Greener Structures

AVB – Air Vapor Barrier
Airtightness and vapor permeability are very different material properties. Combining these to describe a component of a thermal envelope propagates the confusion that they are one and the same, or that are both needed. But an air barrier is explicitly required by the Energy Code and necessary for a well-performing thermal envelope, while the degree to which vapor can diffuse through the material depends, in part, on the material’s location relative to the conductive thermal insulation. Frequently the air barrier should be vapor permeable to allow drying of the envelope component – the complete opposite of a vapor barrier.

Embodied Energy
The proportion of energy used in manufacturing and construction from renewable sources is growing. Solar and wind energy, and energy from heat pumps, have a much smaller carbon footprint than energy from fossil fuels, so knowing the total amount of energy used is not useful as a metric to compare the product’s contribution to global warming. I prefer “carbon footprint.”

Embodied Carbon
There are two completely different meanings to this term: One is the amount of CO2 and other global warming gas emissions resulting from the manufacture of the material or product, and the other is the biogenic, or stored, carbon that came from CO2 in the atmosphere which is physically in the material. For the first meaning, a high number is bad, while for the second meaning, a high number is good. For example, the amount of C02 emitted to manufacture a ton of steel varies from about one to three tons, while a ton of softwood lumber represents about one and a half tons of CO2 that was drawn down from the atmosphere during the growth of the tree that forms the carbon in the wood’s cell structure. I prefer “carbon footprint” and “biogenic carbon,” respectively.

Thermally Broken
The phrase is completely lacking in quantity, and is therefore meaningless. It is sometimes used to specify the performance of a window or door frame, e.g. “The aluminum window frames shall be thermally broken.” Such a term allows a “break” of one-eighth of an inch or less between the interior and the exterior aluminum in the frame, which is a rather poor thermal break. The entire window assembly (not just the glass panes, or lites) should be specified, tested, and labeled to have a U-factor (which is the measured heat flow through the material or assembly) less than a specified maximum value.

Global Weirding
This might be an accurate description of the effect of greenhouse gas emissions on the atmosphere, but its use makes people sound like radical, left-of-mainstream zealots. I prefer “AGW” – Anthropogenic Global Warming, or “ACC” – Anthropogenic Climate Change.

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