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Where should I install my insulation?
Attic: The most common place to insulate your attic is on the attic floor. This is usually most effectively done with loose fill cellulose or fiberglass. You want to insulate the attic to an R-49 level (15 to 20 inches of insulation). The insulation on your attic floor needs to be consistent, with no gaps or squishes.
If you have ductwork running along your attic floor, stack the insulation over it. If you have ductwork running above the floor, and/or your heat pump or furnace is in the attic, it could make sense to insulate the attic roofline. This is typically done with open-cell spray foam. When insulating your attic roofline, you’ll need to seal up all the attic venting.
Basement:
You can either insulate the basement ceiling, or the walls. Typically, insulating the walls is a better option because it makes the basement more comfortable. By making the basement more comfortable, you are increasing the efficiency of any ducts that are running through the basement as well. When insulating the walls, you have two options: build a new wall with studs and fill with fiberglass batts, or glue rigid insulation to the walls.
When insulating the basement ceiling, you can use either spray foam or batts. When installing batts, make sure they are continuously touching the 1 st floor. Any areas where they are sagging down renders the insulation nearly useless.
Whether you choose to insulate your ceiling or walls, pay special attention to the top of the walls (where the exterior walls hit the first floor), often called the joist. In most homes, the joist area has significant air leaks, and is typically insulated poorly or not insulated at all. Air seal the joists with spray foam before insulating.
Crawlspace:
Like the basement, either the walls or the ceiling of the crawlspace can be insulated. When insulating the walls, all vents to the outside need to be sealed. You should always insulate the crawlspace walls if there is ductwork running through the crawlspace. Rigid foam is the most common choice for insulating walls, R-19 fiberglass batts when insulating the ceiling.
Before insulating a crawlspace, you’ll need to make sure a proper moisture barrier is installed on the floor. This is typically done by installing sheets of 5-mil plastic and sealing them together and to the walls. This will help prevent moisture from the ground to cause mold growth along the crawlspace ceiling.
Walls:
Insulating existing walls can be very difficult, if not impossible in many situations. The most common way of insulating existing walls is with cellulose insulation, which is blown into the walls between each set of studs.
Source: Department of Energy – Insulation Fact Sheet
 Duct Sealing
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