An analysis is presented comparing Teflon film with glass for the inner glazing of a double-glazed selective-black, flat-plate solar collector. The effect of spacing between glazings and between the inner glazing and absorber plate is examined. It is shown that a 12.5-micron Teflon film is superior to glass for the inner glazing of a selective-black collector, because the advantage of its high solar transparency overwhelms the disadvantage of its infrared transparency. A too-small spacing between a selective-black absorber and its inner cover short-circuits the desirable thermal radiation resistance offered by a selective-black absorber plate. Account is taken of spectral variations in the radiation properties of glass, Teflon, and the absorber plate. Allowance is made for the fact that critical Rayleigh number is lower for a plastic film inner glazing than for a glass one.

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