A.Julius Ballanco, P.E., president of J.B. Engineering and Code Consulting in Munster, Ind., responds: Unlike solid PVC pipe, cellular-core PVC pipe is a co-extruded product with at least three different layers. The inside and outside walls are solid PVC, while the inner core is cellular (or foamed) PVC, a material that includes tiny bubbles of entrained air. Cellular-core PVC pipe is available in various wall thicknesses, including Schedule 40 pipe for DWV and thin-wall sewer-grade pipe. For pipe manufacturers, the main advantage of cellular-core PVC is lower cost, since it requires less resin to make than solid pipe.

The model plumbing codes permit the use of either solid-wall PVC or cellular-core PVC pipe in residential plumbing systems. However, cellular-core PVC pipe is not as stiff as solid PVC pipe. At 5% deflection, a 4-inch cellular-core PVC pipe has a minimum pipe stiffness of 200 pounds-force per square inch, while a 4-inch solid-wall PVC pipe has a minimum pipe stiffness of 310 pounds-force per square inch.