A contractor we supply asked for help in meeting the new energy code requirements for a residential project. The local building inspector referred him to a Web site that calculates whether the project complies. But one of the pieces of information it asks for is the U-factor of the concrete slab. What is a U-factor and how do you calculate it for a residential concrete slab? This project will use 3000-psi concrete for a 736-square-foot, 6-inch thick slab.

The U-factor is a measure of heat transmission, in this case through a concrete slab, either from the ground into the enclosed space or vice versa. It is expressed in BTUs per hour, per square foot, per degree Fahrenheit, referring to the temperature difference between the slab’s upper and lower surfaces.

The U-factor for any building component is the inverse of its R-value, a more familiar term describing thermal resistance or insulation. Generally speaking, lower density concrete provides better insulation than higher density concrete.

A reasonable R-value for a normal concrete slab can be estimated using an R-value, the thermal resistance per inch of thickness, between 0.1 and 0.2 and multiplying it times the slab thickness. For a 6-inch slab, R-value would be between 0.6 and 1.2.

Calculating the inverse of that, 1/R, would give you a U-factor in the range of 1.67 to 0.83. Remember that a higher R-value and a lower U-factor both indicate better insulating properties.