Slab top-side vapor control — Airspace approach

[diyfun]

Read the "READ THIS BEFORE YOU DESIGN,BUILD OR RENOVATE" doc and found above recommendation on page 13 or 19, Or view it here (click me). Many of you here, including Dana, seems like "poly + foam + plywood" approach for basement floor. Above approach is "dimpled + foam + plywood".

The dimpled is more expensive than poly. is it worth to spend several hundreds more?

[Dana]

I haven't seen any good science that would indicate a need for dimpled membrane rather than 10-mil poly as a capillary break when retrofitting a slab. The wicking potential of either poly or XPS is extremely small, and no-wicking/drying toward the basement==no mineral depletion of the concrete. The slab itself doesn't "need to dry", and encouraging moisture migration through the slab only degrades it. The additional benefit of having an air-space between the extremely low-permeance non-wicking poly sheeting and the concrete is academic.

Dimpled membranes may be useful under ceramic or wood flooring with NO foam with damp slab, where a rip in a poly sheet might allow some wicking of moisture into the ceramic, staining it. But with an inch of (no-wicking low-permeance) XPS that's not a likely scenario.

If the water table is occasionally high and you regularly get wet spots or efflorescence on the slab, use a concrete sealer on it before the poly + foam.

[diyfun]

Do I need to leveling the floor before put poly+ foam?

[Dana]

If you were planning to use a cement based floor leveler it'll bond better and have fewer cracking issues if you apply it to the slab, and not on top of the foam.