(not applicable to Coldwind)
Air boundary layer is used to describe the phenomenon that materials 'entrain' a shallow region of air close to their surfaces. Although on exposed construction materials, prevailing winds will disturb this layer it is nonetheless important to consider its beneficial insulating effects, particularly when materials with intrinsically high heat transfer coefficients (poor thermal insulators) are under consideration. That it always occurs is a certainty, it is however often ignored because it has little effect when included in calculations involving well insulated composite material walls and because of the extra work involved in manual calculations. Mistral's database of composite materials always includes air boundary layers - with 3 metres (approximately 10 ft) per second wind velocity in the case of outside exposed surfaces and 'still' air in the case of interior surfaces.
The thickness of the air boundary layer is assumed to be 6mm (¼") for most practical purposes (it is anyway not possible to attribute a precise dimension as the limit of the air layer region is indefinite. The thickness of the air boundary layer will in practice also vary depending upon surface roughness factors but 6mm (¼") represents a reliable mean value for calculation purposes). When selecting air boundary layers it is of course not possible to have two or more layers adjacent to each other and the program is error trapped to prohibit this.