45 against five inches of closed-cell foam at STC 37; 2x4 open-cell foam at STC 38, the same as R-13 batts.
- The steel stud case, where the gap is largest: the Gypsum Association reports fibreglass and mineral wool beating both spray foams by 8 to 10 STC points on steel studs, because foam stiffens the resilient framing that was doing the acoustic work.
- The Canadian research finding, from NRC's mid-rise wood programme: direct STC fell from 38 to 34 in a non-loadbearing wall and from 42 to 36 in a loadbearing wall when glass fibre was replaced with spray foam.
- Absorption within a room, which is a different measurement again: published NRC absorption coefficients of 0.55 for one open-cell product at 3.5 in. and 0.75 for another, measured to ASTM C423.
- Duct, plumbing and structural flanking paths, which frequently dominate a real complaint and which no cavity insulation of any kind addresses.
- Thermal barrier requirements, since a studio or media room is occupied space and Article 9.10.17.10 applies to any foamed plastic in the wall or ceiling assembly.
How We Would Approach an Acoustic Job.
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Separate the two problems
Traffic, wind and exterior noise arriving through leaks is one problem. A drum kit heard through a party wall is another. They have different solutions and only the first is an insulation question.
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Seal every penetration
Back-to-back outlets, recessed lights, duct boots and the perimeter of the assembly. This is real, measurable work and it is where foam is genuinely the right material.
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Choose the cavity material on the evidence
If transmission loss through the partition is the target, the published test data points at fibre, not foam — particularly on steel studs. We will say that rather than sell against it.
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Point at the parts that are not ours
Mass, decoupling, resilient channel and door and glazing selection do most of the work in a serious sound room. Insulation is one layer of a system that we do not pretend to supply in full.
Frequently Asked Questions About Sound Room / Acoustic Insulation
Is spray foam good for soundproofing?
Not in the way it is usually sold. On sound transmission class through a stud wall, the published test data puts spray foam level with or below fibre insulation: open-cell at STC 33 and closed-cell at STC 31 against fibreglass at about 34 in one 2019 test series, and Johns Manville publishing 2x6 R-19 batts at STC 45 against five inches of closed-cell at STC 37. NRC Canada's mid-rise wood research measured direct STC dropping by 4 to 6 points when spray foam replaced glass fibre. What foam does well is stop noise arriving through leaks and penetrations — which is often what a homeowner actually notices from traffic or wind — and that is the claim we will make.Is closed-cell or open-cell better for sound?
Open-cell performs slightly better and is roughly comparable to fibre; closed-cell is the weakest of the three on transmission loss, because a rigid, dense, well-bonded material couples the two faces of the assembly together. On steel studs the penalty is largest: the Gypsum Association reports fibreglass and mineral wool outperforming both foams by 8 to 10 STC points there, because foam stiffens the framing that was providing the decoupling.So why would anyone use spray foam in a sound room?
Because a sealed room is a quieter room, and because the same assembly usually has a thermal job to do. Sound arriving through an unsealed outlet box, a duct boot or an un-gasketed door is not attenuated by anything in the cavity. If the room is on an exterior wall it also has to meet the Zone 7A thermal requirement, and if it is over an unheated space it has to meet effective RSI 5.02. Foam can do the air and thermal work while a fibre layer or an additional gypsum layer does the acoustic work.What actually improves STC in a partition?
Mass, decoupling and cavity absorption, roughly in that order, plus airtightness so the assembly performs as tested. That is why an extra layer of gypsum, resilient channel or a staggered or double stud wall changes a partition's rating more than swapping the cavity insulation does. We are an insulation contractor and this is beyond our scope to design — but pointing at it is more useful than selling you a cavity fill that the published data says will not get you there.Does foam absorb sound inside the room?
Absorption is a separate measurement from transmission, and open-cell foam does have published absorption values — 0.55 and 0.75 noise reduction coefficient for two products, measured to ASTM C423. But absorption within a room affects reverberation and the room's own character, not how much sound leaves it. And in an occupied room the foam has to be covered anyway under Article 9.10.17.10, which puts an interior finish between the foam and the air.