The paper presents some results from a study of the thermal insulation performance of air cavities facing a thin reflective layer integrated in well insulated floors, walls, ceilings and roofs. The interest for single or multi layer reflecting products for use in building structures has increased, even in regions with cold climate and well insulated building envelopes, partly because of a general lack of conventional thermal insulation material due to high activity in the building industry. Another reason may be an overestimated marketing of the thermal performance of these products due to lack of knowledge of building physics in some enthusiastic manufacturing organisations as well as in some certification bodies. A goal of the study is to show both the possibilities and the limitations of these reflective products used in well insulated building structures. The thickness of the air cavity, emissivity of the reflective layer, R-value of the rest of the structure, indoor- and outdoor temperatures and the direction of the heat flux are all parameters influencing the various heat transfer mechanisms and the resulting R-value of the cavity. The influence of these parameters has been studied both by theoretical calculations and by laboratory measurements on a full scale wall by use of a guarded hot box. The theoretical calculations has been conducted according to ISO 15099 Thermal performance of windows, doors and shading devices – Detailed calculations, which is the most updated international standard for heat transfer in closed cavities. Results from both the theoretical calculations and the hot box measurements will be presented in diagrams. The R-value of a closed air cavity facing a thin reflective layer, being an integrated part of a well insulated timber frame structure, is equivalent with a conventional thermal insulation layer with a thickness of approximately 30 mm for walls and approximately 20 mm for ceilings. In structures with heat flow direction downwards, like in floors, the potential R-value is much higher as the natural convection may bee very low due to thermal stabile air layers.