Mario A. Medina

A Transient Heat and Mass Transfer Model of Residential Attics Used to Simulate Radiant Barrier Retrofits, Part I: Development

This paper describes a transient heat and mass transfer model of residential attics. The model is used to predict hourly ceiling heat gain/loss in residences with the purpose of estimating reductions in cooling and heating loads produced by radiant barriers. The model accounts for transient conduction, convection, and radiation and incorporates moisture and air transport across the attic. Environmental variables, such as solar loads on outer attic surfaces and sky temperatures, are also estimated. The model is driven by hourly weather data which include: outdoor dry bulb air temperature, horizontal solar and sky radiation, wind speed and direction, relative humidity (or dew point), and cloud cover data. The output of the model includes ceiling heat fluxes, inner and outer heat fluxes from all surfaces, inner and outer surface temperatures, and attic dry bulb air temperatures. The calculated fluxes have been compared to experimental data of side-by-side testing of attics retrofit with radiant barriers. The model predicts ceiling heat flows with an error of less than ten percent for most cases.

A Transient Heat and Mass Transfer Model of Residential Attics Used to Simulate Radiant Barrier Retrofits, Part II: Validation and Simulations

A computer program was developed and used to implement the model described on Part I of this paper. The program used an iterative process to predict temperatures and heat fluxes using linear algebra principles. The results from the program were compared to experimental data collected during a three-year period. The model simulated different conditions such as variations in attic ventilation, variations in attic ceiling insulation, and different radiant barrier orientations for summer and winter seasons. It was observed that the model predicted with an error of less than ten percent for most cases. This paper presents model results for nonradiant barrier cases as well as cases for radiant barriers installed horizontally on top of the attic floor (HRB) and for radiant barriers stapled to the attic rafters (TRB). Savings produced by radiant barriers and sensitivity analyses are also presented. The model results supported the experimental trend that emissivity was the single most significant parameter that affected the performance of radiant barriers.

An experimental method for validating transient heat transfermathematical models used for phase change materials(PCMs) calculations

An experimental procedure, herein called the temperature-change hot chamber method (TCHCM), was developed for the validation of transient heat transfer mathematical models that involve phase transitions. Two such mathematical models were created using two well-accepted methods in the field, namely the enthalpy and the effective heat capacity methods. The models were implemented using computers and validated via the TCHCM. It was shown that the results from the models and from the validation experimental data agreed well. Throughout this paper, it is shown that the TCHCM is simple, effective, and can fully validate heat transfer mathematical models that involve phase transitions. In addition, the accuracies of both the enthalpy and theeffective heat capacity methods were evaluated, learning that the accuracy of the effective heat capacity method was superior, but also that the results provided by the enthalpy method could be improved by the proper determination and selection of a specific phase transition temperature range related to the substances being evaluated.

A Simplified Model for Radiative Transfer in Building Enclosures With Low Emissivity Walls: Development and Application to Radiant Barrier Insulation

This paper deals with a simplified model of radiative heat transfer in building enclosures with low emissivity walls. The approach is based on an existing simplified model, well known and used in building multizone simulation codes, for the long wave exchanges in building enclosures. This method is simply extended to the case of a cavity including a very low emissivity wall, and it is shown that the obtained formalism is similar to the one used in the case of the based model, convenient for enclosures with only black walls (blackbody assumption). The proposed model has been integrated into a building simulation code and is based on simple examples; it is shown that intermediate results between the imprecise initial simple model and the more precise detailed model, the net-radiosity method, can be obtained. Finally, an application of the model is made for an existing experimental test cell including a radiant barrier insulation product, well used in Reunion Island for thermal insulation of roofs. With an efficacy based on the very low emissivity of their surfaces and the consequent decrease in radiative heat transfer through the wall in which they are included, the proposed simplified model leads to results very close to those of the reference method, the net-radiosity method.

Effects of shingle absorptivity, radiant barrier emissivity, attic ventilation flowrate, and roof slope on the performance of radiant barriers

This paper presents a parametric study of the effects that shingle absorptivity, radiant barrier emissivity, attic ventilation flowrate, and roof slope have on the performance of radiant barriers in symmetrical residential attics. A heat balance model was developed to investigate these effects. The model was validated against experimental data and was found to predict with good accuracy. Of the four parameters investigated, only emissivity of the radiant barriers had first-order effects on their performance. Variations in the performance of the horizontal radiant barrier (HRB) configuration were minimal in the other three parameters. The truss radiant barrier (TRB) configuration showed slightly more variations because of the presence of uncovered end-gables. This paper presents a brief description of the heat balance model, the parametric studies and conclusions. Copyright

Development and verification of an EnergyPlus-based algorithm to predict heat transfer through building walls integrated with phase change materials

A conduction finite difference algorithm developed for EnergyPlus was used to model building walls integrated with phase change materials. The model was validated by comparing it against experimental data in terms of temperatures, wall heat fluxes, and total wall heat transfer. Experimental data, using two identical test houses of conventional residential construction, were collected for the validation and further analyses. The thermal performance of walls without phase change materials (control house) and with phase change materials (retrofit house) was evaluated. The model showed that the differences between experimental and predicted total heat transfer values were under 5%. The total heat transfer reductions produced by phase change materials could be predicted accurately using the conduction finite difference algorithm in EnergyPlus.

Validation and simulations of a quasi-steady state heat balance model of residential walls

This paper discusses the validation of the model presented in a companion paper [1]. A test house was built, instrumented, and monitored. The results suggested that the model predicted reasonable well with a 9.6% difference when cumulative heat flows were compared. Because of the steady state principles used to construct the model, the measured data and model prediction showed a lag of approximately one hour. In all cases, the model underpredicted the peak heat fluxes by an average of 17%. Simulations were run to estimate total heat transfer in walls as a function of wall orientation and insulation level.

Verification of an energy balance approach to estimate indoor wall heat fluxes using transfer functions and simplified solar heat gain calculations

This paper presents the results of a mathematical and computer model used to predict building indoor wall heat fluxes. The model is based on an energy balance approach that uses conduction transfer functions (CTF) and simplified solar heat gain calculations via solar heat gain factors (SHGF). The model was implemented via computer programming. The purpose of the research was to produce an accurate model driven by only a time series set of measured outdoor and indoor air temperatures. In addition to the model, an experimental test set up was constructed and instrumented to gather data for verifying the model. It was found that the energy balance/CTF/SHGF enabled heat fluxes to be predicted with accepted degrees of accuracy, from which space cooling and heating loads as well as building energy could be estimated.

A quasi-steady-state heat balance model of residential walls

A quasi-steady-state heat balance model was developed to predict heat transfer across the walls of a residential building. Hour of day, day of month, month of year, together with ambient air temperature, outside air velocity, inside-room air temperature, construction data, and location and orientation of the building were input to the model. The output of the model was hourly wall heat fluxes. The model predictions were validated by comparing the predictions with experimental values from monitoring a test house located in Southern Texas, U.S.A. In general, the cumulative heat transfer predictions differed by no more than ten percent when compared to the experimental data. Computer simulations were run to predict total wall heat transfer by taking one years ambient temperature data from the Typical Meteorological Year (TMY) weather files. The experiments, validations, and simulations of the model are presented in a companion paper [1].

Natural Materials for Thermal Insulation: Mulch and Lava-Rock Characterizations

This paper reports on the thermal characterization, via the thermal conductivity, of natural materials, such as mulch and lava rock and their usefulness as building insulation. Experiments were carried out using a scale one monitored wall (i.e. heat flux and temperature sensors) exposed to a heating source on one side and to an air conditioned space on the other. The wall system was composed of an 8.85 cm thick cavity, where the mulch and lava rock were placed. The cavity was enclosed between two layers of pine wood (40 mm thick each). After the experiments and statistical data manipulation, the estimated thermal conductivity of the materials were 0.48 ± 0.001 W.m-1.K-1 and 0.129 ± 0.003 W.m-1.K-1 for mulch and lava-rock, respectively. That is, mulch has a thermal conductivity comparable to that of bulk hemp while lava rock has a thermal conductivity comparable to that of hemp brick. These values indicate the usefulness of mulch, compared to the impracticality of using lava-rocks materials for building insulation.