Flooring puzzle for uninsulated slab + humidity
I have a 1965 Sears cape in coastal Rhode Island (Zone 5, moist). It is built slab-on-grade, and I don’t think there is any insulation or vapor barrier under the slab, because 1965. Currently there is carpet laid on the slab with a pad underneath.
In the Summer, during times of high humidity, the relatively cooler top surface of the slab becomes a condensing surface. This creates moisture in the house and carpet. I would like isolate the top of the slab from the moist interior air, and lay an alternative flooring material that won’t hold moisture. The challenge is that the first floor ceilings are only 7’6″, so a thick insulated floor is not an option. I may be able to add a total of 1″ to the floor without making the ceiling seem too low.
My current thinking is that I need to (a) lay very thin layer insulation on the slab, (b), air seal over that insulation to prevent moist indoor air from reaching the top of the slab, and (c) add a thin moisture-tolerant flooring material that also provides some insulation.
For example, coat slab with liquid-applied moisture barrier. Then a layer of 1/2″ foil-faced polyiso, with taped seams. On this, lay 1/4″ plywood subfloor. Top with marmoleum or cork flooring.
Another option is the Forbo Quick Fit subfloor system, that appears to feature a taped insulating layer under a two-layer subfloor, which is topped with marmoleum: https://www.forbo.com/flooring/en-au/technical/subfloor-underlay/pmty4i#panel_6
I’m open to suggestions.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part