Boss 334 Window & Door Minimal Expanding Gun Foam is a one-component, polyurethane foam used to seal the gap around windows, door frames, & rough openings.   Boss 334 Window & Door Minimal Expanding Gun Foam expands to generate an effective seal, and when used properly is proven not to distort or bow window and door frames. Boss 334 Window & Door Minimal Expanding Gun Foam will achieve a Sound Transmission Class (STC) of 58 when applying it in accordance with ASTM C-919 “Standard Practice for use of Sealants in Acoustical Applications”.