A transient heat and moisture transfer model for building materials based on phase change criterion under isothermal and non-isothermal conditions

Highlights: • PCC was introduced in both energy and moisture conservation equations to simplify model. • Expressions of PCC under isothermal and non-isothermal conditions were presented. • Effects of relative humidity, temperature and temperature gradient on PCC were evaluated. • PCC could be taken as 1.0 when relative humidity in cellulose insulation was less than 60%. • Model with proposed expressions of PCC was more accurate than that with empirical value. In this paper, a transient model for coupled heat and moisture transfer in building materials based on phase change criterion (PCC) was built. The PCC was introduced in both the energy conservation equation and the moisture conservation equation to simplify the model. The relations between PCC and usual material hygrothermal properties under isothermal and non-isothermal conditions were both derived to evaluate the effects of relative humidity, temperature and temperature gradient on the PCC. These relations enabled solving the problems that the empirical determination of the PCC was difficult and the existing models with a constant empirical value of PCC were inaccurate. The numerical results showed that the PCC was significantly affected by the sorption capacity. The effects of relative humidity, temperature and temperature gradient on the PCC could be ignored when the relative humidity in the cellulose insulation was less than 60%, and the PCC could be taken as a constant value of 1.0 within this relative humidity range. When the relative humidity in the cellulose insulation was larger than 60%, the model with the proposed expressions of PCC was more accurate than that with an empirical value of PCC for the hygrothermal performance analysis of the material.