In the last issue of Coastal Connection, we discussed the use of construction adhesives to boost the wind resistance of wood-framed walls and roofs. Besides the structural advantages, however, adhesives can also play a role in the performance of air-tight assemblies in the high-performance energy-efficient house. For an example, we can turn to the Rhode Island house that energy consultant and insulation contractor Ned Reynolds built for himself back in 1995, and wrote about in the April, 1996 "New England Update" section of Hanley Wood's Journal of Light Construction ("Tightest Little House in Rhode Island," by Ned Reynolds). The airtight details in the home included an unusual wall framing system. Reynolds eliminated window headers in favor of a load-bearing band joist for the floor systems. He ran his wall sheathing vertically to eliminate horizontal sheathing joints, lapping the sheathing panels over the wall plates. And he glued the sheathing to the framing, making an adhesive bond between the sheathing and the walls and turning all the wall cavities into individually sealed airtight pockets.
Rhode Island contractor Ned Reynolds built this airtight wall system detail in the mid-1990s, using 8-foot sheets of plywood run vertically and fastened with adhesive as well as nails. Today, the availability of extra-long OSB sheathing in the market would enable a similar detail, but with sheathing spanning across the band joists as well as the walls, for enhanced structural bracing and wind uplift resistance along with airtightness and energy performance. A decade and a half later, reports Reynolds, the careful details have proven their worth. He no longer lives in the home, he says, but for many years he tracked fuel use, and the house consistently used about 450 gallons of heating oil per year for space heating and hot water. "We rarely cooled the house," he says — "we were able to keep the place about 15 degrees cooler than the outdoors in the summer just by opening the windows at night and closing them during the day." After building his house, Reynolds worked for four years as an energy auditor for Conservation Services Group (CSG). He has blower-door tested about a thousand houses himself, and seen the results from thousands of other tests, he says, and over the years he has seen barely a handful of houses that exceeded the airtightness of his own home. "I had 330 CFM of leakage at 50 pascals. That's very, very low for a 2400-sqft house." Reynolds cautions against giving the airtight wall assembly too much credit for the home's exemplary efficiency and airtightness. "I think what really made the difference was that the roof was insulated, and I had furred that out, so I had a 12-inch cavity; and then the basement walls and floor were both insulated. And I paid a lot of attention to the rim and band joist areas." Wall systems aren't the big player in home airtightness, says Reynolds: "When you do blower-door testing of houses, most of the air leakage is in the attic and in the crawl spaces. Walls aren't all that significant." On the other hand, total house airtightness is not the only factor in energy efficiency. Reasonably air-tight drywall may reduce air leakage in and out of the house below any industry standard. But if there's air movement into and out of the stud cavities from the outdoors, that can degrade insulation performance and the effective R-value of the assembly — even if that air movement does not communicate all the way into the living space of the home. Air movement into and out of walls can also create moisture problems. And as Reynolds points out, lapping pieces of sheathing over the floor system band joists and gluing them in place improves the airtightness of those floor system gaps — one of the critical vulnerabilities in any home. So as builders consider the use of adhesives in their framing systems, it may be worth looking at where the structural advantages and the energy-efficiency advantages dovetail, with the use of a relatively cheap material: glue.
