This paper reports a measured relationship between the surface density of droplet condensation on a reflective aluminum surface and the emittance to infra-red radiation. The data can be used to predict the effect of condensation on the thermal resistance of foil lined building cavities. The effect is quite strong with 1 g/m2 of condensate increasing the emittance of bright aluminium from .06 to 0.25. This would reduce the thermal resistance of a 100 mm deep reflec tive cavity with downward heat flow from 1.4 to 0.6 m2 K/W.
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