A batt is a mat of formed, somewhat fluffy material, such as fiberglass or rock wool, typically used to insulate spaces between framing studs in a home or building. Batts are sold in sizes conforming to the width and depth of studs installed according to standard building practices used in most structures. Most batt insulation is manufactured in long rolls, easily cut to length to conform to the length of the stud cavity being insulated.

Batts are sold in two forms: Faced, which includes an attached vapor barrier, typically of treated paper or lightweight plastic; and unfaced, which lacks the vapor barrier. Faced insulation is necessary for preventing condensation or moisture problems in new construction, or in existing construction lacking insulation. Unfaced products are suitable for locations where a vapor barrier already exists.