A: Decades of acoustical testing have shown that fibrous insulations, such as fiberglass, mineral wool, and cellulose, generally increase the sound transmission class (STC) of cavity wall systems and floor-ceiling assemblies. There is very little acoustical testing, however, of construction assemblies with spray foam insulations.
To better understand the effects of spray foam insulation on STC performance, the Gypsum Association sponsored a series of acoustical tests on common wood-framed (UL U305) and steel-framed (UL U465) wall systems with several cavity insulation conditions: no insulation, 3-1/2” fiberglass batts, 3” mineral wool batts, 3” closed-cell polyurethane spray foam, and 3” open-cell polyurethane spray foam. The framing conditions for both the wood-framed and steel-framed wall specimens met the base requirements for UL U305 (2 x 4 wood studs spaced 16” o.c.) and UL U465 (3-5/8” 18 mil steel studs spaced 24” o.c.) without resilient channels or other acoustic treatments.
The test results on the wood-framed wall show that all insulation types provide nominally higher performance (2-4 STC points) than the empty cavity (no insulation) condition. These results suggest that the stiffness of the wood stud framing condition governs sound transmission performance of these systems, and any cavity insulation is slightly better than no insulation at all, but no one insulation type is significantly better, or worse than another.
The test results on the steel-framed wall tell a very different story, with the fiberglass and mineral wool insulations yielding results 8-10 STC points higher than the open-cell and closed-cell polyurethane spray foam insulations. These results suggest that, in construction systems with more flexible or acoustically resilient framing conditions, the stiffening effects of spray-foam insulations can limit sound performance and fibrous insulations are far more effective in achieving high STC.
The scope of the test program summarized above was limited to the basic wood-framed and steel-framed wall systems tested. Sound transmission performance of any construction system or assembly is affected by myriad factors, so it is critical to evaluate performance on a case-by-case, system-by-system basis. As such, when considering the use of spray-foam insulation in a sound-rated assembly, it is important to request relevant information and test data from the insulation manufacturer or consult with a qualified acoustic design professional.