Extra Heat Loss Through Light Weight Roofs Due to Latent Heat

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    This report is one in a series of papers in Task 5 of IEA Annex 24 on how moisture and air movements affect the energy performance of building constructions. The effect of latent heat flow will be demonstrated by means of an example: a light weight flat roof.Latent heat flow is one of three processes by which moisture affects energy performance:Higher thermal conductivityMoist materials have higher thermal con-ductivity than when they are dry. This is because thermally conducting moisture replaces the better insulating air in the pores of the materials. Moisture also enhan-ces the thermal contact between the solid grains of a porous material. Finally, moisture may partici-pate in microscopic heat pipes in a material by which vapour diffuses from the warm to the cold sides of wide pores in the material, and is trans-ported back again by capillary action in adjacent fine pores. This is a latent heat effect that will be described sepa-rately from the macroscopic latent heat flow because, as there is no net mois-ture transfer associated with it, it cannot be distinguished from heat conduction.Other processes which from a macroscopic point of point can be treated as heat conduction, and thus driven by temperature differences, are convection and radiation heat transfers that take place microscopically within a porous structure. Heat conduction might not be the most correct term to express these processes, and therefore thermal transmission (ISO 10051, 1994), or heat flow caused by a temperature gradient in condition of moisture equilibrium (Sandberg, 1986), are sometimes used instead. Thermal transmissivity or apparent thermal conductivity are commonly used terms for the material property associated with these phenomena.ConvectionWhen moisture flows in a material without phase conversion, it carries its own enthalpy from one region of the material to another. However, for normal moisture and heat flow rates in building applications, this enthalpy flow can be neglected when compared to normal flow rates for heat transmission, and the rates of potential heat that is carried with moisture that changes phase at the terminals of its passage.Note however, that convection of air most often will have an important effect on the overall heat flow - but that is a different topic.Macroscopic latent heat transferConsider the following scenario: Initially, moisture is present in its condensed or frozen phase within a certain region of a material. It then changes phase and migrates to another region. Here it changes phase back again to its original state. The heat associated with the phase changes is consumed at one place, and released at the other, and the whole process signifies a transfer of latent heat. This heat transfer phenomenon, which can sometimes be quite important, is the main subject of this paper.
    Original languageEnglish
    Title of host publicationProceedings, IEA-Annex 24 on Heat, Air and Moisture Transport in Highly Insulated Envelope Parts, Closing Seminar
    Place of PublicationLeuven, Belgium
    PublisherLabo. Bouwfysica, Katholieke Universiteit Leuven
    Publication date1996
    Publication statusPublished - 1996
    EventIEA-Annex 24 on Heat, Air and Moisture Transport in Highly Insulated Envelope Parts, Closing Seminar - Helsinki, Finland
    Duration: 1 Jan 1996 → …


    ConferenceIEA-Annex 24 on Heat, Air and Moisture Transport in Highly Insulated Envelope Parts, Closing Seminar
    CityHelsinki, Finland
    Period01/01/1996 → …

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