The space heating energy needed during the winter heating season in Seattle Washington, USA, was monitored over a 15-year period, 1987-2002. Two single family residence houses were constructed to building code standards in force at the time of construction and two to standards calling for envelopes with improved thermal resistance. Space conditioning energy needs are strongly affected by occupant behavior, but generally simulations ignore the behavior of the occupants in estimating the energy needed for heating and cooling. The data suggest that estimates of energy savings can be based upon envelope thermal resistance for moderate occupant behavior. For such behavior, the space heating is well characterized by the daily average difference between house average space temperature and outside air temperature. Characterizing in terms of indoor temperature, outdoor air temperature, wind speed, and insolation gives a slightly better representation but requires more information than is usually available. However, vigorous conservation tactics, which produce a thermal response that is highly transient, can lead to substantially different energy needs. No correlation could be established from the measured space heating when aggressive conservation made use of thermostat setback at every opportunity. (c) 2005 Elsevier Ltd. All rights reserved.