Setting Realistic Design Indoor Conditions for Residential Buildings by Vapour Pressure Difference
Abstract
Indoor relative humidity is commonly used to characterize the indoor environment for hygrothermal simulations, chamber studies, analysis of monitoring data, or test hut studies of buildings without recognition that indoor relative humidity and condensation potential depend on concurrent outdoor temperature and relative humidity. This can lead to the use of unrealistic boundary conditions for hygrothermal simulations and test programs which may result in misleading conclusions.
In buildings operating without mechanical dehumidification, the indoor air moisture level (vapour pressure) is directly related to the outdoor vapour pressure, moisture sources in the space, and the level of ventilation. Mathematics suggests that one can expect buildings with similar operation, occupancy, and construction, but affected by different weather conditions, to have a similar difference between indoor and outdoor vapour pressure.
The paper published in the Journal of ASTM International (2009) on this topic provides a foundation for selecting appropriate and realistic boundary conditions for the design of residential buildings that are based on vapour pressure difference with the aim to eliminate any significant bias for a particular climate. The ASTM paper presents the following:
Discussion of current standards that provide some guidance to selecting appropriate indoor moisture levels based on vapour pressure difference.
Moisture balance equations will be used to show the impact of ventilation and moisture generation rates on the vapour pressure difference.
Monitoring data for six multi-unit residential buildings in two Canadian climates (Toronto and Vancouver) showing the relationship between the outdoor temperatures and vapour pressure difference.
Analysis of seasonal indoor moisture conditions and their impact on HAM modeling based on assumed indoor relative humidity (RH) and conditions derived by a constant vapour pressure difference.
Exploration of the concept that vapour pressure difference and indoor RH is limited by moisture removal on windows.
A precursor to this publication is a paper called “Modeling of Uncontrolled Indoor Humidity for HAM Simulations of Residential Buildings” that is part of the proceedings for the Thermal Performance of the Exterior Envelopes of Whole Buildings X International Conference (2007).
