Integrating　phase　change　materials　(PCMs)　into　building　envelopes　can　improve　the　heat　storage　ability　of　buildings　and　prevent　overheating　when　the　outdoor　air　temperature　and　solar　radiation　are　high.　However,　simply　adding　a　PCM　to　a　building　does　not　ensure　that　the　temperature　of　a　room　can　be　controlled;　hence,　a　suitable　design　method　is　necessary.　Nevertheless,　the　heat　storage　mechanism　of　PCM　envelopes　is　very　complicated　because　it　is　affected　by　several　factors,　including　the　climate,　building　parameters,　and　the　properties　of　the　PCM　employed.　This　study　develops　a　mathematical　model　for　passive　buildings,　and　the　factors　affecting　the　heat　storage　process　of　PCM　envelopes　in　summer　are　analyzed　to　determine　an　efficient　method　to　control　the　temperature　of　the　room　with　PCMs.　A　theoretical　analysis　demonstrates　that　PCMs　reduce　the　indoor　temperature,　but　not　the　average　room　temperature.　The　effectiveness　of　PCMs　in　buildings　is　influenced　by　both　climate　and　building　parameters.　If　the　average　outdoor　air　temperature　is　higher　than　the　boundary　of　the　humans＇　comfort　level,　overheating　will　occur　even　when　using　PCMs.　Before　using　PCMs,　the　average　room　temperature　should　be　calculated　and　previewed　using　the　proposed　theoretical　model.　If　the　effectiveness　of　PCM　is　limited,　the　average　room　temperature　is　reduced　via　ventilation.