Insulation
 Insulation in the walls, probably the most important guard against heat loss or gain, is hard to install after a house is built. If you don't know if you have insulation in your exterior walls, you can either remove an outlet cover (careful!) to look into the wall cavity for insulation or you can drill two 1/4 inch holes into a wall inside a cabinet or closet, one 4 inches above the other. Look into one hole while shining a flashlight into the other. If you can see insulation, you're golden. Otherwise, bring in an insulation contractor to blow cellulose or fiberglass into the walls.
In an unfinished basement, you want to insulate between the floor joists with unfaced fiberglass batt insulation. You should cover the underside of the joists with a moisture permeable air barrier like Tyvek® or Typar®.
A heated basement needs insulated walls. Build a 2X4 frame on the inside, insert fiberglass insulation, and cover with drywall. Be sure you don't have drainage problems. Or admit you have two left thumbs and hire a contractor.
In your unfinished attic, check to see if you have a full foot of fiberglass or cellulose insulation on the floor. If not, put down some loose fill or unfaced batts to make up the difference. If the top of the joists are still exposed, add an additional layer of unfaced batts across the joists.
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