Carpet padding
The cushion layer installed between the subfloor and the carpet backing, which absorbs foot pressure, extends carpet life, and is the primary source of wick-back when saturated with prior spills.
Also known as: carpet cushion, carpet underlay
Carpet padding is the cushion layer that sits between the subfloor and the carpet backing. It absorbs foot pressure, insulates the room, dampens sound, and materially extends the useful life of the carpet by protecting the fiber structure from being ground against a hard floor. Most residential padding is bonded urethane foam; commercial and specialty installations use fiber, rubber, or waffle-rubber constructions.
From a cleaning standpoint, padding is where the wick-back mechanism lives. Any spill that soaks through the carpet backing into the padding does not simply disappear — it saturates the pad, and as the carpet later dries after any cleaning, moisture in the pad wicks upward and carries residue back to the surface. Pet urine is the classic worst case: urine saturates the pad, the pad wicks the contamination upward for months, and no amount of surface cleaning by itself resolves what is actually happening below the fiber.
Professional cleaning approaches heavily contaminated padding differently than surface soiling. Sub-fiber saturation cases may require dedicated deep-extraction techniques, sub-carpet treatments, or in the most severe cases pad replacement. Setting expectations at pre-inspection about what the padding is or is not carrying is what allows honest scoping — extraction of the carpet alone cannot fix a saturated pad.