Fan-Fold Foam Board

Fan-fold foam board is widely marketed as an easy fix for improving the thermal performance and water resistance of walls when re-siding. But if not applied properly, this option could lead to moisture problems, according to leading building-science experts.

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Source: REMODELING Magazine
Publication date: September 1, 2005

By Clayton DeKorne

Best practice calls for stripping off existing siding, then installing a good weather barrier.

Fan-fold foam board is widely marketed as an easy fix for improving the thermal performance and water resistance of walls when re-siding. But if not applied properly, this option could lead to moisture problems, according to leading building-science experts.

At about ¼-inch-thick, fan-fold foam board adds only R-1 to R-1.5 to the wall system, a negligible insulation improvement. Foil-faced products promise higher R-values, but only when installed with a minimum ¾-inch air space in front of them, an option that requires siding over strapping. According to foam board manufacturers, the advantage of fan-fold is really that the board provides a consistent surface over the old siding, eliminating cracks and crevices that lead to energy-robbing drafts. All manufacturers recommend taping the foam to help stop air leaks and prevent water that gets through the siding from leaking behind the foam.

Photo: Courtesy THE JOURNAL OF LIGHT CONSTRUCTION
Certainly, the foam board provides a smooth working surface for installing new siding, but whether there is a significant improvement in thermal performance is hotly debated. “The danger is that you are putting an impermeable layer over the wall,” explains Steve Easley, a building consultant who specializes in helping building.

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