The ins and outs of insulation

I am a total geek for all things Building Science — the study and application of improved building performance — but often struggle to form coherent sentences when trying to explain the basic concepts. Building Science is, after all, about physics, and being a right-brained individual it’s no wonder I stumble over the concepts of heat, air and vapor. Unfortunately, without an eloquent and concise presentation it’s hard to convey the imperative of creating more durable and efficient buildings.

The green movement, where it relates to construction, has praised the benefits of energy efficiency while pouncing on the ills: mold, carbon emissions and indoor air quality among them. But rarely is there room in the taglines to explain the relationship between them. This becomes apparent when a client asks, “Why can’t they build houses like they used to?” Or a contractor contends, “Don’t build it too tight! A house needs to breathe.” The contractor (or homeowner) who insulated my partner’s 1920’s Tudor in North Minneapolis — the “country house,” if you follow my digressions regularly — must have believed this because he or she carefully stapled fiberglass insulation between the rafters only to cap the ridge with a whirling attic vent.

Insulation works by holding air still. It’s measurement, known as the R-value, is actually its resistance to heat loss. In effect, the slower the heat loss, the more efficient your home. Of course, installation is critical.

Understand that South Minneapolis, and Minnesota at large, is considered a cold climate. Obviously! But in technical terms it means the majority of our conditioning days require heating, mechanically warming air inside our homes while the cold is knocking at the door. Warm air also carries moisture, the physics of which require it to move from hot to cold, and damp to dry. Gaps in insulation allow air to move freely, resulting in not only heat loss but also the transfer of moisture in the form of vapor. So much so that 30 quarts of water can be collected through a 1-inch square opening in a typical heating season. Thankfully, when allowed to dissipate, water vapor is relatively harmless. But when it encounters something that is cold and resistant to its advances, it’s downright vindictive.

In this enabling, co-dependent relationship, vapor-rich warm air finds the cold very attractive and will move in its direction the first chance it gets. Older homes are generally immune to this fatal attraction because without adequate insulation their heating systems essentially bake the moisture out of highly permeable materials like wood and stucco. Today’s houses are built tighter to use (and lose) less energy, with new products designed to use less wood — OSB, for instance — but these materials are often less permeable. When that same vapor-rich warm air encounters an impermeable surface colder than itself, vapor will condense on the cold surface.

As of the 2000 U.S. Census, 168,624 housing units exist in the city of Minneapolis. Many of these homes were built out of wood with little or no insulation, making them incredibly resistant to moisture damage, but expensive to heat, and in need of regular maintenance. As the incentive to improve a home’s efficiency increases, so does the need to understand the basic principles of thermal dynamics, because chances are good, one of these housing units is your home.

Bryan Anderson lives in Stevens Square. He works for SALA Architects on East Hennepin.