Using hygroscopic damping of relative humidity and vapour pressure fluctuations to measure room ventilation rates

Abstract

A field study (carried out for other purposes) indicated that relatively air-tight rooms have small relative humidity fluctuations in response to fluctuating indoor temperature and outdoor vapour pressures. This effect is due to hygroscopic damping and has been quantified here to allow the derivation of a simple formula giving room ventilation rates as a function of the amplitude of sinusoidal fluctuations in external vapour pressure, and fluctuations in room vapour pressure and humidity. It is found that the room air-change rate is proportional to the room relative humidity fluctuations, and inversely proportional to the difference between the indoor and ourdoor vapour pressure fluctuations, allowing for phase differences. Calculated results agree well with field data.