Adding insulation to the walls is a more intrusive process than adding attic and crawlspace/basement insulation.
As you have already completed the attic insulation, consider the crawlspace and basement.
- With a crawlspace or unfinished basement, this would consist of adding insulation between the floor joists from underneath, either with a product that can be stapled up against the first floor, or spray foam, which would also help with sealing the house.
- Of course, the foam is a petroleum product and not a green product in itself; however, over a long period of use, 's superior insulation and air-sealing capacity make it the better product in my opinion.
Insulate the walls if you have a finished basement
If you have a finished basement that you are continously heating like the rest of the house, insulation between first floor and basement does not help much, though, and you would need to move on to insulating the walls.
In order to insulate walls, access needs to be made to the stud frame, the space behind your drywall. Your house is probably framed with 2x4 studs, which have an actual depth of 3.5 inches, so this is the maximum thickness of insulation you can add to the walls (you could also build up the walls in thickness to the inside, but this is pretty expensive and rarely done).
- As you do not want to take the entire sheetrock off the inside of the walls in order to install the insulation, you will need to select an insulation product that can be applied through a hole or a narrower strip opening that can be cut into your sheetrock.
- Cellulose blown in insulation is a nontoxic option here, but will only give you about R-11 in the wall. Spraying in foam can give you walls of up to about R-21.
Either option is still pretty messy, though, as the openings will need to be sheetrocked again, taped, mudded and painted.
For more information: