Adding insulation to your attic is a great idea, but the power of spray foam insulation is its ability to stop air infiltration, along with its insulating strength. Insulating just the attic with spray foam insulation does not take advantage of those properties.
Because I am assuming the walls and floors are not insulated or are under-insulated, I would not recommend the use of home foam insulation unless you are going to insulate the entire house to reduce air infiltration from all sources. The cost of spray foam insulation, whether it be biobased polyurethane or regular polyurethane or icynene, is higher than conventional or even some non-conventional insulation. The cost/benefit of installing foam insulation in the attic does not make sense.
I would recommend cotton batts as the best attic insulation for your situation.
- Cotton batts have the same basic R-values as fiberglass or blown-in insulation without the environmental issues associated with fiberglass and, in my opinion, without the mess of blown-in insulation.
- Cotton batts contain no harmful airborne particulates, no formaldehyde and no VOCs, eliminating health concerns regarding particulates in the surrounding environment.
You will have to install a vapor barrier against the heated side of the structure (ceiling). If budget is an overriding concern, installing fiberglass batt insulation is a final alternative, due to the environmental concerns.
According to the Department of Energy's insulation recommendation map, you live in an area similar to me in New Jersey, so I can give you an idea of the insulating power you may need. Each portion of your home will require a different level of insulation.
- All the insulation types have about the same R-value per inch of about 3.5 except for the polyurethane spray foam, which is higher.
- In my area, I usually specify walls to have an R-value of 13 to 15; ceilings in attics an equivalent of R-30 or R-38; floors over an unconditioned crawl space or basement the equivalent of R-30 or R-38.
- These numbers can vary depending on the amount of glass in the house, orientation of the house, etc.
The level of R-49 is impractical, and for the most part unattainable by conventional insulation (fiberglass) within conventional construction. The amount of blown-in needed to attain a level of R-49 is approximately 14 inches, which is also excessive. The added thickness of insulation brings added cost, and the return on your investment drops after a specific level of insulation.
Cotton batts installed to a reasonable thickness give you a good insulation layer in your attic, without the indoor air quality issues of fiberglass, the mess of blown-in, or the added cost of spray foam insulation.
You can have a quick and inexpensive energy model of your house done by an architect in your area using the Department of Energy's "Res Check" program to simulate levels of insulation required to calculate compliance with federal energy standards of new or renovated homes. Using this program will tell you what levels are appropriate specifically for your home in your area.
For more information:
Read Ian MacLeod's Q&A "What is the safest and most effective insulation for our attic crawl space?"