A 2004 house without any insulation in the attic? How is that even possible -- do they not have a building code and inspections down there?
- First question -- is one of your HVAC units in your attic?
- If not -- forget the spray foam and simply get a good air-sealing job done at your ceiling and install blown in insulation (cellulose or fiberglass) to R-38.
- If you do have an HVAC unit and/or ductwork in the attic, you may want to consider a semi-conditioned attic -- where home foam insulation is one of the few options.
Although rare, I do hear of issues with reactions to spray foam, so if you are sensitive to plastics, you may want to avoid using it.
There are other ways to accomplish the same thing, or you can consider an HVAC "penthouse" to bring the unit itself into the thermal envelope of your home. Be sure your ductwork has an R-8 insulation on it. Insulating at your ceiling level in conjunction with a radiant barrier installed on the bottom of your rafters is another option that is similar in cost and performance.
For more information:
Check our best attic insulation Q&A to see what other homeowners and contractors are saying about spray foam insulation, etc.