Various methods exist for room acoustic measurements. To increase the signal-to-noise-ratio (SNR), averaging of repeated room impulse responses (RIR) can be performed under the assumption of time-invariant systems. Associated with a study of the Paris Notre Dame cathedral, time-variances due to minute temperature changes were observed which confounded results of averaged RIRs, producing significantly shorter reverberation times. A correction method for such time-variance of the acoustic system is proposed, based on time-stretching. Following correction, reverberation times of averaged RIRs were comparable to expected tolerance ranges for measurements, well within one just noticeable difference, while also exhibiting a mid-frequency SNR gain of 8 dB.