Isolating a new slab from existing sill
RedfordHenry
| Posted in Construction Techniques on
Have an addition coming up that will have a frost wall and a slab flush with top of the new foundation. The existing house has sturdy granite ashlar walls for a foundation but they are only about 6 inches above grade. The top of the new foundation wall, as well as the slab will be about 3 inches above the top of the granite ashlar blocks which means they will be adjacent to the existing mudsill (the existing mudsill is 6×6 handhewn oak and is as solid as the granite upon which it sits). My concern is the upper three inches of slab that will be directly against the existing mudsill. Was thinking of isolating the new slab from the existing mudsill with 2″ of XPS foam. Any other ideas on how to keep the new concrete off the old mudsill?
Replies
Thinking of pouring the frost footing right up against the granite?
If any of this 3" overlap is exposed to the weather, you'll destroy that nice oak real quick.
Are both sides of the oak conditioned space?
Depending on the complete situation you may want anything from;
Bins, Kilz, Oil based primer, latex primer, or even Automotive spray enamel. They are all bond breakers and short term water barriors. That may be all you need.
Wax paper is a better bond breaker and a short term vapor and moisture barrior.
15# felt is not a vapor barrior, and in this application would not be a moisture barrior, but does allow some slow evaporation.
Cardboard, with the currogations vertical is a long term air gap and short term water barrior. Leaves a nice gap at the top for seal caulk.
Stick on Ice and Water is a permanent H2O barrior.
SamT
This is basically a rectangular addition so the top of the 4' frost wall foundation will go up against the granite blocks in two locations. Eventually, nothing will be exposed to the weather. The existing oak mudsill sits on top of the granite and is covered with sheathing/siding, it is not within conditioned space (this is an old house sitting over a shallow crawl space). The addition is a three season room, not heated. The sill will never be within conditioned space although the new slab will be insulated with 2" extruded polystryrene. All theory says the mudsill should be pulp by now but it's not, and the idea of using it to form one edge of the new slab doesn't sit right. I like the idea of I&W, maybe another use for my buddy, Mr. Vycor.