A properly insulated attic can reduce energy costs by up to 40%. But insulating an attic isn’t as simple as adding some fiberglass batts or cellulose blown in between the joists. You must also seal all air leaks through the ceiling into the attic to ensure the insulation performs as designed. This is particularly important in older homes that may have suffered from moisture problems or sagging that can compromise the R-value of the insulation.

Depending on the type of attic, you have a few options to consider:

For unfinished attics, insulating the rafters can be accomplished using spray foam or rigid board insulation. Both methods require careful planning to protect recessed light fixtures, electrical wires, soffit vents and other obstructions. The insulation must be sealed to the soffit to prevent heat loss and to help stop ice damming at the eaves.

The most common attic insulation is fiberglass batts. This is available precut in a variety of R-values. It’s inexpensive, readily available and effective, although its airborne fibers can cause irritation if not properly installed. You can use it alone or in combination with a polyethylene air barrier or low-permeable foam insulation board, which provides a more durable, water resistant and longer-lasting solution.

If you’re concerned about the risk of fire in an attic, cellulose insulation is an alternative. It’s made of recycled paper and has an R-value up to 3.7 per inch. Cellulose manufacturers add fire-retardant additives such as ammonium sulfate or borate to make the product safe and fire-resistant.

Another option is loose-fill cellulose or mineral wool. This is a green alternative that uses recycled glass, shredded newspaper and cardboard or industrial waste for its components. This form of attic insulation can have a higher R-value than the fiberglass batts and will last longer since it doesn’t lose density over time.

Both loose-fill materials require a thorough attic cleaning before installing to remove any existing insulation and dirt, as well as to allow for proper installation. It’s a good idea to hire an experienced contractor for this work, as both methods are labor-intensive and there are many things that can go wrong if you’re not careful.

If your attic is finished, you can install insulation over the drywall and soffit vents. This method is best for a home with limited headroom, but it can still be done with a little creativity and attention to detail. For example, you can use cut pieces of extruded polystyrene to cover recessed lights and other obstructions.

You can also install a layer of loose-fill or blown-in insulation over the attic floorboards and soffit vents. This is an excellent way to maximize the R-value for a small attic space and to help prevent thermal bridging through the joists and floorboards. This technique should not be used for a truss roof that has joists spaced more than 12 inches on center, as the attic floor will not be structurally sound. Lastly, you can have a professional blow in dense-pack cellulose or spray foam for an even more efficient attic.