To reduce future energy bills, some builders are willing to include above-code levels of insulation. Each additional inch of insulation saves energy — but with each additional inch, the savings per inch diminishes. At some point, the cost of adding more insulation because hard to justify.
Three energy experts recently examined the question, “How much insulation is too much?” These experts wanted to determine whether the thick levels of insulation required by a superinsulation standard developed in Germany, the Passivhaus standard, were justified. All three reached the same conclusion: they are not.
The experts presented their findings at this year’s BuildingEnergy 16 conference in Boston. They were trying to answer two questions: At what point are envelope improvements a waste of money? And what metrics or rules of thumb should we use to determine when enough insulation is enough?
When should we quit?
The three presenters were David White, an energy consultant from Brooklyn, New York; Marc Rosenbaum, director of engineering at South Mountain Company in Massachusetts; and Rachel Wagner, a designer at Wagner Zaun Architecture in Duluth, Minnesota. (All three experts have published articles on GBA or in Fine Homebuilding; see, for example, In Defense of the Passive House Standard and Double-Stud Walls.)
Introducing the session, Ben Southworth said, “We will be talking about building assemblies. How much insulation is enough? How much is too much? Left to your own devices, what do you specify and why?”
The traditional net-zero approach
David White addressed these questions by introducing a case study: a new construction project in Huguenot, New York, a Climate Zone 5 location with about 6,000 heating degree days. (White was a consultant on the project.) The owners were aiming for net zero energy.
White started with the classic method used by designers of net-zero-energy homes: he compared the annual energy…