Slab to foundation wall: Air Barrier and capillary break detail with interior rigid foam
We’re planning on insulating our basement walls with 2 inches of Thermax (foil-faced polyiso) rigid foam, installed with adhesive on the interior of the wall. We’ll also have 2 inches of XPS beneath the 4 inch slab, and would have these different types of foam connect right on top of the footer, similar to the detail shown at:
So between the slab and the foundation wall there will be the 2 inches of Thermax. Question is … how should we handle the air barrier at this line where the slab butts up against the foil-faced Thermax on the wall. Would any particular type of sealant be recommended for this application (foil to concrete)? Or should we maybe just seal between the XPS and the Thermax, before the slab is poured? Or is this all unnecessary – will the concrete against the two types of foam be sufficient as a seal? Note: We will not be furring or framing up the walls or finishing the basement for now (maybe later.)
Also, in reference to above linked detail, if anyone would like to explain the implied installation of the capillary break, that’s also confusing to me. We will have some type of capillary break, yet to be determined, but were thinking it would just be installed (or liquid applied) between the footing and the foundation wall. This detail seems to show it lapping up above the sub slab foam. I guess part of this is to prevent capillary movement of moisture up from footing to the foam on the wall? But why would it need to lap over the poly under the slab? And wouldn’t this make any sort of sealing between the sub slab foam and the wall foam a lot trickier? Could we just extend the capillary break flat on the footing so that it would also be under the end of the Thermax? Wouldn’t that be good enough to prevent any wicking?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part