On a recent build, Matt Risinger leaned on the adhesive capacity of two-part, closed-cell polyurethane foam to provide increased racking resistance to the frame walls and uplift resistance and waterproofing to the framed roof.

University of Florida researchers tested three levels of spray-foam roof underside retrofit: a “fillet” application at the joint, a one-inch application across the whole cavity (as might be used in a "flash and batt" application), and a three-inch application filling the bay. The thicker foam application provides the most uplift strength (far stronger than nails alone). But the thicker foam also tends to trap moisture in the sheathing in the case of a roof leak. However, applying self-adhering membrane roof underlayment to the top of the sheathing can sharply limit this moisture risk, the researchers report.
University of Florida researchers tested three levels of spray-foam roof underside retrofit: a “fillet” application at the joint, a one-inch application across the whole cavity (as might be used in a "flash and batt" application), and a three-inch application filling the bay. The thicker foam application provides the most uplift strength (far stronger than nails alone). But the thicker foam also tends to trap moisture in the sheathing in the case of a roof leak. However, applying self-adhering membrane roof underlayment to the top of the sheathing can sharply limit this moisture risk, the researchers report.

Risinger is on solid ground when he claims that closed-cell foam can increase the structural capacity of wood framing. The basis for the structural properties of closed-cell spray foam stems from research conducted at the University of Florida, led by Professor David O. Prevatt, a P.E. and Ph.D. who is an expert in wind engineering.

JLC reported on Prevatt's early work in 2008 and followed up in 2013 with a report on a number of studies led by Dr. Prevatt. This work found that closed-cell spray foam can double or triple the strength of a wood-frame assembly and improve a building's performance against high winds. The research examined three application types, including the "fillet" (or what Matt called "picture framing" the building cavity) and a 1-inch-thick application suitable for the "flash and batt" method that Matt uses.

In addition to structurally enhancing the frame, Matt's crew sprayed each of the joints between roof sheathing panels, sealing the gaps between panels from below. This will provide backup water protection if a high-wind event should ever rob the house of its roof.

Of course, this is not just a structural or waterproofing system, it's an insulation and air-sealing system, as well. On this house, Matt is using Rockwool as continuous insulation on the exterior walls and is filling the remainder of the stud cavities with a Rockwool batt, so he ends up just shy of R-40. In the roof, he fills the rafter bays with a less expensive open-cell spray foam to get an R-40 roof, too. And, as long as the closed-cell foam is applied 1-inch-thick, it will provide an excellent air barrier.