STC Rating Calculator

Because spray foam is applied as a rigid, fully-adhered layer that bonds to both wall faces, which can transmit vibration across the cavity almost like a solid bridge. Fiberglass batt and mineral wool sit loosely in the cavity and absorb sound energy through friction rather than bridging the gap, which is why this calculator scores mineral wool highest (6 points), fiberglass batt next (4 points), and spray foam lowest of the three insulation options (3 points) for STC purposes even though spray foam often wins on thermal performance.

Staggered/double studs alone add 10 points and resilient channel alone adds 8, but combining them on the same wall caps the total decoupling contribution at 13 points (10 from staggered studs plus a reduced 3 from resilient channel), not 18. Once the wall faces are already substantially decoupled by staggering the studs, adding resilient channel on top provides a smaller additional benefit -- diminishing returns, not simple addition.

No -- in this calculator, both only apply a bonus when Wall Type is wood stud or steel stud. Concrete block and poured concrete walls rely on mass for sound isolation rather than the wall-face decoupling that resilient channel and staggered studs are designed to provide, so selecting either on a masonry wall type produces no change to Estimated STC.

No. This calculator applies a published rule-of-thumb points system to estimate STC from assembly components; a certified rating comes from measuring transmission loss across a set of one-third-octave frequency bands per ASTM E90 and fitting the results to the reference contour defined in ASTM E413. Use this estimate for early design comparisons, not as a substitute for a lab-tested or manufacturer-published assembly rating on a real project.

Each additional drywall layer beyond the first adds 4 STC points in this calculator, up to a maximum of 3 layers per side (2 additional layers, or 8 points). It is a straightforward, uncapped-relative-to-other-bonuses way to raise STC, but the resilient-channel and staggered-stud decoupling options generally deliver more points per construction step for stud walls specifically.