Heat Blockers
Not technically an insulation, radiant barriers keep houses cool by reflecting thermal radiation. These thin sheets of shiny aluminum bonded to foam board, bubble wrap, or sheathing are often installed in attics to block heat from the summer sun.
To be effective, the barrier's reflective surface must always face an air space at least an inch thick and be installed shiny side up if laid on the attic floor, shiny side down if attached to the rafters. Tests show that a radiant barrier in an insulated attic can lower attic temperatures by as much as 30 degrees. While radiant barriers are a boon in warm climates, they're less useful in cold-weather regions because they prevent beneficial solar heat gain in winter. To see if it makes economic sense to put a radiant barrier in your house, visit the Department of Energy's Website (www.ornl.gov/sci/roofs+walls/radiant/).
Vapor Blockers
Once insulation gets wet, it's hard to dry it out. "It will sit there like a sponge, leading to mold problems and rot," says Tom. Vapor barriers—sheets of plastic or kraft paper—keep water vapor out of the wall cavity, so the insulation stays dry. Not every type of insulation needs a vapor barrier. But if it does, the barrier should face inside in northern, heating climates, and outside in humid southern climates.
