Laboratory studies were conducted to investigate the influence of fiber and fabric characteristics and relative humidity on the thermal insulation behavior of drapery fabrics. Flat drapery fabrics were sealed to a full-scale window built into are insulated wall between a cold chamber, which simulated outdoor winter conditions, and the inside of a test room, which was maintained at indoor winter conditions. Thermal transmittance (U value) of the window/fabric system was measured by transducers mounted on the surface of the window glass. The temperature of the cold chamber, the temperature and relative humidity of the test room, and the heat transfer were monitored by a desk-top computer. Transmittance values of tern experimental fabrics were measured at two indoor relative humidity levels. The effects of relative humidity, fiber hydrophilicity, fabric weave openness, and physical characteristics of the fabrics on thermal transmittance are discussed.