U Definitions

Definition Categories


U-factor is a measure of a material’s capacity to allow conductive heat flow. The U-factor is the reciprocal of the R-value. A lower U-factor indicates a higher resistance to conductive heat flow. The U-factor is most often used to describe the thermal efficiency of windows, glazed and opaque doors, and skylights. Chapter 11 contains an alternative compliance method based on the U-factor values of the building’s components.


(1) Underlayment is a smooth material laid on the subfloor that serves as a base for a finish flooring material such as vinyl or linoleum. In modern homes, the underlayment is often plywood or particleboard.

(2) Underlayment is a waterproof material placed under roof coverings including fiberglass shingles and tile. It is usually either 15 or 30 pound (per 100 square feet) roofing felt.

Ungrounded conductor

An ungrounded conductor is one that intentionally carries electricity. The common term for an ungrounded conductor in residential electrical systems is the “hot” or “live” wire or conductor. Ungrounded conductors are usually insulated with red or black colored insulation.