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Insulation: a Blanket for Your House

Insulation slows down the heat flow through a building's envelope. It works year-round to make your home more comfortable and energy efficient. In the winter, it slows heat loss and condensation buildup. During summer months, insulation reduces heat gain and helps keep your home cool.

Adding insulation to your home can cut heating and cooling costs 15 percent or more, depending on factors such as the amount of existing insulation in your home, house size, air leaks, personal energy use, and living habits. Many variables affect the exact amount you’ll save, but insulating your home is often a wise energy investment.

While every house is different, the basic rule of insulating is the same for all homes: Install insulation on any surface separating a heated space from an unheated space. Even if your home already has some insulation in these areas, there can be great benefits in adding more insulation, especially in your attic.

Recommendations vary for the amount of insulation necessary in particular locations for peak energy savings, depending on factors such as climate conditions, the sections of your home being insulated, and the kinds of materials used in your home’s construction.

Insulation is rate by R-values

The R-value (or thermal resistance) of insulation is a measure of its ability to resist heat loss or heat gain. The higher the R-value, the better a material insulates. It’s important to note that an insulation’s R-value is based on its performance with no air movement. Therefore, the effective R-value of a particular insulation may be much lower than its rated R-value, especially if the insulation is not properly installed—or if air leaks are not sealed before the insulation is added.

When you go shopping for insulation, it’s important to remember that the product with the highest R-value per inch may not be the most cost-effective one. For example, when insulating a basement wall to an R-12 value, using 3 inches of an R-4 per inch insulation material might be less expensive than using 2 inches of an R-6 per inch product. To get the most insulating value for your money, compare the total costs of insulating an area to a specific R-value.

In addition, blown or loose-fill insulation materials will settle after installation, reducing their effective R-value by 10 percent or more. (Dense-pack cellulose for use in wall or floor cavities, however, has negligible settling.) Be sure to check the manufacturer’s specifications before you buy insulation, and follow supplied charts to ensure adequate long-term coverage.

Finally, some types of insulation—such as dense-packed cellulose, polyurethane, and polyisocyanurate—combine both an air-sealing barrier and insulation in one step. Some insulation products also serve as a vapor retarder when properly installed. When comparing products, carefully consider the costs and benefits of doing the entire job: insulation, air-sealing, and vapor retarder.


Loose-fill vermiculite or perlite may be found in older homes but is no longer used for home insulation. Because of the risk of asbestos in some vermiculite, it is strongly recommended that homeowners do not disturb these materials. In some cases, insulation can be installed over vermiculite in attic locations—but only by trained insulation contractors. Should vermiculite removal be necessary, a licensed asbestos removal company must be engaged.

Knob & tube wiring

Although less frequently seen in homes as wiring has been updated, there are still some places where the old “knob and tube” wiring exists. Consisting of cloth-insulated wires that run over and through ceramic knobs and tubes in wall studs and joists, this type of wiring can be very hazardous. Over time, the wire insulation deteriorates, thus exposing bare wires. Additionally, if the wires are covered by insulation, heat can build up, leading to a potential fire. In fact, it is illegal to install insulation over knob and tube wiring in an attic and other locations. The only remedy is to completely replace the entire wiring system, allowing for proper insulation—and increased safety.