Institute of Fundamental Technological Research

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building energy dynamics, and the thermal response characteristics of building components, this paper contains unpublished results from past studies that focused on innovative roof and attic configurations.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.

roofs and attics, thermal performance, numerical analysis, field testing, dynamic thermal response, peak load management, thermal storage, phase change materials

Residential attics has the potential to be one of the most energy efficient building components by combining thermal processes of attic floor insulation, attic air space, ventilation in attics, and solar collecting roof decks. Large amounts of solar energy collected by the roofs in cooling-dominated and mixed climates generate excess cooling loads, which need to be removed from the building by the space conditioning systems. This paper investigates potential ways to improve the thermal design of the residential home attics to minimize the cooling energy consumption in the cooling-dominated and mixed climates. Dynamic thermal characteristics of thick attic floor insulations and blends of phase change materials (PCMs) with insulations are analyzed. Both approaches can provide notable reductions of thermal loads at the attic level. In addition, a significant time shift of peak-hour loads can move a major operation time for air conditioning system from the daytime peak hours to nighttime low demand hours. A reverse heat flow direction can be observed during the day in the case of really thick layers of bulk insulation or PCM-enhanced insulations, compared to the rest of the building envelope components. This effect may provide free passive cooling to the building, and can be very useful in locations of double electrical tariffs with high daytime peak-hour electric energy rates and less-expensive off-peak energy cost. In both of the above cases, an addition of PCM to the bulk insulation brings substantial performance enhancement not available for traditional insulation applications. This paper presents a short overview of dynamic material characteristics and energy performance data necessary for future dynamic applications of different configurations of the attic floor insulation and PCM-insulation blends in residential homes. A series of whole-building scale and material scale numerical simulations were performed on a single story ranch house to analyze potential energy savings and optimize location of PCM within the attic insulation.

Building envelopes, Attics, Thermal mass, Insulation

Experimental and theoretical analyses have been performed to determine dynamic thermal characteristics of fiber insulations containing microencapsulated phase change material (PCM). It was followed by a series of transient computer simulations to investigate the performance of a wood-framed wall assembly with PCM-enhanced fiber insulation in different climatic conditions. A novel lab-scale testing procedure with use of the heat flow meter apparatus (HFMA) was introduced in 2009 for the analysis of dynamic thermal characteristics of PCM-enhanced materials. Today, test data on these characteristics is necessary for whole-building simulations, energy analysis, and energy code work. The transient characteristics of PCM-enhanced products depend on the PCM content and a quality of the PCM carrier. In the past, the only existing readily-available method of thermal evaluation of PCMs utilized the differential scanning calorimeter (DSC) methodology. Unfortunately, this method required small and relatively uniform test specimens. This requirement is unrealistic in the case of many PCM-enhanced building envelope products. Small specimens are not representative of PCM-based blends, since these materials are not homogeneous. In this paper, dynamic thermal properties of materials, in which phase change processes occur, are analyzed based on a recently-upgraded dynamic experimental procedure: using the conventional HFMA. In order to theoretically analyze performance of these materials, an integral formula for the total heat flow in finite time interval, across the surface of a wall containing the phase change material, was derived. In numerical analysis of the southern-oriented wall the Typical Meteorological Year (TMY) weather data was used for the summer hot period between June 30th and July 3rd. In these simulations the following three climatic locations were used: Warsaw, Poland, Marseille, France, and Cairo, Egypt. It was found that for internal temperature of 24 °C, peak-hour heat gains were reduced by 23–37% for Marseille and 21–25% for Cairo; similar effects were observed for Warsaw.

A method of derivation of the conduction z-transfer function coefficients from response factors, for three-dimensional wall assemblies, is described. Results of the conduction z-transfer function coefficients calculations are presented for clear walls and separated details which are listed in ASHRAE research project 1145-TRP: ‘‘Modeling Two- and Three-Dimensional Heat Transfer Through Composite Wall and Roof Assemblies in Hourly Energy Simulation Programs’’. Resistances, three-dimensional response factors and so-called structure factors, have been computed using the finite-difference computer code HEATING 7.2. The z-transfer function coefficients were then derived from a set of linear equations, constituting relationships with the response factors, which were solved using the minimum-error procedure. Test simulations show perfect compatibility of the heat flux calculated using three-dimensional response factors and three-dimensional ztransfer function coefficients, derived from the response factors.

Heat transfer, Thermal response, z-transfer function, Simulation, Building envelope

Two methods are proposed of the wall specimen time constant estimation, for the hot box apparatus testing. Directions of the American standard ASTM C 1363-97 are discussed. First method assumes numerical calculation of the response factors and deriving time constant from their ratios. The second one makes use of the approximate relation between the time constant and the product of resistance, capacity and the structure factor. Correlations between time constants and structure factors are examined.

A method of derivation of the conduction z-transfer function coefficients from response factors, for three-dimensional wall assemblies, is described.Results of the conduction z-transfer function coefficients calculations are presented for clear walls and separated details which are listed in ASHRAE research project 1145-TRP: “Modeling Two- and Three-Dimensional Heat Transfer Through Composite Wall and Roof Assemblies in Hourly Energy Simulation Programs”. Resistances, three-dimensional response factors and so-called structure factors, have been computed using the finite-difference computer code HEATING 7.2. The z-transfer function coefficients were then derived from a set of linear equations, constituting relationships with the response factors, which were solved using the minimum-error procedure.Test simulations show perfect compatibility of the heat flux calculated using three-dimensional response factors and three-dimensional z-transfer function coefficients, derived from the response factors.

This paper is focused on the energy performance of buildings containing massive exterior building envelope components. The effect of mass and insulation location on heating and cooling loads is analyzed for six characteristic wall configurations. Correlations between structural and dynamic thermal characteristics of walls are discussed. A simple one-room model of a building exposed to periodic temperature changes is analyzed to illustrate the effect of material configuration on the ability of a wall to dampen interior temperature swings. Whole-building dynamic modeling using DOE-2.1E is employed for the energy analysis of a one-story residential building with various exterior wall configurations for six different US climates. The best thermal performance is obtained when massive material layers are located at the inner side and directly exposed to the interior space. # 2002 Elsevier Science B.V. All rights reserved.

Building heat transfer, Structure factors, Frequency response, Thermal stability, Dynamic thermal performance

In most whole building thermal modeling computer programs like DOE-2, BLAST, or ENERGY PLUS simplified, one-dimensional, parallel path, descriptions of building envelope are used. For several structural and material configurations of building envelope components containing high thermal mass and/or two- and three-dimensional thermal bridges, one-dimensional analysis may generate serious errors in building loads estimation. The method of coupling three-dimensional heat transfer modeling and dynamic hot-box tests for complex wall systems with the whole building thermal simulations is presented in this paper. This procedure can increase the accuracy of the whole building thermal modeling.
Current thermal modeling and calculation procedures tend to overestimate the actual field thermal performance of today’s popular building envelope designs, which utilize modern building technologies (sometimes highly conductive structural materials) and feature large fenestration areas and floor plans with many exterior wall corners. Some widely used computer codes were calibrated using field data obtained from light weight wood frame buildings. The same codes are used now for thermal modeling of high mass buildings with significant heat accumulation effects. Also, the effects of extensive thermal shorts on the whole building thermal performance is not accurately reflected by the commonly used one-dimensional energy simulations that are the current bases for building envelopes and systems designing.

Thermal modeling, Thermal bridges, Hourly energy simulation programs

This paper investigates how the spatial arrangement of thermal insulation influences the overall thermal resistance of concrete and masonry wall systems. Multi-dimensional finite difference modeling was used for this purpose. Concrete masonry units (CMUs) are commercially produced in various geometries and with different weight concretes. Although insulation inserts can increase a CMUs thermal performance, thermal bridging through the solid webbing of the CMUs can greatly reduce the effectiveness of the integrated insulation. Different commercially available CMU geometries and concrete weights were investigated using finite difference modeling to show the impact on overall CMU R-value and to determine the thermal efficiency of the insulation inserts.

thermal insulation, building envelope, masonry, concrete, thermal performance

PCM–insulation mixtures functionas light weight thermal mass components.It is expected that these types of dynamic insulation systems will contribute to the objective of reducing energy use in buildings. In this paper, dynamic thermal properties of a material in which phase change occurs are analyzed, using the temperature-dependent specific heat model. Integral formula for the total heat flow in finite time interval, across the surface of a slab of the phase change material was derived. Simulations have been performed to analyze heat transfer through a light-weight wall assembly with PCM-enhanced insulation, in different external climate thermal conditions. Results of simulations indicate that for cyclic processes, the effect of PCM in an insulation layer results in time shifting of the heat flux maxima and not in reduction of the total heat flow. The heat gains maxima, resulting in high cooling loads, are shifted in time by about two hours and reduced upto 22% for not very high external sol-air temperatures.

Different types of Phase Change Materials (PCMs) have been tested as dynamic components in buildings for at least 4 decades. Most of historical studies have found that PCMs enhance building energy performance. The PCMs store energy and alter the temperature gradient through the insulated cavity because they remain at a nearly constant temperature during the melting and solidifying stages. The use of organic PCMs to enhance the performance of thermal insulation in the building envelope was studied at the Oak Ridge National Laboratory during 2000 – 2009. PCMs reduce heat flow across an insulated region by absorbing and desorbing heat (charging and discharging) in response to ambient temperature cycles. The amount of heat that can be stored in PCMs is directly related to the heat of fusion of the material, which is between 116 J/g to 163 J/g (or 50 to 70 Btu/lb) for the most - popular micro encapsulated paraffinic PCMs, or fatty acid materials used in this research. This paper presents experimental and numerical results from the long-term thermal performance study focused on blown fiber glass insulation modified with a novel spray-applied micro encapsulated PCM. Experimental results are reported for both laboratory - scale and full - size building elements tested in the field. Test work was followed by detailed whole building Energy Plus simulations in order to generate energy performance data for different US climates.

The ability for reduction of whole-building energy consumption depends, in large scales, from correct predictions of building thermal loads with the building’s envelope characteristics being one of the most important factors. Since most of today’s building envelopes are complex three-dimensional networks of structural, insulation, and finish materials, the potential for correct predictions of their thermal performance depends on availability of acceptable, scientifically valid, consensus procedures for accurately implementing a building’s envelope thermal characteristics into whole-building energy simulation programs.
This paper is discusses a joint LBNL and Fraunhofer CSE project, focused on the upgrade of the already existing THERM program and its integration with EnergyPlus, a whole-building energy simulation tool. It is expected that these two programs, combined together, will eliminate typical analytical limitations of most of existing whole building energy tools, capable to simulate only simplified one-dimensional envelopes. The main research challenge is the design of an easy to implement upgrade of the THERM numerical tool to allow analysis of complex building envelope structures. The new version of THERM needs to be able to modify thermal characteristics of the complex three-dimensional (3-D) wall assemblies, in a way to enable their use in whole building energy simulation programs. It will be achieved through an application of the unique theoretical procedure, which will allow a generation of the simplified one-dimensional (1-D) wall geometry and material characteristics to fully and accuratly capture the dynamic effects of thermal bridges.
At this stage of the project, the research team focuses on development of theoretical bases for necessary changes in the THERM framework. This paper explains the theoretical methodology which is used and presents some results from the series of steady-state and dynamic heat transfer simulations performed on building envelopes architectural components, to illustrate the accuracy limitations associated with thermal calculation methods recommended by building energy codes worldwide.