Two different approaches to the determination of the thermal response of buildings are possible: deterministic models and methods based on equivalent thermal parameters (ETPs) of a building. While the former are computer applications of heat transfer theory, the latter consist of data oriented techniques that infer the ETPs of a particular building by multiple correlation of indoor temperature and weather. The ETP method is convenient to provide a rank ordering of different houses by their thermal performance and to assess the overall effects of retrofits on a house. Like deterministic methods, the ETP method can also predict accurate free-floating indoor temperatures and heating loads as a function of weather.

A convenient set of ETPs is established for a residential townhouse by means of a simple, single thermal mass model. Multiple step regressions of actual data on indoor temperature and weather yield estimates for the ETPs. The model tracks the measured data well. The regressed ETPs agree with what is expected from theoretical calculations and are consistent with the result from a different, constant-indoor temperature experiment.