R-Value? Don’t Forget About Air-Sealing


When we talk about an energy efficient building, everyone wants to know about the R-value.

“R-value” (that’s the inverse of the BTUs of heat transferred per hour per across a square foot of surface area per difference in temperature, in degrees Fahrenheit, on either side of the surface, for your science-types) is a measure of how well an insulating material slows the eventual loss of heat through the exterior surfaces of a building.

It is a fundamental law of thermodynamics that all of the heat that we contribute precious fossil fuels to put into a building will eventually leave it, for the same reason a cup of hot coffee left on a table at room temperature will eventually cool off. But, if a building is built with materials that have higher R-values, it will lose that heat more slowly—like putting that coffee in an insulating thermos. In an effort to reduce the energy use in buildings, most states in the US (but not all) have minimum building code requirements for how much R-value each portion of a building should have. Green building practices often aim to well exceed those code requirements.

R-value is well and good—but it’s not the whole story, nor even the most important part of the story. Heat transfer comes in multiple forms, and one of the most important strategies for energy efficiency construction comes from paying attention to an entirely different method of heat loss, far too often overlooked in the construction industry.

That’s the heat you lose when the heated air itself migrates outside, through all of the myriad cracks and holes within a building that allow it to escape.

And most buildings, even new ones, are full of holes for air to leak through.

6/30/2013 8:31:05 AM

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