Attic and Crawlspace Bypasses
Attic and crawlspace bypasses are penetrations into the living spaces. Pipes, ducts, flues, and electric wires are the most common reason for these penetrations, and the best way to seal them is often (but not always) with ccSPF. Because ccSPF expands and seals, it does an excellent job of filling voids that allow conditioned air to escape. However, ccSPF should not be used to seal around high-temperature areas such as combustion appliance flues.
The Structural Properties of Foam
Recent research conducted at the University of Florida has demonstrated that closed-cell spray foam (ccSPF) applied to the underside of roof decking effectively bonds the sheathing to the framing, significantly increasing uplift resistance. The study, conducted by Dr. David O. Prevatt and funded by Honeywell and Huntsman, two makers of ingredients that go into ccSPF, found that 3 inches of the foam sprayed between framing members provided a threefold increase in uplift resistance as compared with traditionally installed roof sheathing panels. While these results sound impressive, Dr. Prevatt points out that the increase provides the same benefits as increasing the nailing schedule to a 6/6 schedule (every 6 inches along panel edges and every 6 inches in the field) from the usual 6/12 schedule. What was perhaps most impressive is that using only spray foam to glue the sheathing to the framing provided almost as much resistance (178 to 209 psf) to uplift as does 8d common nails (205 psf) installed at the 6/6 schedule. This suggests what may be the biggest structural advantage of a foamed roof assembly — reducing the likelihood of a roof blowoff when the sheathing doesn’t get nailed off with enough nails or when too many nails miss their mark.
The uplift study also evaluated the benefit of installing a “fillet”: a 3×5-inch bead of ccSPF in the corners between the sheathing and the roof framing. The fillet method effectively doubled the uplift resistance of the baseline assembly of 2×4 framing on 24-inch centers sheathed with 1/2-inch OSB nailed on a 6/12 schedule. The uplift study is one of several recent studies of the structural properties of ccSPF. Tests conducted by Building Science Corporation (BSC) to evaluate the impact resistance of wall systems showed that conventional wood-framed walls do not have the same impact resistance as impact-resistant windows. (That is, walls consisting of studs, 1/2-inch OSB sheathing, housewrap, and siding cannot sustain the impact required by the ASTM E1886 and E1996 missile test, which hurls a 9-pound 2×4 at 50 feet per second.) The only test panel in the BSC demonstration capable of resisting the required impact load included a layer of 1/2-inch OSB sheathing between 1-inch foam insulating sheathing and 2 inches of ccSPF sprayed between 2×6 studs. Surprisingly, BSC found that a wall with foam sheathing, housewrap, and ccSPF (no OSB) performed better in impact tests than a wall with housewrap and OSB sheathing.
The BSC study notes that walls may not have to be built to the same standard as windows, despite these surprising results. When a window fails under impact, the resulting hole in the wall (the entire window) is relatively large, providing a big enough hole to internally pressurize a home, which often leads to catastrophic failure. When a wall fails, the zone of impact is marginally bigger than the impacting face of the projectile. Such an opening may not be large enough to have a catastrophic effect. — Clayton DeKorne
Living Spaces Over Garages
Living spaces over garages create conditions that demand careful attention to insulating the floor. Yet it is difficult to support the insulation in this cavity, and oftentimes the insulation falls onto the garage ceiling. This separation between the insulation and the living space floor creates a thermal bypass that compromises the value of the insulation. Air easily infiltrates in at the band-joist area over the top of the insulation, which scavenges away heat. This often freezes plumbing pipes, creates cold floors, and can lead to major mold and water damage. Builders often try to solve the problem by supplying
forced-air heat near the plumbing, but this only succeeds in pressurizing the space with warm, humid air. As this air exfiltrates through the exterior cracks, it can condense and lead to even worse moisture and mold problems at the band-joist areas.