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The Total Effective R-Value performance of ICFs consists of three factors:
(1) the R-Value of the expanded polystyrene,
(2) the thermal mass of the concrete, and
(3) the enormous reduction in air leakage (infiltration).
First, the R-Value of BuildBlock's polystyrene alone is R-20, compared to wood frame's R-9 to R-15. So ICF walls are expected to cut the conduction losses through foundation and above-grade walls by about half.
Air leakage accounts for as much as 40% of the heat load requirements of a wood framed structure. Nothing blows through solid concrete! ICF walls reduce air infiltration by about half, compared to wood frame.
But ICF walls do more than cut down on the biggest types of energy loss. The concrete gives them the heat-absorbing property "thermal mass." This is the ability to smooth out large swings in temperature. It keeps the walls of the house a little warmer when the outdoor temperature hits its coldest extreme, and keeps the house a little cooler when the outdoor temperature is hottest.*
*Energy loss graphs and thermal mass information from Portland Cement Association, Technology Brief 1
As a result, with the combined performance of these three factors, ICF walls actually perform as high as R-50 in some areas of the country.