In the unsteady calculations of room thermal environments, two simple and effective methods were introduced to reduce computer efforts through two case studies. One method(method-A) was applied to a passive solar room analysis(caseA) and another method( method-B) was applied to estimation of energy consumption in an air-conditioned room(caseB). In method-A, flow fields are calculated intermittently, namely, calculated every N ts time step while temperature fields are calculated every time step,here,N ts > 1. In method-B, only flow fields are calculated every N ts time step as method-A, however, several flow patterns stored in the computer mass storage in previous calculations are used according as the supply air temperature level. In the calculations, CFD code 'SCIENCE' developed by authors was used. In both cases CPU times were saved efficiently without distorting results.