Foam Shrinks, and Other Lessons

Engineer and building scientist Joseph Lstiburek did a deep-energy retrofit on a barn 16 years ago. A problem that surfaced four months later--a clear melting pattern of frost on the roof--led to years of wondering about other possible problems with the retrofit. Lstiburek eventually decided to take the barn apart, see how its components had performed, and put it back together again using the best commercially available technology. In this article, he shares some of the lessons he learned from this experiment: The paint on properly prepped siding that is installed over a vented rain screen can last for a long time, and furring strips used in the rain screen don't need to be made from pressure-treated wood. It's better to use multiple layers of rigid foam, with seams staggered, than to use one thick layer. Over time, EPS foam shrinks, cantilevered window boxes don't sag, and peel-and-stick membrane wrinkles if the top edge isn't sealed with contractor's tape. Window film is a poor substitute for the right glazing. Carpenter ants will find any locations that have water-management problems. Epoxy-coated screws last as well as stainless steel.