Moisture Buffering and its Consequence in Whole Building Hygrothermal Modeling

Abstract

Moisture absorption and desorption of materials in contact with indoor air of buildings can be used as a passive, i.e., nonmechanical, way to moderate the variation of indoor humidity. This phenomenon, which is recognized as `moisture buffering', could potentially be used as an attractive feature of building products to improve indoor air quality and to save energy. Of interest therefore is to establish a unit to appraise this quality of building products and to investigate the importance of moisture buffering when it is considered in whole building hygrothermal simulation. This paper will illustrate both. A new test method specifies a protocol for determination of what has recently been termed the Moisture Buffer Value (MBV) of building products. The paper presents the definition of MBV and introduces a test protocol which has been proposed for its experimental determination. The MBV is primarily meant as a value to characterize the ability of building products to moderate the variations of humidity in air which is in contact with the products, since it indicates the rate of flow of moisture over the product's surface when exposed to a certain humidity excitation. Hygroscopic interaction between air of the indoor climate and materials in the building envelope is taken into account in a model for whole building heat and moisture simulation. By means of an example, it will be investigated if: 1. it is possible to use the benefits of moisture buffering to save energy by reducing the requirement for ventilation if indoor humidity is a parameter for controlling ventilation rate, 2. it is possible to improve the perceived acceptability of indoor air, as judged by the temperature and humidity of the air, by using moisture buffering to control the indoor humidity. The results of the whole building hygrothermal simulations show that it is possible to rather significantly reduce the amplitudes of indoor relative variation when the moisture buffering effect of building materials is taken into account, compared to a situation with moisture tight interior building surfaces. The modeling also shows some possible benefits on energy consumption if humidity-controlled ventilation is employed, as well some benefit on the indoor acceptability with the hygrothermal quality of indoor air. However, both benefits seem to be somewhat limited in the model room used for analysis.