NYSERDA’s rules for "Insulation For Homes Heated with Natural Gas or Electricity" (September 2016) identifies insulation measures that are approved for the Assisted Home Performance (AHP) Subsidy and contractor incentives. It states that insulation measures that are outside the parameters of the rules cited on the Policy will be “loan only and the project must pass loan cost effectiveness rules.” In the section regarding crawlspaces over non-conditioned space, which are cited as eligible for HEMI and the AHP Subsidy, the Policy states, “This excludes insulating basement ceilings, where the basement is tied to moderated earth temperatures (i.e., semi-conditioned).”
In “Residential Energy” (ISBN 978-1-8801-09-5) John Krigger and Chris Dorsi’s state “The thermal and air boundary must be in alignment.” A contractor must determine whether or not the basement should be included within the thermal boundary. As Krigger and Dorsi note, a thermal boundary may surround only conditioned spaces, or it may include unconditioned spaces also’ (emphasis added).
The Department of Energy recommends insulating the basement walls rather than the ceiling: “A properly insulated basement can save you money on heating and provide a dry, comfortable living space. In most cases, a basement with insulation installed on its exterior walls should be considered a conditioned space. Even in a house with an unconditioned basement, the basement is more connected to other living spaces than to the outside, which makes basement wall insulation preferable to ceiling insulation.” (http://energy.gov/energysaver/where-insulate-home).
Bruce Harley in his book “Insulate and Weatherize” (ISBN 978-1-60085-468-2) states, “A basement with a furnace or boiler is typically warm enough that insulating the floor won’t save you much, if anything.” (p.85). In the NYSERDA video “Guidelines for Air Sealing Basements” Bob Kahabka recommends including a basement that includes mechanical systems within the thermal boundary.
This practice is consistent with NYS codes. Section 402.4.7 relates to basement walls and does allow for basement walls to be insulated when associated with unconditioned space as follows (emphasis added):
402.2.7 Basement walls. Walls associated with conditioned basements shall be insulated from the top of the basement wall down to 10 feet (3048 mm) below grade or to the basement floor, whichever is less. Walls associated with unconditioned basements shall meet this requirement unless the floor overhead is insulated in accordance with Sections 402.1.1 and 402.2.6.
However, a crawlspace vented to the outside would best be treated by insulating the ceiling. This is an exception already identified in the Policy (emphasis added): “Insulation of conditioned floors over garages, ventilated crawlspaces, and floors, overhands and cantilevers over non-conditioned space is permitted.”
Resources for additional information: Section 6 of the 2016-2017 Contractor Resource Manual