Journal of Building Physics

With the adoption of the Montreal Protocol last year, the search for al ternate blowing agents has intensified. HCFC-123 and HCFC-141b have been iden tified as two possible alternatives for CFC-11 in rigid foam applications. Although neither product is commercially available today, evaluations of both have begun in laboratories and on production equipment.

Performance of foams manufactured on production equipment will be discussed. HCFC-123 and HCFC-141b were run in typical polyurethane/polysocyanurate for mulations on production-scale laminate board equipment. Again, physical properties and thermal conductivities will be discussed and compared. Full-scale flammability test results (i.e., ASTM E-84 Standard Test Method for Surface Burning Characteris tics of Building Materials) will be discussed.

Despite the fact that HCFC-123 and HCFC-141b are not currently available to rigid foam manufacturers, this study gives good base line data on the performance of the two most promising alternate fluorocarbons to replace CFC-11 in the future. The studies demonstrate that either product is a viable blowing agent for the rigid foam industry.

Thermal insulation with thermal resistivities of at least 20 (h ft ² F/ Btu in.) have been shown to have the potential for significant energy conservation if employed in residential and commercial refrigerator/freezers Many materials and systems have been proposed which may be incorporated into refrigerator/freezers to achieve this potential.

The current state of the art for advanced evacuated insulations which may achieve resistivities of 20 was established by reviewing data on many materials available in the open literature on the dependence of the thermal performance on internal pressure. The costs for the powdered, fiber, foam and multilayer materials were then obtained from the manufacturer of the products Possible candidate materials for m- clusion into refrigerator/freezers are described and ranked based upon their thermal properties and costs.

Several materials were found that may be used to make super-insulation panels with material costs of less than $1.00 per board foot, if a plastic laminate is used for the container required to maintain the necessary vacuum. Materials falling in this category included. Beverly silica dust, open-cell polyurethane foam, fine perlite, 2.7 lb/ft³ fiberglass and 3.6 lb/ft³ fiberglass opacified with vapor-deposited alu minum, precipitated silica, and mixtures of precipitated silica and fly ash. Mate rial costs approaching $3.00 per board foot will result if metallic contamment is required.

The fundamental mechanisms of heat transfer through rigid polyure thane (PUR) foam are reviewed in terms of three distinct elements, λ _g , λ _s and λ _r , which represent heat transfer by thermal conductivity through the gas phase, solid phase and by radiation respectively. It is demonstrated that values for λ _g and λ _s can be estimated, and from the measured λ-value of the foam, an approximation of λ _r ob tained by difference.

This shows that there is considerable scope for improving the thermal insulation efficiency of these materials. Small reductions in λ-value can be obtained by op timisation of λ _g and λ _s elements, but the main scope for improvement is by reduction of λ _r . This can be partially achieved by the production of foam with very fine cells. However there are strong indications that the cell windows are transparent to radiative heat transfer and several other factors need to be taken into account in re ducing λ _r to its minimum practical value. Combining all these factors, laboratory foams can be produced consistently with λ-values 20-30% lower than typical com mercial material.

λ-value ageing, which results from air ingress into the cells of the foam, can be substantially eliminated by covering the foam with air impermeable barriers. How ever recent developments have shown that by optimisation of foam formulations, the rate of ageing and the aged λ-value can also be significantly reduced in uncovered foams.

The excellent insulation properties of PUR foams compared with competitive ma terials is a consequence of the low value of λ _g which results from the encapsulation of the Refrigerant 11 blowing agent inside the foam cells. Supplies of Refrigerant 11 may become limited in the future, with potential adverse effects on the insulation efficiency of PUR foam. In such a situation the indications are that any losses in in sulation efficiency can be minimised and be more than compensated for by further optimisation of the approaches discussed in this paper.

Two alternative chlorofluorocarbons, CFC-123 and CFC-141b, have been proposed as substitute blowing agents for CFC-11 in rigid urethane foams. Both of these have similar boiling points to CFC-11 but substantially reduced ozone depletion potentials. Mobay Corporation has evaluated both of these in a variety of systems to determine their potential use as blowing agents for rigid urethane foams in appliance applications.

Due to the different molecular weights compared to CFC-11, there is theoretically 11 weight percent more CFC and 15 weight percent less CFC-141b required in foam formulation to obtain equivalent gas volumes. Using the same gas volumes as CFC- 11, CFC-123 and CFC-141b gave poorer flowability resulting in higher foam densi ties. With CFC-123, depending on the formulation, the nature of the isocyanate, the chemical structure and on the technique of foam preparation, there was a remarkable difference in densities and physical properties. The highest densities and the poorest physical properties were found in PMDI polyurethane foams because of a plasticiz ing effect of the CFC-123 in the polymer. However, in TDI polyurethane foams, densities and properties were more comparable to foams with CFC-11. CFC-123 also showed good performance in rigid polyisocyanurate foams. In all systems, CFC- 141b was closer to CFC-11 m overall properties.

Both alternatives produced as theoretically expected, higher k-factor foams than CFC-11. In corresponding foam systems, we measured up to 15 percent higher ther mal conductivities. The k-value retention of foams made with CFC-123 and CFC- 141b appeared to be similar to CFC-11.

The formulations tested were developed originally for use with CFC-11. To improve the performance of the alternatives as blowing agents in rigid polyure thane appliance foams, adjustments and optimization of the formulations will be nec essary.

The effects of air movement on the thermal performance of permeable insulation systems are reviewed. The paper addresses air convection in and around porous insulation exclusively. Air infiltration through the insulation, from uncondi tioned to conditioned space is not covered The results reported indicate that the thermal insulation actually provided by a porous material may differ substantially from that which is calculated by simple application of R-values (as reported in the lit erature), due to air convection effects The application of an airtight membrane can reduce this R-value degradation.