Question: How much of a gap should I leave at the tops and the bottoms of the foil to allow for proper ventilation?
Ideally you want about 3 to 6 inches of space at the top and 3 to 6 inches of space at the bottom. Attic ventilation is simple: you have holes in the bottom of your attic, and holes in the top. The goal is to install the AtticFoil® Radiant Barrier in such a way that it doesn't change the attic ventilation from its natural course.
The key is to give the air a free, clear path to flow in to the attic at the bottom via soffit vents and out the top via your exit vent; with the staple up method you create an air channel behind the foil (between the foil and the roof deck) that will allows this path to exist.
By leaving a gap, some of the air behind the foil will come in to the attic, but that is OK. The hot air will travel the path of least resistance to make its way out of your attic. When you install AtticFoil® with the proper air gaps, it allows for maximum ventilation between the roof and the foil and it also gives you the benefit of full coverage.
What happens if you install the foil and you have a space that is less than a full sheet width? Do you cut another piece to fit or do you leave it as-is?
It’s really up to you. If you have a smaller attic and the space uncovered is a good percentage of your total coverage, then it would probably be worth the extra effort to cut a piece of foil to fit the space. If, however, your attic is larger and the space left uncovered is a small percentage of your roof area, then you could just leave it as-is. With any radiant barrier, the more coverage you can get, the better the overall results will be. Just like if you were parking your car outside on a sunny day – parking it in an area with as much shade as possible will help the car stay cooler.
You certainly want adequate ventilation, but more is not always better. Having good attic ventilation helps to reduce the air temperature in your attic, but having a radiant barrier helps reduce the surface temperatures in your attic. This is why radiant barrier combined with proper ventilation works to make your home more comfortable and energy efficient. From our experience, most people have a good amount of exit vents, but not nearly enough intake vents. Your soffit vents along the bottom of the roof line are the intake to allow air into the attic.
If you have several points of exit for the air, but not enough intake vents, then some of those outtake vents can actually turn into intake vents, completely disrupting the natural cycle of airflow. Air always travels the path of least resistance so by mixing exhaust vents, you can actually have the strongest vents dominate as the outtake and all other vents will become a point of air intake. Again, this is important because the natural airflow is for air to enter the attic at the bottom, and flow out of the top. Your best bet is to make sure you have plenty of clean and obstacle free soffit vents. We have seen cases where a soffit vent was attached to a home, but the holes were actually never cut out; take time to make sure yours are done correctly and save yourself a lot of time later. As for the exhaust vents, they should be at the top of the roof (or close to it) and at least a few feet apart, if not more.