Glottal closed phase estimation during speech production is critical to inverse filtering and, although addressed for radiated acoustic pressure analysis, must be better understood for the analysis of the oral airflow volume velocity signal that provides important properties of healthy and disordered voices. This paper compares the estimation of the closed phase from the acoustic speech signal and the oral airflow waveform recorded using a pneumotachograph mask. Results are presented for ten adult speakers with normal voices who sustained a set of vowels at a comfortable pitch and loudness. With electroglottography as reference, the identification rate and accuracy of glottal closure instants for the oral airflow are 96.8 % and 0.28 ms, whereas these metrics are 99.4 % and 0.10 ms for the acoustic signal. We conclude that glottal closure detection is adequate for close phase inverse filtering but that improvements to detection of glottal opening instants on the oral airflow signal are warranted.