Moisture migration and drying rates for low-slope roofs: Preliminary results.

摘要

The Large Scale Climate Simulator at the US DOE Roof Research Center was used to impose steady and diurnally varying temperatures on nine roof sections simultaneously. One test determined the effect of water trapped in sections containing polyisocyanurate, extruded polystyrene or fiberglass. A second test allowed downward drying through solid polyethylene, polyethylene with holes, a Hygro Diode, or no vapor retarder. In the first test, the thermal performance of the foam insulations was not severely degraded by adding water. But, the fiberglass allowed moisture to move freely, which affected the heat fluxes. Water condensed and evaporated throughout the insulation and, to some extent, on heat flux transducers in the middle of the fiberglass. In the second test, fiberglass under plywood and expanded polystyrene over lightweight concrete were used with the various vapor retarders. Heat flux transducers placed above and below the fiberglass rather than in the middle gave more easily interpreted results. Computer modeling helped to understand the effects of condensation and evaporation on the heat flows. It allowed latent and sensible effects to be separated which showed if the measured heat fluxes were consistent with expectations. In wet permeable materials, the measurements and model showed that peak heat fluxes are twice what is expected in the same materials when dry. The second test also explored the potential for downward drying of wet roofs without venting. Gravimetric techniques were aided by electronic moisture probes to yield drying rates. The two vapor retarders which allowed liquid to escape had drying rates between those for solid polyethylene and no vapor retarder. The results show that downward drying can be appreciable while retarding vapor upflow. 5 refs., 12 figs.