How Much Insulation Is Too Much?
Energy Nerd: Net-zero case study, space heating, cold climates.
Adding insulation in a house saves energy, but with each extra inch, the savings per inch diminishes. At some point, the cost of adding more insulation becomes hard to justify. At this year’s BuildingEnergy conference in Boston, three energy experts explored two questions regarding high-performance houses: At what point are envelope improvements a waste of money? And what metrics should we use to determine when enough insulation is enough? One point to these questions was to determine whether the thick levels of insulation required by the Passive House standard, an approach to superinsulation developed in Germany that is gaining traction in the United States, were justified. Because of the declining cost of PV, all three reached the same conclusion: They are not.
The three presenters were David White, an energy consultant from Brooklyn; Marc Rosenbaum, director of engineering at South Mountain Company in Massachusetts; and Rachel Wagner, a designer at Wagner Zaun Architecture in Duluth, Minn.
The net-zero approach
White introduced a case study of a new construction project in Huguenot, N.Y., a location in climate zone 5 with about 6000 heating degree days. The owners were aiming for net-zero-energy use, where a combination of insulation and site-produced energy create an energy-independent house. White was a consultant on the project. White started by comparing the annual energy savings attributable to a proposed envelope improvement — for example, upgrading from 4 in. of subslab foam insulation to 6 in. — with the annual energy production of a PV system costing as much as the proposed envelope improvement. If the proposed envelope improvement would save less energy than the energy produced by a PV system with the same price tag, then the proposed improvement wouldn’t be worth it.
In this case study, the criteria were somewhat flexible. “We were aiming for a marginal payback of…