Rain Screen under Exterior Insulation

A homeowner working on an old house asked a building forum a practical question while re-siding: after stripping old siding, installing taped and sealed Zip sheathing (a structural panel that also acts as a water and air barrier), and putting in new windows, does rigid foam insulation added to the outside of the wall need a "rain screen" gap behind it?

A rain screen is a small air gap that lets any water that gets behind the siding drain and dry out, instead of sitting against the wall. The answer from another forum member was clear: the gap needs to go on the outside of the foam insulation, not between the sheathing and the foam. Putting the gap on the inside, next to the sheathing, would let air move behind the insulation layer, and that air movement would undercut the foam's ability to keep heat in. The insulation needs solid, uninterrupted contact with the wall to work as intended; the drainage gap belongs between the foam and the siding.

This is a good detail to know if you're planning your own re-siding or exterior insulation project on an older home. Getting the layers in the right order (sheathing, foam insulation, rain screen gap, siding) affects both how well the wall insulates and how well it sheds water over time, so it's worth confirming with whoever is planning the work on your house before the insulation goes up.