As recommended by the myhomescience.com action plan, the best place to add insulation hands-down is the attic. In winter, air that you’ve paid to heat finds its way to the attic by natural convection, literally forcing its way through the tiniest wall and ceiling leaks. From there it is vented to the outdoors. A corresponding amount of cold air rushes in through leaks in the lower part of the house to replace it. The result is a cold, drafty, and costly home.
To reduce ongoing heat loss, insulation may be added between attic floor joists or over the floor of the attic. If you have plans to convert the attic to “conditioned” (heated or cooled) room – such as a home office or extra bedroom — Install the insulation between the rafters and/or over the underside of the rafters.
Once you’ve insulated the attic, turn to your basement or crawlspace. As noted above, they are major sources of cold air infiltration. It is especially important to seal the area where the house framing rests upon the foundation, often called the rim joists.
For both attic and basement, many professional insulation contractors choose an insulation that seals against air leaks as well as resists conductive heat flow (high R value). Spray foam and rigid foam board insulation are excellent choices.