I'm re-insulating my attic and was wondering about the reflective foil barriers put on the underside of the roof. While I know they're good for preventing downward heat, does this pose any problem for the roof in terms of making it hotter and causing it to prematurely age?
- Dan M.
Dan, I'm not aware of any instance where installing reflective foil in the attic space adversely affects the lifespan of the roof sheathing or the roofing material. Of course, the material is still fairly new so there really aren't any long term test results yet. However, they have done some testing in Florida, where it can get fairly hot during the summer months. Even on hot, sunny days, the asphalt shingles on homes with the barrier were only 2 to 5 degrees hotter than those without the reflective foil. That small increase shouldn't affect the sheathing or asphalt shingles in any detrimental way.
One of the most important things to remember when installing reflective foil is that for it to be most effective, it should be installed perpendicular to the roof surface. That means that if you have a pitched roof, as most homes do, the foil should follow the underside of the trusses or roof framing so it has that same slope.
The other big issue is dust. Many unfinished attics aren't cleaned on a regular basis and if dust accumulates on the reflective foil, it reduces the area able to effectively reflect heat. If it gets too dirty, the foil may no longer function at all. I have read that some reflective foils work just as well when the reflective side faces down, so that's something you may want to consider when choosing a foil. A material that faces down isn't going to accumulate as much dust on the reflective side of the sheets.
If you want the best of both worlds, there are double sided reflective foils available. However, to function efficiently the "up" side of the material should be cleaned on a regular basis.
One last point that's worth mentioning is that reflective foils work better in hot climates than they do in those where heating is more important than cooling. The government's energy.gov site estimates that when reflective barriers are used in hot, sunny climates, they may be able to reduce cooling cost by as much as 5 to 10 percent. Using the barriers in conjunction with a cool roofing material may lower your costs even more.