Air　change　rate　(ACR)　significantly　influences　the　indoor　thermal　environment　and　indoor　air　quality.　Moreover,　as　a　matter　of　fact,　ACR　varies　with　time　during　natural　ventilation.　However,　to　date　only　a　few　studies　have　accounted　for　the　dynamic　ACR　for　the　actual　in-use　buildings.　To　address　this　issue,　a　field　measurement　of　outdoor/indoor　environments　and　the　window　state　was　conducted　in　a　dormitory　in　Beijing　in　spring,　summer,　and　autumn.　Transient　Mass　Balance　Equation　(TMBE)　method　associated　with　the　extended　Kalman　filter　(Hereinafter　referred　to　as　Kalman　filter　method)　was　adopted　to　obtain　the　dynamic　results　of　the　ACR.　Then　the　correlations　of　outdoor/indoor　environments　with　the　ACR　and　the　probability　of　window　opening　were　investigated.　The　results　confirm　the　validity　of　Kalman　filter　method　for　tracking　the　dynamic　nature　of　natural　ventilation　in　residential　units,　and　further　demonstrate　that　occupants＇　interaction　with　windows　does　not　simply　refer　to　opening/closing　windows,　but　also　refer　to　adjusting　the　extent　of　the　window　being　open　in　response　to　occupants＇　ventilation　needs.　Therefore,　in　addition　to　the　probability　of　window　opening,　ventilation　needs,　i.e.,　the　dynamic　ACR,　as　the　underlying　trigger　for　window　opening　behavior,　should　be　used　as　an　indispensable　part　of　describing　occupants＇　interaction　with　windows.　This　study　provides　a　complete　framework　to　describe　occupants＇　interactions　with　windows　by　combining　the　dynamic　ACR　and　the　probability　of　window　opening.