The thermal transmittance of a material or assembly (especially windows) Quantitative measure of ease of heat flow expressed as an equivalent conductance per unit area and per unit temperature difference, the reciprocal of R-value. While building scientists will use R-values for measures of the resistance to heat flow for individual building materials, U-factor is usually used as a summary metric for the ease of heat transfer through building assemblies.

Related terms: U-factorR-value

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