FHB Podcast Segment: XPS vs. Polyiso for Continuous Insulation

A recent Fine Homebuilding podcast episode tackled a question homeowners doing exterior renovations often face: which rigid foam insulation to use on the outside of a house. A listener remodeling a 50-year-old home in North Carolina wrote in asking whether to use XPS or polyiso foam board as continuous insulation (a layer of foam that wraps the whole house to cut heat loss through the wood framing) under new siding, along with a rainscreen, which is a small air gap behind the siding that lets moisture drain and dry out. Both foam options cost about the same at the 1-inch thickness being considered, so the choice comes down to performance rather than price.

The podcast hosts discussed the tradeoffs between the two foam types, since each behaves differently depending on climate and how it is installed. This kind of decision matters mainly to homeowners who are re-siding a house or adding exterior insulation as part of a larger renovation, since the type of foam chosen affects how well the wall resists moisture and keeps heat in or out over time.

If you are planning a similar project, the choice of foam type is one of several details worth understanding before work begins, alongside the rainscreen approach itself, which is increasingly common in siding replacements. Local climate plays a role in which product tends to work better, so it's worth learning how these materials perform in a climate similar to your own before settling on one.