In the United States most builders still subscribe to the idea that roofs should be vented in order to allow for moisture to escape. This would mean that you would not want to use foam roof insulation since it provides a moisture barrier. However, this belief is coming into question. Foam roof insulation is commonly used in the United Kingdom, and people are starting to doubt the necessity of vented roofs in the U.S. as well.
For those with vented attics who would like to use foam roof insulation, it should be sprayed on the floor of the attic in between the floor joists just like other types of insulation. This will still mean that the attic will be able to get very hot in the summer. However, the foam roof insulation should help to keep that heat from transferring to the living areas of the house.
If you happen to have an unvented roof, you would want to spray the foam roof insulation all around the attic including the ceiling and the floor. This will keep both the attic and the house in general much cooler in the summer and decrease your heating and cooling costs throughout the year. You will also be less likely to have problems with roof leaks.
There are a number of companies that sell foam roof insulation. You can get it either in stiff sheets or in a spray form that expands. Difficult to reach spaces are easier to insulate with the spray variety which expands to fill any cracks and spaces where air or moisture could get through. Dust and loose insulation are also less likely to somehow find their way down to your living area with this type of insulation.
One product that is available for people who would like to take a do-it-yourself approach to foam roof insulation is Handi-Foam. A kit that contains enough to cover 605 square feet with a one inch coat of foam costs about $605. There are also smaller sizes available. However, large areas like attics are best done by professionals who have the right equipment to finish the job quickly and easily. Although foam roof insulation will cost more than fiberglass batting, you will make up the difference in cost through savings on your energy bill.
