Variation of Insulating Properties of Closed-Cell Foam Insulation

Heat is transferred through closed-cell foam insulation by conduction through the solid polymer making up the cell structure, conduction through the low conductivity gas (blowing agent) inside the cells, and radiation. The effective thermal conductivity of closed-cell foam panels changes with the foam age due to diffusion of air components into the foam and diffusion of the blowing agent out from the foam. The change in the composition and thermal conductivity of the gas mixture was modeled as a function of the aging time. The change in the composition is computed by solving the transient diffusion equations for each gas species in the foam. The thermal conductivity of the gas mixture is computed from an empirical correlation proposed by Lindsay and Bromley.

A computer program was developed to numerically solve the governing equations. The effective diffusion coefficients of the air components and the blowing agent are the main code inputs. The numerical prediction of the change of the effective conductivity of foam panels for a given age is within 6% or better of the conductivity obtained in long-term tests under three different thermal environments.