ASHRAE LO-09-007

Abstract

<strong>INTRODUCTION</strong> <br>Although it is well known that the heat, air and moisture flows (called HAM) generated by building use and entering from outside, that the HAM flows traversing the enclosure and that the HAM flows injected by the HVAC system are in permanent and mutual balance, simulation tools and designers hardly consider that reality. Building designs are scrutinized on energy needed for heating and cooling, while HVACsystems are dimensioned to deliver the power needed to keep the indoor temperature at comfort level even under extreme outdoor weather conditions. Indoor relative humidity however is mostly kept free floating, as it is perceived as being less important except when the building's function imposes full air conditioning. Few designers detail the envelope taking into account the full hygrothermal load from inside and outside, while hardly anyone considers the whole heat, air and moisture balance that develops between the building's interior, its envelope and the outside environment. This is a pity as air pressure gradients inside the building and between the building and the outside generate airflows that may change the heat, air and moisture response of the envelope and the building drastically, while buffering effects dampen indoor water vapor pressure fluctuations significantly compared to the outside. Resulting air ingress, possible rain penetration and moisture deposits in the envelope could not only negatively affect energy consumption but also trigger the envelope's durability. Simultaneously, inside relative humidity, if not well managed, may affect perceived indoor environmental quality and become a driving force for mold and dust mite infection. <br>Clearly, whole building heat, air, moisture response has impact on human comfort, indoor environmental quality, energy consumption and envelope durability, reasons why in 2003 an IEA-ECBCS Annex, termed Annex 41, Moist-En was initiated (Hens, 2003).