摘要：Reflective insulation systems (RIS) are generally described as enclosed reflective air spaces. RIS provide resistance to heat flow across enclosed airspaces with low‒emittance surfaces on at least one of the surfaces perpendicular to the direction of heat flow. The low‒emittance surface is typically provided by aluminum foils or metalized films that have thermal emittances in the range 0.03 to 0.05. The low emittance significantly reduces the transfer of heat from the warm side to the cool side of the enclosed air space. Steady‒state heat transfer between large parallel planes bounding an enclosed has been thoroughly described in the literature1–4 and consensus standards have been published.5–7 In most cases, part of the heat transfer across an enclosed air space will be due to free convection which on the orientation of the air space. The greatest convective contribution occurs when the heat flow direction is upward (lower surface is hot) and the least convective contribution occurs when the heat flow direction is downward (upper surface is hot). In hot‒humid climates, the dominate heat flow direction across roof assemblies is downward from the building roof. This is the optimum orientation for RIS. RIS performance is characterized by a thermal resistance or R‒value (often RSI when SI units are used). RSI values measured in laboratories are for steady‒state, one dimensional heat flow. RSI values with units m2. K/W have the same meaning and use as RSI values for mass insulations such as mineral fiber of cellular plastic insulations.
