Moisture Modeling: Effective Moisture Penetration Depth Versus Effective Capacitance

This paper compares two moisture models in the context of residential whole-building simulations - the effective moisture penetration depth (EMPD) model and the effective capacitance (EC) model. Previous research has shown that the EMPD model provides a more physically realistic response to humidity-load fluctuations than the EC model. But it is unclear how the choice of moisture model affects overall conclusions informing building design and equipment selection. In this study, the models are evaluated based on their ability to predict relative humidity, predict energy use, and to select HVAC equipment. Whole building simulations indicate that the EC and EMPD models are not equivalent, and that combining the capacitance of the building materials with the indoor air capacitance in the EC model is the root cause of this discrepancy. It means that the EC model cannot dampen humidity loads of different frequencies. This study also showed that a simulation using a certain value of EC in one city can agree with the EMPD results, but that the same EC value in a different city may not agree with the EMPD results. However, the results showed that some conclusions drawn from simulations using the EMPD model and the EC model may often be the same, even if the results differ.