Best practice for stone siding over stick-built walls?
We are planning to apply 4 inch thick rubble stone siding over the lower level of our daylight basement home. We had originally planned for ICF at the lower level walls, partly to mitigate rain water and inward vapor drive risk brought on from the stone (which is south facing). As it turns out the ICF represents a substantial cost increase and we are considering alternative wall options (stick-built) for the above grade portion of the lower level wall.
Due to the irregular, non-dimensional geometry of our stone, the use of a conventional rain-screen does not seem feasible. If we go the stick-built route, we are considering a Delta-Dry or similar engineered rain-screen layer instead; with some type of sheathing (rigid foam?) that would allow a parging coat and stone to be directly affixed over it. We would need to accommodate shear requirements where indicated and still maintain our R-value. Where would our water resistive and vapor impermeable barrier/barriers fit into this? Can you recommend a detail that would best address this scenario?
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A "conventional rainscreen" can be an air gap anywhere from 1/4" to 5" deep between the drain-plane or insulating foam sheathing and the stone. It doesn't have to be a smooth, even dimension, just an air gap. It's primary function with a masonry cladding is to provide a capillary break, and any amount of air gap will do.
A secondary but still important function with stone cladding would be to allow a convection path to exchange potentially humid air in the gap with outdoor air.
Here is a link to a GBA detail that includes for stone veneer, and air space, rigid foam, and a stud wall: GBA detail with stone veneer.
The GBA detail library contains many other details with stone veneer and rigid foam.
There are other ways to do it, of course. Integrating a layer of Delta Dry makes sense.
When building a wall with natural stone, you aren't really going to be adhering the stone to any kind of substrate. Instead, you will be building a self-supporting wall. This requires some skill. The stone wall can be tied back to the wall framing with masonry ties, as shown in the GBA detail, but it's still important to have an air gap between the stone masonry and the rigid foam (or the WRB), as with a brick veneer wall.
OK, so if the air gap needs to be directly behind the stone I may need to re-think the stone we had planned to use (free beautiful and on-site). I’ll get a mason’s perspective but I’m worried that a freestanding wall with irregular shaped stones and only a 4 inch cross-section would be tricky even with ties. I had hoped to apply the stone against a rot impervious substance and place a rain-screen/channel behind that. You are advising against that, Yes? Would your recommendation be the same if we were to stay with the ICF wall design we had originally planned for?
Q. "I’ll get a mason’s perspective."
A. Good. That is essential.
Q. "I’m worried that a freestanding wall with irregular shaped stones and only a 4 inch cross-section would be tricky even with ties."
A. You're right to be worried. I've done a lot of stone masonry, and I prefer my stone walls to be at least 8 inches thick.
Q. "I had hoped to apply the stone against a rot impervious substance and place a rain-screen/channel behind that."
A. You can do that with thin manufactured stone, but I don't think that you can do that with natural stone. But talk to your mason.
Q. "Would your recommendation be the same if we were to stay with the ICF wall design we had originally planned for?"
A. An ICF wall has a layer of EPS on each side, and the EPS wouldn't provide as much structural support for a thin stone wall as would a concrete wall. (If you were installing the stone against a concrete wall, it would be easy to add mortar between the stones and the concrete -- and this approach would mean that the concrete wall would lend structural support to the stone wall.) But it's possible that you could install relatively thin stone directly against EPS if you had an ICF wall, and it's possible that the EPS could supply some structural support. But frankly, I've never done that.