My client wants to install old tin on their porch ceiling. This porch roof is on the West face of the house, receiving unfiltered afternoon sun in south Mississippi. My concern is that without insulating the ceiling and venting the attic space (the rest of the house is spray foam insulation), that the metal ceiling will act as a radiant heater when heated by high attic temperatures in the summer. Any experiences or advice on this?

  • Does the space above the porch ceiling communicate with the rest of the attic to begin with? – ThreePhaseEel Nov 22 '17 at 4:12
  • You seem to think metal radiates a lot, but non-metal like plywood doesn't. This is not the case. If an object is hot and you touch it, then it will tend to burn you more if it is a metal than a non-metal would. Everyone is familiar with the effect of touching the sheet metal of a dark painted car which has been sitting out in the sun. We know that a piece of dark painted plywood will not do the same. What is happening is that heat is conducted in the metal so that a lot more heat flows into your hand. We have a light painted tin ceiling in our kitchen; it does not radiate heat from the attic – Jim Stewart Nov 22 '17 at 10:32
  • I would beg to differ on heat dissipation properties of different objects. In my experience as a thermal imaging specialist, heat the heat dissipation properties between metal and wood vary greatly in the infrared range, as well as thermal capacity. Your kitchen ceiling does not radiate heat because it is not hot, due to the insulation in your attic, and the fact that your attic is vented. – JMoudy Nov 22 '17 at 22:47

I think you are seeing a problem where there is no problem. Any additional minimal heat that makes its way to the metal ceiling will long be dissipated before is makes anyone uncomfortable. If makes you feel better put a thin layer of expanded polystyrene under the metal. P.

  • Do you intend to nail the tin panels (usually 2' x 2') to the ceiling or use construction adhesive? I used 18 ga brads to attach our tin ceiling in the kitchen and set the pressure on the brad nailer low enough so that the heads would not be driven through the panels. To do this I screwed 10 mm thick plywood over the drywall into the ceiling joists, but presumably your existing ceiling will hold brads. – Jim Stewart Nov 22 '17 at 10:43
  • I second the idea of a thin layer of insulation under the metal. Would one use polystyrene or polyurethane? There used to be a fiberglass board that would not be as good an insulator as the plastic, but would not be a hazard in a fire. Some of this fiberglass board had reflective metal foil on one side which would raise the question of whether the foil side should be up or down. I would think up. – Jim Stewart Nov 22 '17 at 11:07
  • Paul, why would the heat from an averheated attic space that directly contacts the metal be minimal? The ambient temperature from an unvented, uninsulated attic space would quickly heat thin gauge metal, which should radiate that heat 360 deg, which would include downward, onto my clients. – JMoudy Nov 22 '17 at 22:50

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