Recessed lighting
Recessed light fixtures ("cans") may have two designations, "IC" for Insulation Contact and "AT" for Air Tight. However, even those designated AT aren't totally air tight. For thermal and air leakage reasons, recessed lights aren't a good idea for placing in a ceiling under an attic space or in a cathedral ceiling with roofing over it in new construction. Aside from the loss of heat by direct conduction and by air leakage, which otherwise would help heat the house in winter, leakage of air into a roof assembly, carrying inside humidity with it, can lead to condensation under the roof deck and the mold and rot damage that can result from that moisture. Also, one doesn't want to add heat to a roof surface in winter. Most instances of ice damming and the damage resulting from that are due to too much heat leaking into the attic or into an improperly built cathedral ceiling.
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