Long-Term Thermal Resistance of Closed-Cell Foam Insulation ...

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3rd Global Insulation Conference and Exhibition, Barcelona, 16-17 October 2008What is Closed-Cell Foam Insulation?The basic structure of closed-cell foam insulation consists of two major components (1)polymer matrix, and (2) gas-filled closed cell (Figure 4). The captive gaseous blowingagent(s) fills up the closed-cells, surrounded by polymer matrix, during themanufacturing process of the foam. The thermal conductivity of blowing agent is usuallylower than air. In effect, the presence of blowing agents inside the closed-cell reduces theair conduction component of thermal conductivity significantly depending on the type ofblowing agent used in the closed-cell foam insulation, as shown in Table 1 (Wu and Eury,2002). The thermal conductivity of most of the commonly used blowing agents is lowerthan half of the thermal conductivity of air.Table 1 – Thermal conductivity of blowing agentsBlowing AgentThermal ConductivityBlowing AgentThermal Conductivity[k gas @ 25 (°C) mW/m.K][k gas @ 25 (°C) mW/m.K]CFC-11 8.7 Hydrocarbon n-C 5 15HCFC-141b 9.7 HFC-134a 13.6HCFC-142b 11.5 HFC-245fa 12.2HCFC-22 11.0 HFC-365mfc 10.6Hydrocarbon c-C 5 12.0 Air 26In general closed-cell foam insulation has many desirable properties as insulation and aconstruction material such as:1. High r-value per unit thickness2. Fire resistance3. Moisture resistance and water repelling characteristics4. Dimensional stability5. Efficient filler for cavities, and6. Resistance to solvents commonly found in construction adhesives.However, it is also true that the thermal conductivity of the closed-cell foam insulation,due to the presence of captive blowing agents inside the closed-cell, increases over timeas air diffuses into and blowing agent(s) diffuses out of the closed-cells (Figure 5). Therate at which this diffusion (also called aging) occurs depends on a number of parameterssuch as properties of foam matrix, geometry and structure of the cells, exposure
3rd Global Insulation Conference and Exhibition, Barcelona, 16-17 October 2008conditions, density of material, manufacturing process, thickness of the insulation,chemical composition and permeability of the surfaces etc. This aging process of thefoam insulation is very rapid just after the manufacturing and slows down with the timeand almost reaches to an equilibrium state after a certain time. Hence, for the thermaldesign purposes, unlike traditional fibrous insulation materials, the long-term thermalresistance (LTTR) value is to be considered as the effective thermal resistance of theclosed-cell foam insulation. The researchers at the National Research Council (NRC)Canada, Institute for Research in Construction (IRC) have been researching on thevarious aspects of the LTTR characteristics of the closed-cell foam insulation for morethan 20 years now. A brief summary of these research activities and the technicalchallenges are presented in the following paragraphs.Solid Polymer MatrixClosed Cell (Blowing Agent)Figure 4 – Basic structure of closed-cell foam insulationPermeableBAAirFacersImpermeableFoamClosed-cell Polyiso Foam FoamBoardFigure 5 – Aging of closed-cell foam insulation
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Long-Term Thermal Resistance of Closed-Cell Foam Insulation ...

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