You can’t talk green building without talking about insulation: it plays a critical function in energy efficient design. Insulation acts like a blanket or wool sweater for a house: it slows down the gradual loss of heat through conduction, thus allowing you to enjoy the warm house you used all those fossil fuels to create for longer, before more fossil fuel input is required to bring up the temperature once again. In the summer, insulation helps keep heat out of your home.
How Much Insulation?
Just like putting on a thicker coat keeps you warmer than a thinner one, more (properly installed) insulation has greater energy efficiency than less insulation—so the answer to the question “how much insulation is enough” is something of a moving target. In the olden days, homes were often built without installing any insulation at all. In the past two decades, however, most state building codes began requiring a certain amount of insulation in the walls, ceiling or attic, and floor. In the past decade, the codes have become stricter and stricter, requiring more and more insulation.
The goal of an exceptionally green house—what we’ve dubbed a “Pretty Good House” in our conversation—should be to exceed code requirements. That’s the “super” in “super-insulation.” The trick is to arrive at an insulation balance somewhere between “more is always better” and common sense, based on the goal of the home: to have much lower energy bills than standard new construction, perhaps as much as 2/3s lower. In Green Building Advisor’s original conversation on the “Pretty Good House” the “R10-R20-R40-R60” rule was posited as best for cold climates. (More R-value means more insulation.)
This rule means going with R10 insulation values underneath slab foundations, (building code often requires none), R20 insulation values in foundation walls (building code requires R10-R15), R40 in exterior walls (building code requires R13 to R20, depending on your climate, with R13 being the maximum R-value you can get with batt of that “pink stuff” installed in a 2×4 wall), and R60 in the attic (building code requires R38 in most “mixed” climates).
In mixed-humid climates, experience has shown me that values can be relaxed some, and I propose that a “10-15-30-50” rule is a good starting point: a minimum value that can be achieved at a relatively good cost payback. If budget and space permits, higher values in exterior walls would still make good sense. Innovations like a double stud wall or a raised heel truss help increase the amount of space one has in which to fit all of this extra insulation.
Insulation Alone is Not Enough
Most insulation products are only as good as the wall or ceiling in which they are installed—and are designed to be installed uncompressed, evenly, and often in an enclosure that is air-tight. Cramming an R19 batt into space for an R13 batt does not equal R19 walls, and a pile of loose-fill cellulose insulation in your attic does nothing to stop heat loss from any sections that are uncovered. Paying attention to the proper installation for each type of insulation product is absolutely crucial.
Another aspect of the building envelope that is often forgotten in the quest for maximum R-values is the total air-tightness of the home—as warm air leaking out and cold air leaking in are an entirely different source of indoor temperature change that must be corrected with a heating or cooling system. Pretty Good Houses should be exceptionally air-tight, with any and all cracks and penetrations in the exterior of the building sealed. Yes, this kind of detail work takes extra time and attention, but is just as important, if not more so, than the R-values you choose to put into your super-insulation.
Next up in our southeastern climate “Core Green Design” series: “Right-sized” Homes and Outdoor Living Spaces