I’m preparing to do a foam-insulated, ventilated, rainscreen retrofit on an old house and have a couple of issues.

First is the fact that the studs have erratic spacing which is often more than 24 inches. I’m maintaining the original 3/4 inch fir cladding inside and out or I would probably just add some additional studs. I’m going to use fiber cement lap siding and have discussed this with at least one supplier engineer who maintains that the exterior fir cladding (to which I have added additional screws to insure its integrity) is sufficient to attach to when a stud is not available. He stressed that plywood or OSB cladding would not be sufficient. So the idea would seem to be to run screws through the siding, the furring, the foam and into the fir cladding and a stud where possible. That seems reasonable. I’m not a contractor and this is my own house so I’m not so concerned with siding warranty issues – just that it would work. Any thoughts?

The second issue that springs to mind (and which was reiterated recently by a contractor friend) is the idea that the screws penetrating the foam seem counterintuitive with respect to my use of the foam as a primary WRB (not to mention the thermal issue). There will be felt behind the foam as a secondary WRB but I can’t help wonder why it isn’t an issue with respect to possible water penetration. I have thought about putting a strip of self-healing flashing over the foam for each row of screws to go through. A more elaborate scheme would seem to be to use some kind of furring that is sufficient to support the siding. It would be minimally screwed through the foam into cladding and studs and then the siding would only be screwed into the furring thus eliminating a large number of penetrations. I’m planing on using 1/2 PT ply strips which obviously would not serve the purpose but some kind of metal channel might serve. Is it the additional cost and effort involved in such a scheme that keeps it from being typically used? Or is it just not a real concern?