Insulation traps tiny air pockets between insulating solids such as fibers or foam. This slows heat flow through a space by inhibiting conduction (heat transferred through liquids or solids, in this case the insulation material) and convection (heat transferred through liquids or gases, in this case the trapped air pockets).



Insulation doesn't stop heat flow; it just slows it down, and better insulation slows it down more. And because heat rises, if you don't have good attic ceiling insulation you'll lose a lot more heat through your attic than you will through poorly insulated walls.

The effectiveness of insulation is measured in R values, which tell you how well the insulation slows down the natural transfer of heat. If you have several types of insulation touching one another you can add the R values together. For example, if you have ten inches of fiberglass batt insulation between joists, at R-3 per inch (total of R-30) and you blow in another 5 inches of fiberglass at R-2.5 (total of R-12.5), you have R-42.5, except that if you add insulation in a way that compresses it or insulation previously installed below it, you'll lower the overall R value. For instance, if you put a 10-inch R-30 rated batt between nominally 10 inch joists (which are usually 9.25 inches) and press it down to joist level, you are probably cutting the R-value down to around the R-25 mark.

You can install your own batts between joists; choose batts whose thickness will make them flush with the top of the joists. If the batts protrude above the joists and there are gaps between the joists, the batts aren't providing much insulation value above that level. You can install a second layer of batts over top of batt insulation in the joists; layer it crosswise to the joists and don't leave any gaps between the batts. Never lay batt insulation over top of blown insulation.

Blown insulation typically has a lower R-value than batt insulation and needs to be installed with a blower. A professional insulation firm is your best bet, but you can sometimes rent blowers from your local building center or equipment rental outlet. Still, I would recommend hiring a pro; a given R-value of blown-in attic ceiling insulation, say R-48, often costs less to have professionally installed than it would cost you just to buy the batts for that R-value, without counting your time spent installing it!

Higher R values mean higher energy efficiency. The US Energy Efficiency and Renewable Energy (EERE) provides a heating map that divides the country into six heating zones. EERE recommends at least R-49 for the three coldest zones (1-3, which makes up about 90% of the country's land mass) and R-38 for the warmer zones; for electric heat, R-49 is recommended country wide. My recommendation is to go for at least R-49 regardless of where you live, and R-60 or more for colder zones. You might as well save as much on heating and cooling as possible.

Make sure that when new insulation is installed, you don't cover up any existing vents in your attic roof, side walls, or soffits. You might even want to add soffit or roof vents if there aren't already enough, as better ventilation prevents heat build-up in your attic in summer, and prevents moisture build-up in winter. Even a well-insulated attic can create heat gain inside an air-conditioned house (remember, the insulation slows heat transfer, it doesn't stop it). Moisture can reduce fiber insulation R-values temporarily (because high-humidity air spaces between the fibers transfer heat better than low-humidity air spaces) and permanently, if the humidity causes the fibers to become compressed.

If you have a professional install blown-in insulation, you'll want to check that they filled to at least the agreed-on level. Before the insulation is installed, nail thin strips of wood around the attic against the joists, so that they reach at least 8 inches above the level of attic ceiling insulation you are paying to have blown in. Mark the agreed level of insulation on each stick in red marker, and then mark two lines above and two lines below that line in another color. You can then inspect the completed installation to ensure that on average the insulation level made it at least to the agreed level. These markers will also ensure that the contractor is careful to install what he agreed to install.

There's no single best material for attic ceiling insulation. Avoid foam insulation because you do not want a vapor barrier between your attic and your living space. The key is to choose a material that gives you the highest overall R value possible.

If your attic currently has vermiculite insulation, you need to be very careful. Vermiculite is a naturally occurring mineral that expands into a lightweight material when heated (kind of like popcorn). Unfortunately, one of the main vermiculite mines in the US until 1990 was found to have a natural asbestos deposit in the same mine, which means vermiculite insulation installed in North America before or during 1990 may be contaminated with asbestos, an extremely toxic fiber. Although this mine was shut down I still don't recommend vermiculite - asbestos might be found in some other vermiculite mine later, and vermiculite has a lower R-value than most other materials.

If you do have vermiculite installed, leave it alone, or hire someone to test it for asbestos contamination before you replace it or even disturb it. If it does contain asbestos it is safe if undisturbed, but to remove it you'll need to hire people who specialize in asbestos removal.