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    Heat and Moisture Transfer in a Glass Fiber Roof-Insulating Material

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    The transport of heat and moisture has been investigated in a mediumdensity wetted insulation. Experimental and analytical investigations were made on samples up to 50 mm thick with fixed average moisture concentrations. Moisture and temperature distributions were measured under transient and steady conditions.

    The experiments were conducted in a guarded hot plate test chamber with moist test specimens scaled inside highly impermeable vapor barriers to simulate the thermal response of wet roof insulation. Experimental apparatus, procedures, and problem areas associated with measuring moisture and temperature distributions under representative conditions are described. The experiments consisted of subjecting uniformly wetted and initially isothermal test specimens to a fixed applied temperature difference. It was observed that up to several weeks could be required to approach a steady-state moisture distribution, while the temperature distribution approached a “quasi-steady” condition in a few hours. The thermal conductance of the specimens was more strongly dependent on moisture distribution than on the mean moisture content. The results obtained were compared with existing simplified theories for the “effective” thermal conductance of wetted materials.

    An analytical model was developed for correlating the experimental results and investigating dominant transport phenomena. For moderate liquid contents, the primary moisture transfer mechanism resulted from the applied temperature difference and consisted of evaporation, vapor diffusion, and condensation. Conduction and latent heat effects were the important energy transfer mechanisms. The experimental observations supported the analytical results that transient effects are significant in applications where temperature boundary conditions vary with time.


    moisture migration, heat transfer, roofing insulation, fibrous materials

    Author Information:

    Thomas, WC
    Virginia Polytechnic Institute and State University, Blacksburg, Va.

    Bal, GP
    Mechanical Engineering Department, Virginia Military Institute, Lexington, Va.

    Onega, RJ
    Virginia Polytechnic Institute and State University, Blacksburg, Va.

    Committee/Subcommittee: C16.16

    DOI: 10.1520/STP29473S