Walls, floors, or ceilings without insulation offer the best opportunity for a quick payback on your insulation investment. You might be surprised to learn that it’s more important to insulate small uninsulated areas (for example, attic and crawl space hatches) than it is to add insulation to large areas that already have some insulation.
The more extreme your climate (hot or cold), the more you can benefit from insulation. See the Department of Energy (DOE) Recommended Total R-Values map to determine what climate zone you live in and the recommended insulation levels for each part of your house. Consider the low end of each R-value range as a minimum; once you’re insulating an attic, it doesn’t cost much to add a few more inches.
Roofs and attics. If you plan to reroof, you may want to add insulation to the roof deck before you install new roofing—especially if you have a flat roof or an unvented vaulted ceiling, which are typically underinsulated. If you have a vented attic, you can add insulation almost any time—just be sure to air seal the attic first.
Attics are usually insulated with batts, blankets, or blown insulation. Blown insulation fills cavities more completely than batts or blankets, and will fit around ducts, wiring, and truss cords. When blowing insulation into a vented attic, use baffles around soffit vents to maintain adequate airflow and keep wind from displacing the insulation.
In uninsulated attics, some installers like to use a hybrid system. This consists of about 1 inch of sprayed foam on the ceiling deck for air sealing and enough of the less-expensive blown insulation on top to make up the desired R-value.
Skylight chases. In houses with skylights, chases through unconditioned attics should be insulated to the same level as the exterior walls. This is often difficult with batts or blankets; a good alternative is to use sprayed foam or box in the chase with rigid insulation.
Floors. The floors over vented crawl spaces and unconditioned basements can be insulated any time after the crawl space or basement is air sealed. The hybrid system described above for attics works well. For best performance, floor insulation should be in substantial contact with the floor above. It should also be well supported so it doesn’t sag or fall out over time, allowing air to circulate around it and diminish its R-value.
Crawl spaces. If you are changing from a vented to an unvented crawl space, the new crawl space should be insulated.
Concrete slab-on-grade floors. Insulate around the perimeter of the slab, from the top of the slab down a minimum of 2 feet, or to the footings. Rigid foam insulation is appropriate, but it must be protected with a durable material, such as cement board, stucco, or metal flashing, wherever it is exposed aboveground.
Basements. In cold climates, walls below ground level that enclose conditioned space should be insulated. Exterior insulation is ideal, but the required excavation is usually too costly in a retrofit situation, so interior insulation may be your best bet. However, if you’ll be excavating around the foundation for other reasons, consider applying exterior rigid foam insulation board to the outer side of the foundation wall. Be sure to protect the insulation from sun and weather wherever it is exposed. If termites are a problem in your area, include a termite barrier to keep them from moving through the foam into the wood framing.
Ducts. Any ducts outside your home’s conditioned space should be well sealed and insulated to avoid losing much of your investment in heated or cooled air. Several types of insulation are made for ducts, including rigid fiberboard, flexible fiberglass, flexible foam, and reflective insulation.
Plumbing. Insulate plumbing pipes outside conditioned space to protect cold-water pipes from freezing and hot-water pipes from losing heat.