Q: How do I fasten cedar shingles to sidewalls covered with rigid foam?
A: Paul Eldrenkamp, owner of Byggmeister, a custom remodeling firm in Newton, Mass., responds: That depends partly on the thickness of the foam. If it's 1 1/2 inches or less, we apply Cedar Breather over the taped foam to provide back venting, and fasten the shingles to the structural sheathing beneath with 2 1/2- or 3-inch stainless steel nails. That approach has worked fine in our area, but we aren't subject to big wind loads. If you're not comfortable using the surface of the foam as the drainage plane, you could apply housewrap or asphalt felt under the Cedar Breather.
If you're adding 2 inches of foam or more, the easiest option is to use cedar-shingle panels, which consist of shingles laminated to a plywood backing. They can be applied over vertical battens, just like back-vented clapboards. Despite the premium pricing for the panels, this may be the most cost-effective strategy when labor costs are factored in.
Another approach is to apply a layer of 1/2-inch sheathing over the foam, fastened through to the framing with long screws. We then cover the sheathing with asphalt felt and Cedar Breather, which I feel offers better drainage and back-venting than one-step drainage wraps like Tyvek DrainWrap.
In a few cases, we've actually installed horizontal battens for each course of shingles. That can go pretty fast if the shingle exposure is 6 inches or more and you're dealing with large expanses of mostly uninterrupted wall, but it's impossibly time-consuming if you're dealing with cheek walls of dormers or other chopped-up areas. In my experience, there's no need to cut kerfs in the backs of the battens to provide drainage - we just leave a gap between the butt ends of the strapping. In cases where we've opened up such walls years later (we did the first one this way in 1989), we've found that the assembly was performing just fine.
