Carpet underlay grades
10mm or 11mm PU underlay is the residential workhorse, quiet, warm, the right firmness for power-stretching. 9mm or 11mm crumb rubber is the heavy-domestic and stair-rated choice. 8mm sponge rubber is a budget option for landlord refits. Contract underlay (firm, dense, low-profile) is what goes under carpet tiles and broadloom in offices.

