Kohler Underscore RL 60" x 32" Alcove Soaking Bathtub

Kohler Underscore RL -- the alcove soaking benchmark

The Kohler Underscore RL (K-1139) is the most-specified alcove soaking bathtub in Kohler's residential lineup. "RL" designates the right-hand drain configuration; K-1140 covers the left-hand variant. At 60 inches by 32 inches with a soaking depth of 17.5 inches, it fits the standard 60" alcove opening found in the majority of US homes and delivers a bath experience that sits measurably above the builder-grade fiberglass shells that typically occupy that same space.

The distinguishing engineering is Kohler's integral lumbar support -- a contoured backrest molded directly into the acrylic shell rather than added as an accessory. That backrest positions the bather at a reclining angle that keeps the head above the waterline while submerging the body to the shoulders, which is the functional definition of a "soaking" tub (as opposed to a standard residential tub where the soaking depth is only 12-14 inches). The net result is a bath that requires about 45 gallons to fill to soaking depth versus 80+ gallons in a whirlpool or freestanding design -- lower fill time, lower water heating cost, and lower refill frequency.

Construction and materials

Shell material is Kohler's proprietary acrylic -- thicker than industry-standard builder-grade acrylic and reinforced with fiberglass backing. The surface is non-porous, resists staining from bath oils and tub cleaners, and carries a limited lifetime warranty from Kohler. The integrated apron (front panel) is included, reducing the total cost of a bathroom renovation by eliminating the need for a tile or stone apron build-out. Drain location is left or right (verify by facing the tub from the room); the K-1139-0 variant ships in white, -96 in biscuit, -95 in ice grey.

Installation requirements

The Underscore RL installs in a standard three-wall alcove with the open face toward the bathroom. Framing requirements: 60" rough opening width (+/-1/4"), minimum 32" depth, minimum 14" rough-in height for the drain connection. The acrylic shell requires a mortar bed or adjustable leveling feet for support -- Kohler supplies a leveling kit, but most installers prefer a mortar bed for long-term rigidity. Water supply rough-in for the faucet is a separate purchase; the K-1139 is the tub only, not a faucet kit. Deck-mount faucets are compatible on the ledge deck; wall-mount faucets require blocking in the framing before tile-up.

Compared to builder-grade alternatives

The typical alcove competition at lower price points (Sterling, Aquatic, American Standard Cadet fiberglass) uses thinner acrylic or fiberglass construction with shallower soaking depths of 12-14 inches. The Underscore RL's 17.5" depth and lumbar support justify the price premium for owner-occupied baths where the tub will be used for soaking rather than just showering over. For rental or flip projects where soaking depth is irrelevant, the American Standard Cadet or Sterling Ensemble at $350-$450 installs identically and carries a lower installed cost.

Who should buy the Kohler Underscore RL

Primary bath renovation where soaking depth matters, standard 60" alcove opening, and the homeowner wants a US-brand lifetime-warranty acrylic tub without the footprint or floor-reinforcement cost of a cast-iron soaker. At $650-$850 unit cost plus $300-$600 installation, the all-in cost of $950-$1,450 is the sweet spot between builder-grade fiberglass ($700-$900 installed) and premium stone-resin freestanding ($3,500-$8,000 installed).