Cooling a Hot Flat Roof
Taming the heat under the Michigan sun
We are slowly fixing up a 1937 International-style house built by architect Louis Rossetti. Although the modern design might seem more at home in Miami Beach than Michigan, we love it. But we do have a problem: In the summer the sun beats down on the flat roof and roasts us in the bedrooms below. The roof, which is covered with
silver-painted neoprene and regularly maintained, is only about 10 years old, so we'd hate to tear it off. Also, there's very little insulation in the ceiling. We thought of putting in skylights to vent the heat but hate the idea of cutting into a roof that doesn't leak. Help!
— Carolyn, Grosse Point Park, MI
Tom Silva replies: Cutting into a flat roof is something you do only when there is absolutely, positively no other choice. Besides,
skylights won't do a thing to stop the main problem: heat radiating through the ceiling. In fact, they would probably make it worse by allowing the sun's rays to heat up interior surfaces.
There are several strategies for blocking radiant heat gain in a house. According to Danny Parker, a research scientist at the Florida Solar Energy Center, "You can insulate heavily, install a radiant barrier, or make the surfaces more reflective." He also says that shading surfaces is effective, although I don't see any realistic way to do that in your case.
The insulation I generally recommend under flat roof decks such as yours is a spray-on open-cell foam insulation, which is very effective and doesn't have to be ventilated. But you'd have to rip out your ceilings to do that—not a great option. Adding a radiant barrier, which has a coating of reflective aluminum, would be a great solution if you had an attic but isn't practical on a flat roof.
Your best option, therefore, is to change the reflectivity of your roof simply by painting it with a white elastomeric coating. Parker says that this paint dramatically increases a roof's reflectivity and reduces the amount of heat it absorbs.
By the way, Parker says you don't need the ceramic beads that some manufacturers add to their elastomeric paints to improve insulating value. Extensive testing conducted by the Oak Ridge National Laboratory in Tennessee determined that the energy efficiency of "ceramic paints" is no better than standard elastomeric coatings.