Enclosed airspaces to reduce heat flow have been recognized for well over 100 years. Airspaces with one or more reflective surfaces define reflective insulation (RI) assemblies, a product type used in walls, roofs, windows with multiple panes, curtain walls and skylights. The thermal resistance (R value) of airspaces depends on the emittance of all surfaces, airspace dimensions and orientation, heat flow direction and surfaces temperatures. The modeling of RI now includes CFD coupled with radiation to quantify the total heat transfer. This study compares a validated model for airspace R values with existing methods such as ISO 6946 and hot-box results that provide the R values in the ASHRAE Handbook of Fundamentals. The existing methods do not include an airspace aspect ratio. This study showed that the aspect ratio can impact the R value by a factor of two. The impact of aspect ratio was calculated for double airspaces variation such as that for single airspaces. The present calculations are two-dimensional and also consider all the bounding airspace surfaces, while previous methods are one-dimensional and do not include surface temperature variations or detailed radiative transport.

This is an overview of a Key Note lecture; the quote for this lecture is from T.S. Eliot: “We must not cease from exploration and at the end of all our exploring will be to arrive, where we began, and, to know the place for the first time”. This quote highlights that the process of scientific development goes in circles, yet each of them goes above the previous circle, building up the ladder of knowledge. Closing one circle and opening the next may be either be a quiet, unnoticeable event or a roaring loud, scientific revolution. Building science (physics) was started about 100 years ago, but only now are we closing its second circle. Perhaps, because of building physics’ role in the fourth industrial revolution, this discipline itself is undergoing a scientific revolution The first industrial revolution was based on steam generated by burning coal, the second was based on petroleum, and the third on electricity and concentrated electricity production. The current one, i.e., the fourth, is based on distributed energy sources combined with information technology.

The determination of thermal resistances (R-values) for enclosed airspaces, including those with one or more low-emittance surfaces, has advanced from one-dimensional heat transfer between large parallel planes to multi-dimensional heat flow in a wide variety of physical configurations. The key elements in this advancement, however, are the evaluation of the heat flow due to conduction-convection and the solution for radiation that includes all surfaces bounding the region of interest. The model used in this study has been validated against several sets of laboratory test data, including the data from the U.S. National Bureau of Standards for the thermal resistance of airspaces, which has been the basis for handbook values for reflective airspaces for five decades. In addition, this model has been previously used to determine the reductions in the R-values of reflective insulations assemblies due to imperfect installations and internal defects in multilayer reflective systems with cross-airflow between the layers. In this study, the model is used to examine the impact on R-value of air intrusion of different air changes per hour (ACH) at various exterior air temperatures into reflective insulation assemblies with a range of effective emittance from 0 to 0.82. Two cases are considered in this study. In the first case, called “infiltration”, exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the cold side of the assembly. In the second case, called “wind washing”, the exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the same side of the assembly. To quantify the reductions in the R-values due to infiltration and wind washing conditions, a reference case is included in this study for the case in which no air intrusion occurs in the reflective insulation assemblies. Finally, consideration is given to investigating the effect of the aspect ratio on the R-values of reflective insulation assemblies without air intrusion and with air intrusion of different ACH at various exterior air temperatures. The results show that the aspect ratio has a significant impact on the R-value of reflective insulation assemblies with and without air intrusion. Additionally, the results show that the impact of infiltration and wind washing on R-values of reflective insulation assemblies increase as the difference between the exterior air temperature and the undisturbed temperature of the cavity increases.

Most recent improvements in building envelope technologies suggest that in the near future, residences will be routinely constructed to operate with very low heating and cooling loads. In that light, the application of novel building materials containing active thermal components (e.g., phase change materials [PCMs,] sub-venting, radiant barriers, and integrated hydronic systems) is like a final step in achieving relatively significant heating and cooling energy savings from technological improvements in the building envelope. It is expected that optimized building envelope designs using PCMs for energy storage can effectively bring notable savings in energy consumption and reductions in peak hour power loads. During 2006/07, a research team at Oak Ridge National Laboratory (ORNL) performed a series of laboratory and field tests of several wall and roof assemblies using PCM-enhanced cellulose insulation. This report summarizes the test results from the perspective of energy performance. The ORNL team is working on both inorganic and organic PCMs; this report discusses only paraffinic PCMs. A limited economical analysis also is presented. PCMs have been tested as a thermal mass component in buildings for at least 40 years. Most of the research studies found that PCMs enhanced building energy performance. In the case of the applicationmore » of organic PCMs, problems such as high initial cost and PCM leaking (surface sweating) have hampered widespread adoption. Paraffinic hydrocarbon PCMs generally performed well, with the exception that they increased the flammability of the building envelope. « less

Enclosed airspaces of various effective emittances exist in the building envelope in walls, roofs, and double/triple glazing windows, curtain walls and skylight devices. Assessing the energy performance of a building component with enclosed reflective airspaces requires evaluation of all modes of heat transport inside the airspace. The thermal resistance (R-value) of the enclosed airspaces depends on the dimensions and orientation of the airspace, and the emittance and temperature of all surfaces that bound the airspace. To the best of our knowledge, the existing methods used around the world to evaluate the airspace thermal performance (e.g., ISO 6946, AUS/NZ 4859, and methods based on the U.S. NBS data) are one-dimensional and assume isothermal conditions on the hot and cold surfaces. In actual applications, however, the temperatures of both the hot and cold surfaces vary (i.e., they are non-isothermal), and the heat transfer takes place via conduction, convection and surface-to-surface radiation. In some cases, convection is absent or negligible, as is the case for downward heat flow or situations with a small temperature difference between the hot and cold surfaces. Radiation transport is significantly reduced by the presence of a low-emittance surface in the heat flow path. One of the goals of this study is to answer the question “is it a good assumption to use the isothermal conditions on both the hot and cold surfaces for determining/reporting the R-value of enclosed airspaces for different building applications?” A complete evaluation of the thermal performance of enclosed airspaces that includes the impact of bounding materials can now be undertaken in multiple dimensions with convective transport described by computational fluid dynamics. In addition to providing a relatively complete evaluation of the thermal performance of the enclosed airspaces, the adequacy of popular simplifying assumptions can be evaluated. This paper describes the computational process used, along with examples of the variation in thermal resistance with the airspace aspect ratio, realistic thermal boundary conditions and radiative heat transport on all surfaces that bound the airspace. The analysis completed in this research shows that the assumption of isothermal hot and cold surfaces affects calculated R-values for enclosed reflected airspaces by less than 3%. This was demonstrated for the five conventionally considered heat-flow directions and effective emittances from 0 to 0.82.

Among all of the internal fabric and external enclosure components of buildings, sloped roofs and adjacent attics are often the most dynamic areas. Roofs are exposed to high temperature fluctuations and intense solar radiation that are subject to seasonal changes in climatic conditions. Following the currently rising interests in demand-side management, building energy dynamics, and the thermal response characteristics of building components, this paper contains unpublished results from past studies that focused on innovative roof and attic configurations. The authors share unique design strategies that yield significant reduction of daytime roof peak temperatures, thermal-load shavings, and up to a ten-hour shift of the peak load period. Furthermore, advance configurations of the roofs and attics that are discussed in this paper enable over 90% reductions in roof-generated peak-hour cooling loads and sometimes close to 50% reductions in overall roof-generated cooling loads as compared with traditionally constructed roofs with the same or similar levels of thermal insulation. It is expected that the proposed new roof design schemes could support the effective management of dynamic energy demand in future buildings.