Last week I was lucky enough to attend a workshop on Residential Retrofits for Energy Efficiency and Sustainability at the NESEA Building Energy Conference. The workshop was led by Larry Harmon of Air Barrier Solutions, an incredibly knowledgeable veteran expert in the building field. The word "sustainability" had for me conjured images of integrated solar panels, wind turbines and who knows what sort of space age technology; so I don't know what magic I was expecting. Nonetheless it wasn't a huge surprise when I realized that most of the workshop was dedicated to two problems: air sealing and insulation, and those mainly in the contexts of the basement and the attic. As it turns out, the science just keeps coming back to wearing a good hat and keeping your boots dry.
It's not quite that simple, of course. A house is a complicated, interconnected system. One of these complications was brought to my attention at the workshop, regarding air sealing and insulation, which I had thought I understood pretty well. Harmon divided what I had always referred to broadly as the "building envelope" into an air (or pressure) boundary and a thermal boundary: separate entities working together, like Butch Cassidy and the Sundance Kid. One screws up, the other gets shot. The air boundary, I learned, should in northern climates be inside the thermal boundary; in the South it's the reverse (as to whether this shift occurs precisely at the Mason-Dixon Line, I have yet to ascertain). One area in which this distinction between the thermal boundary and the air boundary is particularly important is in the attic. Heat rises, as we know, and it also carries moisture. So if there's no air boundary between the interior of the house and the thermal boundary (attic insulation), the thermal boundary gets wet. This leads to problems: 1) moisture compromises the effectiveness of insulation, so you wind up with a colder, more expensive house, and 2) the moist, warm air enters the cold attic (the attic's outside the thermal barrier) and either condenses or freezes, so you wind up with a wet attic - which eventually becomes a rotten, moldy attic. So how to prevent that? First, by understanding the duality of the building envelope. Second, by air sealing - inside the thermal barrier, remember, if you're north of the Mason-Dixon Line.
So go ahead and saddle up with your caulking gun; the Butch Cassidy and Sundance Kid of your building envelope are lying wounded in a damp building, discussing how they could get to Australia where the mold never grows, and they could really use your help.
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