Superinsulating attic for summer cooling?
I just came back from a day at a "green building" forum. Unfortunately, I was not able to attend the whole forum and talk to presenters, informally at breaks, as much as I'd have liked.
One presenter talked about a house he'd built for himself in which - besides insulating his exterior walls very well - he had put a vertical 17" of insulation (mixed materials) in his attic, above the ceiling. He said his house needs hardly any heating in winter beyond the heat provided by his electric lights, his fridge's extracted heat, and the body heat of the occupants. He said it stays cool in summer.
I've got a 30-year-old house with R-20 walls (fiberglass insulation), and with about 10 inches of vermiculite between the second-storey ceiling joists in my attic. The attic is ventilated at the gable ends. The roof is metal we put on as a replacement ayear ago. The house is pretty easy to heat in winter by comparison with most of my neighbors' places... that's partially due to the insulation and partially to the excellent heating equipment we've installed.
But in summer, the darkish green of the roof metal may be attracting and transfering solar heat more than did the cedar-shake roof we had until spring 2005. The house can get a bit uncomfortably warm by 3:00 in the afternoon.
What I'm wondering is whether our house would be easier to keep cool in summer from another layer of insulation above the joists - say, six-inch-thick fiberglass bats. Any ideas?