R-value (insulation) typical straw bale houses have very thick walls and thus are well insulated. Snow is roughly R-1 per inch. Brick has a very poor insulating ability at a mere R-0.2 per inch; however it does have a relatively good thermal mass. Note that the above examples all use the U.S. (non-SI) definition for R-value. When determining the overall thermal resistance of a building assembly such as a wall or roof, the insulating effect of the surface air film is added to the thermal resistance of the other materials. In practice the above surface values are used for floors, ceilings, and
R-value (insulation) with no surrounding air movement. Since these are ideal conditions, the listed R-value for insulation will almost certainly be higher than it would be in actual use, because most situations with insulation are under different conditions A definition of R-value based on apparent thermal conductivity has been proposed in document C168 published by the American Society for Testing and Materials. This describes heat being transferred by all three mechanisms—conduction, radiation, and convection. Debate remains among representatives from different segments of the U.S. insulation industry during revision of the U.S. FTC's regulations about advertising R-values illustrating the complexity of the issues.