For homeowners in heating climates, a cool roof can be a good energy-saving option. Architect Linda Reeder outlines the reasons to consider a cool roof — largely, to save on cooling costs — and describes the way cool roofs work. Tile, metal, and asphalt all can be good cool-roof materials. Cool roofs aren’t foolproof; they can be prone to mold growth in hot, humid climates. Also, they may cost a little more than a traditional roof, depending on the roofing material you choose.
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In Vermont we sometimes do "cold roofs" which are meant to prevent ice dams at the eaves. On an addition to my home I laid 1x3 t&g roughsawn spruce over recycled antique handhewn rafters. Over this went a 6 mil vapor barrier. O top of each rafter I put a 2x6 on edge and carefully fitted rigid foam insulation panels between these rafter extensions, leaving an inch of air space above the foam. One inch rough sawn boards sheathed the roof with recycled roof slates above. The eaves and ridge are vented. While the R value of the roof is not up to current standards, I have never seen an ice dams on this roof.
39 years in Key West FL has proved to me that emmissivity is just as important as reflectivity. An unpainted galvalume metal roof in the summer gets much too hot to touch, while you can lay naked on the same roof painted white. Ever heard the saying "cat on a hot tin roof"? And calling asphalt shingles with white sprinkles on them a cool roof material, well it just makes me hot. Nothing beats white metal over an air space and adequate insulation.