Basement Slab Design

A homeowner building a new house in a cold northern climate (zone 7A, found in parts of the upper Midwest and similar regions) asked where to place a plastic vapor barrier in a basement floor that will have built-in radiant heating. The floor design stacks a concrete slab over two layers of rigid foam insulation (EPS, a common foam board), with a layer of crushed rock below for radon venting. The question was whether the plastic sheeting should go above or below that foam.

A contributor with building science expertise answered that in this climate zone, a basement wall built with 2x6 framing needs roughly 3 inches of foam insulation to avoid condensation problems if there's no separate vapor barrier inside the wall. Using just 2 inches of foam, as this homeower's plan calls for, means a vapor retarder is still needed, either as a plastic sheet or as faced insulation batts. For the floor itself, the plastic generally belongs above the rigid foam, not below, so it does not float out of place during the concrete pour. Because this slab will have radiant heat tubing embedded in it, the response suggested using foam panels with built-in bumps for anchoring that tubing, since many of these come with a vapor barrier already attached, simplifying installation.

For anyone planning a heated basement slab in a cold climate, this points to checking that wall foam thickness and vapor barrier placement meet recommended minimums for the local climate zone before pouring.