Thermal insulation of roofs by using multiple air gaps separated by insulating layers of low infrared emissivity

Abstract

In recent work we have demonstrated the ability of a new thermal insulation design based on creating air cavities between parallel layers of insulation material with a low infrared emissivity (Expanded Polystyrene, 0.6) and the multimodal heat transfer involved (conduction, convection, and radiation) was modeled for walls. Now, this modeling is developed for roofs, regarding the different heat convection mechanisms involved in both slender air cavities, the vertical (walls) and horizontal (roofs) ones. Another difference is that the air convection is null in a top-heated cavity (roofs in summer) meanwhile this mechanism is active in a bottom-heated cavity (roofs in winter) and so, the optimized design is different in both cases. We studied two designs based on multiple EPS layers of 1 cm thickness, namely: EPSW, which takes advantage of the low emissivity of the white EPS (0.6); and EPSF, which adds an aluminum foil of very-low emissivity (0.04) to each EPS layer. Considering roofs in winter, the EPSW design achieves thermal transmittances ranging from 0.447 W/(m2·K) to 0.096 W/(m2·K) for 4 to 17 layers, respectively, obtaining material savings up to 53% compared to solid EPS insulation. On the other hand, the EPSF design leads to thermal transmittances ranging from 0.238 W/(m2·K) to 0.040 W/(m2·K) for 4 to 17 layers, respectively, obtaining material savings up to 81% regarding solid EPS insulation. For summer conditions, the improvement in insulation is limited by the presence of infrared radiation and very hot roof temperatures in warm climates. An intended thermal transmittance of 0.1 W/(m2·K) can be hardly achieved by using 14 layers of EPSW, whereas it can be obtained with just three EPSF layers, although it implies additional burdens in embodied energy and contaminated emissions to the atmosphere. Therefore, we conclude that the simplest EPSW design is suitable for cold climates meanwhile the EPSF design is preferred for warm climates.