why foil?

I've had fun testing barriers for the past few years. I wanted one in my house (built in 1976),so I bought the foil (I have seen a low, medium and high grade) and stapled away. Later I found the paint version. When I moved to a new house, I left one section bare, used the paint for another section, foil for another, and finally did both the paint AND foil for yet another section. When I say "section" I'm referring to the vertical running rafters right next to each other. That way I catch the same sun/angle/plywood/etc. I ran up into the attic every hour during the hottest portions of the day for 3 days and then did a spreadsheet to figure out what was actually working.

The results were that the paint dropped the temperature by 11 degrees. The foil dropped the temperature by 15 degrees, and by doing both, it actually dropped the temperature by 26 degrees. now, someone smarter than me can feel free to explain how that works. All I know is that it works and my bill went from $425 to $245 from July to August. I like to think of it as the "Sherman Tank school of engineering". Yes, it is overkill, but if you do all of the work yourself, you are going to get a payback within 2 years. That's a heck of an ROI.

The advantage of going with foil is that it has a full sheet of aluminum instead of just aluminum particles floating around in a paint solution and sprayed on the plywood. The foil is usually either polyethelyne or polypropelyne with a sheet of aluminum glued to either side. The advantage is the strength that the backing material provides. There are different grades of foil but I highly recommend getting some that you cannot tear with bare hands. The cheap stuff rips as you are trying to install it (this makes for a truly annoying experience).