As a contractor during the1980s energy crisis, I directed my energies toward building the beta version of the super-insulated home. I experimented and struggled with my share of "cutting edge" strategies, with mixed results. It became quickly apparent that some of the experimentation — like site-built skylights — was less than successful. What was not so obvious at the time was that, in spite of beefing up the insulation levels in everything I built, I wasn't really getting the thermal performance I would have expected.
However, as more building diagnostic tools became available, I was able to see the flaws in my previous insulation strategies. I also saw a business opportunity: troubleshooting the all-too-common efficiency and comfort problems in buildings.
Over the last couple of decades, energy prices have continued to rise and energy codes have gotten stricter, but many of the same flawed insulation strategies are still being used in the field. Meanwhile, the consumer's demand for increased comfort often goes unanswered. Many of the calls we get involve existing homes that have new replacement windows and are already insulated. So what else is left to do?
Fortunately, advances in building science are helping the industry take a fresh look at how buildings really perform and what types of improvements are most practical and effective. The critical first step is to understand how a building loses heat. This may sound obvious, but there are still many misconceptions.
Probably the biggest misconception involves where a house loses most of its heat and what types of details can stop these "hidden" drafts. In most building configurations, fiberglass batt insulation — or even loose cellulose blown into an attic — doesn't do a good job of controlling air movement. Many studies have shown that air moves through fiberglass batts and degrades their R-value. And while cellulose can stop air movement when it's blown into closed cavities at densities above 3.5 pounds per cubic foot, loose-fill cellulose blown in an attic will not stop air leaks.
Most of the homes I visit have attic insulation, but they also have many air leaks from inside the house into the attic — at mechanical penetrations and plumbing chases, along partition-wall top plates, at framing intersections like soffits or other changes in ceiling height, around chimneys, and so on. In a typical leaky attic, upward air pressure from the stack effect — the tendency of warm air to rise — can replace all of a home's heated inside air with cold outside air in just two or three hours.
Homes with knee walls or side attics have the further complication of horizontal heat loss and infiltration between heated floor cavities and the eaves (see Figure 1). In many cases attic ventilation only makes this worse, by connecting the interior to the outside and allowing wind to move far into the heated space.
Even though they're often insulated, attic knee walls are a common thermal problem area. Because there is typically no blocking below the knee-wall bottom plate, cold air from the attic or outside air moving through the insulation at the eaves chills interior ceiling cavities, drawing heat away from the living space.