Adding insulation good way to battle heating costs

May 27, 2007

Our home is oil heated and it is getting expensive to heat. We are adding insulation to the ceiling and the floor over the basement. I understand that when it comes to insulation, more is always better than less, but at some point it no longer makes good economic sense to add any more. How much is enough insulation for my home? My wife suffers from allergies, so I was thinking of installing batt insulation over the loose fill between the rafters in the attic. Is there any problem with doing it this way? Will it smash down the existing too much to place it over the top?

While the heating season is about over, this is a good time to think about things that can help reduce heating bills next winter. One proven way to fight the rising cost of home heating is to make your home more efficient by adding insulation, sealing air leaks and purchasing energy-efficient appliances as older ones need replacing. You are on the right track by increasing the insulation levels of your home. If installed correctly, higher levels of insulation will slow heat loss from your home and reduce the amount of heating and cooling you need to remain comfortable.

How much insulation you add will depend on the cost of the fuel you use to heat your home. From an economic standpoint, the less you pay for heating fuel, the less insulation it makes sense to install. That is why homes that were built in the 1960s and 1970s have so little insulation in them — the cost of fuel didn’t justify much of an investment in insulation. As the costs of heating increase, so does the economic justification for adding more insulation.

The Washington state energy code sets minimum levels of insulation for newly constructed homes and remodeled existing homes. The insulation requirements in the code are based on the cost-effective savings for the region over the life of the home from a specific level of insulation. Currently, all new homes are required to have R-38 insulation in the ceiling (15 to 18 inches of blown insulation) and R-30 insulation in the floor (a 10-inch batt of insulation) over unconditioned space.

These levels are based on the minimum cost-effective investment in insulation in a new home; your individual tolerance for increases in monthly heating and cooling costs is your own. These are “minimum” code, or minimum acceptable building practice, levels of insulation.

Depending on the amount of insulation already installed in your attic and the floor over the basement, your home’s construction may preclude installing this amount of insulation. This becomes an issue in attics with very low roof pitch and in floors where the framing will not accommodate the amount of insulation suggested.

Floor joists built of two-by-eight lumber may not provide enough room to install a 10-inch R-30 insulation batt. This is very typical in older homes. A high-density R-21 insulation batt that will fit in a 2-by-8-inch floor joist is an adequate substitute.

Depending on the amount of insulation already in your attic, placing a batt directly over blown insulation may compress the existing insulation, rendering it less effective. However, if the insulation level is no higher than the top of the joist, laying unfaced batt insulation across the joists will minimize compression of the insulation between the joists. If the insulation between joists lofts above the level of the joist, I recommend blowing more insulation over the existing rather than installing batts.

Before adding more insulation to your home, consider doing other work that will be hidden or difficult to work on once the insulation is added — air duct sealing, electrical wiring, television cables, etc. Also, seal holes around plumbing and wiring penetrations in the attic floor and basement ceiling with caulking or foam to minimize air leakage. This will make the insulation more effective.

Older recessed lights and exhaust fans may not be rated for covering with insulation; remember to hold the insulation a minimum of three inches away from these heat-producing fixtures.

While you are in the attic, be careful to step only on the top of the joists, never between them. This will prevent you from falling through the ceiling. I recommend taking a 2-by-8 piece of plywood to span the joists and give you a wider platform to stand and kneel on while installing the insulation.