Mark Bomberg

Semi-Empirical Model of Heat Transfer in Dry Mineral Fiber Insulations

A semi-empirical model for calculating heat transfer in dry mineral fiber in sulations takes into account parameters that characterize both the structure of the fibrous material and the boundary conditions in standard laboratory testing. It permits adjustment of experimental data for changes in specimen density or thickness, mean temperature or temperature gradient, i.e., all the uncertainties involved in sampling and testing procedures.

Thermal Performance of Sprayed Polyurethane Foam Insulation with Alternative Blowing Agents

A test methodology that uses thermal resistance-time curves deter mined on thin shces of the foam and the scaling technique to relate aging time to the specimen thickness, was applied to evaluate long-term thermal performance of six polyurethane foams manufactured with the same polymer but different blowing agents. The blowing agents employed were: CFC-11 with 0, 0 5, 1 0 or 1.5% water and HCFC-123 or HCFC-141b.

Integrated Methodology for Evaluation of Energy Performance of Building Enclosures: Part II — Examples of Application to Residential Walls:

It is often forgotten that the building sector consumes more energy than the transportation sector. To meet expectations and needs of our society, one must seek significant improvements in the efficient use of energy for this purpose. In many instances our normal approach based on conventional testing methods is not comprehensive enough. For instance, the thermal performance of a wall is defined by tests performed on dry materials, without considering the air and moisture movements. The energy performance of materials and building assemblies is affected by moisture and air flows. The authors believe that a more precise means of evaluation of the thermal performance of assemblies must be used to guide us in developing construction practices that lead to better performance. This should include consideration of air and moisture transfer under field conditions. The previous part of this study describes the limitations of conventional thermal resistance testing using calibrated hot boxes and explains that the effect of climate on thermal performance must also involve use of computer models that are capable of simultaneous calculations of heat, air, and moisture (HAM) transfer. In this study, the integrated testing and modeling methodology proposed is applied to a few selected residential walls to highlight the magnitude of air flow effects compared with steady-state thermal resistance without air flows. Effectively, to characterize energy performance of the building enclosure, one must use an integrated methodology that uses both testing and modeling. The study represents a first step in this direction.

Scaling Factors in Aging of Gas-Filled Cellular Plastics

One of the techniques used to determine aging of gas-filled cellular plastics (GFCP) is to correlate the thermal resistance of thin material layers with that of full thickness boards Either models of aging or scaling factors may be used for this purpose. This paper discusses the concept of scaling factors relating aging of thin and thick layers of GFCP, and points out their limitations.

Towards an Engineering Model of Material Characteristics for Input to Ham Transport Simulations - Part 1: An Approach

Heat, Air and Moisture (HAM) modelling of building performance is a quite young research subject but the experimental determination of material properties is often based on classical methods. One should review the manner in which we define characteristic material parameters and there is a need to develop an approximation used to generate the required material functions for input to HAM-transport simulations. The paper presents such an approach, called an engineering model for hygrothermal material characterisation. The paper poses the question, how to arrive at input data that can be used for a model based on thermodynamically defined potentials (Only such a model allows introduction of new potential components (freezing depression, osmotic pressure, air pressure, overburden envelope pressure)) (e.g., Grunewald, J. (1997) and Grunewald, J. (1999)) and yet the respective functions used to describe changes in the material response as a function of the variables of state. Such functions should have a reasonable precision and goodness of fit while the number of measured points must be reduced to a minimum. Those measurements should be relatively easy to perform (i.e., they would not require determination of temporal and spatial profiles of moisture). This discussion paper highlights steps already taken (Part 1), and lists issues that need to be resolved before reaching this goal (Part 2).

Evaluation of Long-Term Thermal Resistance of Gas-filled Foams: State of the Art

Some cellular plastics (foams) incorporate a chlorofluorocarbon (CFC) to improve their thermal insulating properties. During the service life of this type of insulation air diffuses into the foam cells and the CFC gas diffuses out, each at a rate that depends on type of polymer and temperature, eventually reducing the effectiveness of insulation. This effect is known as aging. Although literature on the aging process is extensive, there are no generally accepted procedures for evaluating long-term thermal resistance of new gas-filled foams. The mechanisms of the aging process are reviewed, and integrated approach to the evaluation of aging of gas-filled foams is proposed.

Measurements of the Rate of Gas Diffusion in Rigid Cellular Plastics

This paper describes a method for measuring gas diffusion through cellular plastics A specimen is placed in a chamber, which is then scaled and pressur ized with the test gas. The pressure increase causes some gas to enter the specicmen, and this entry results in a reduction of the chamber pressure By following the change in chamber pressure with time an appropriate transport characteristic for the material is determined An application of this method to a core specimen of an extruded polystyrene board is presented.

Influence of Moisture and Moisture Gradients on Heat Transfer Through Porous Building Materials

The total heat transfer through moist, porous building materials is the sum of a number of heat-transfer phenomena. This paper presents experimental data on heat transfer through specimens of moist aerated concrete and mineral fiberboard. The aerated concrete specimens with known initial moisture distributions were exposed to a temperature gradient. This temperature gradient caused a redistribution of moisture which in turn changed the heat flow through the material. The changes were monitored by means of thermal probes, heat flowmeters, and thermocouples. The results of measurements were compared with calculations based on a simplified analytical model and with measurements made by other investigators. A second series of tests was made on a moist specimen of high-density mineral fiberboard to demonstrate the extent of the dependence of the total thermal resistance on the distribution of moisture in the material.

Analysis of Selected Water Absorption Coefficient Measurements

For isotropic materials with a significant fraction of micro-pores, the cumulative water intake per unit of inflow surface area typically yields a linear function of the square root of time elapse. The authors postulate that this dependence has a limited range of validity. The validity of this approximation starts from an initial period that is inversely proportional to the rate of water intake and ends much before the material reaches the capillary moisture content. Experimental investigation presented here uses a differential presentation of the cumulative water inflow and clearly indicates that material such as calcium silicate or brick belong to a broad class of materials characterized by a constant water absorption coefficient (A-coefficient). Initial period varies from ½ to 4 minutes. On the other hand, materials with a multiple pore-system such as an Aerated Autoclaved Concrete (AAC) may display a systematically varying A-coefficient. Authors propose a test procedure limited to 1-hour duration that can be used to derive a practical and reproducible value of A-coefficient.

Integrated Methodology for Evaluation of Energy Performance of the Building Enclosures -Part 1: Test Program Development

As a result of increased concern with energy consumption in the industrial world, it is only natural to look towards the building sector to seek significant improvements to meet expectations of the society. After all, the building sector consumes more energy than the transportation sector. Yet, the procedures that are used to define the thermal performance of, for example a wall, are typically based on the tests performed on dry materials, without consideration of air and moisture movements. In other words, these tests represent arbitrary rating conditions because we know that the energy performance of materials and building assemblies are affected by moisture and air flows. It is believed that to improve their energy performance one must have more precise means of evaluation of their field performance that would also include the consideration of air and moisture transfer conditions. In the first part of this article a background for the evaluation of thermal performance by traditional testing with calibrated boxes shows that use of these tests is limited. The average heat flow that they measure is sufficient to rate the wall assemblies but insufficient to calculate its thermal performance under field conditions. To include the effect of climate on thermal performance one must use computer models that are capable of simultaneous calculations of heat, air, and moisture transfer. Effectively, to characterize energy performance of the building enclosure one must simultaneously use assembly testing and modeling, i.e., an integrated methodology. In the second part of the article, this integrated testing and modeling methodology is applied to a few selected residential and commercial walls to highlight the magnitude of air flow effects on the steady-state thermal resistance. The integrated methodology proposed by Syracuse University includes several other aspects of hygrothermal performance evaluations. Those aspects will be addressed in later parts of this article series.