Bonneville Power Administration
|BPA Home EE Home Energy Tips Do-It-Yourself Home Weatherization Guide About Insulation|
Insulation is simply a method for slowing the movement of heat. Insulating materials work in the same way that goose down works-by trapping air in tiny pockets that restrict it from moving. Heat transfer that would normally be accomplished through natural air movement is slowed down because the air can't move as freely.
The thickness of an insulating material isn't the only factor in determining its effectiveness. Some materials trap air more effectively than others, and produce the same insulation value with less material thickness. One inch of mineral wool insulation, for instance, has the same "heat resistance" abilities as 46 inches of concrete.
The heat resistance capacity of a material is called the "R-value". R-values of standard insulating materials range from 2.1 to 7.7 per inch. Common building materials such as sheetrock, shakes, and concrete blocks have R-values of 0.5 to 1.0 per inch.
Adding more insulation increases the R-value. To determine the depth required to reach a specific R-value, divide the recommended R-value by the R-value per inch of the material you're using. For instance, to insulate a ceiling to the recommended R-38 level with rock wool batts (R-value: 3.5/inch) you would need 10.8 inches:
Your utility representative will be able to tell you the R-value of any existing insulation in your ceiling and whether you should install additional insulation.
Vapor Barrier vs. Ventilation
Insulation material is available in three basic types-bans (or rolls), loose fill and rigid board. Some ban and rigid board insulation comes with an attached moisture-proof facing of foil or kraft paper called a vapor barrier. Vapor barriers are always installed toward the heated space, so that they are placed between the heated room and the insulation.
Vapor barriers are important in most insulation installations, because they protect the insulation from moisture produced in a heated house. The moisture produced by the everyday household activities of cooking, bathing, and washing can condense in the insulation or on the walls, floors, and ceilings in cold weather. Wet insulation is much less effective in preventing heat loss. The additional weight could also cause structural damage by exceeding the weight-bearing capacity of your ceiling.
In most cases, if the insulation doesn't come with an attached vapor barrier, you must install one as described in this book. However, if you already have some insulation in your attic, do not install a vapor barrier over the old insulation. This will cause moisture to build up in the old insulation. Instead, you may increase ventilation above the insulated ceiling. To find out more about preventing moisture build-up in the insulated walls, ceilings, and crawl spaces, read Section IX, Vapor Barriers and Ventilation.
Types of Insulation
1. Batt or blanket insulation is made of mineral wool or fiberglass (Fig 1.1 Fig 1.2). It is available faced (with a vapor barrier) or unfaced (without a vapor barrier). Batt insulation is best used for crawl spaces or unfinished walls. It can also be used in ceilings where little or no insulation is already installed.
How Much Insulation To Order
1. Find the area, in square feet, of the wall, floor or ceiling space to be insulated by multiplying its length times the width (in feet). When calculating for floors or ceilings, refer to the sections on attics or floors first. Depending on the way your house is constructed, different surfaces should be insulated.
2. Subtract the total square footage of doors, windows, and other spaces which won't be insulated from the number found in Step 1.
3. If you are insulating an uninsulated ceiling with batts, remember that you will need a second layer of unfaced R-19 batts to reach the recommended R-38 ceiling insulation level shown on Table B, so double the area to be insulated in your calculation.
4. The resulting figure is the total square footage to be insulated. Each bag or bundle of insulation is labeled with its square-foot coverage. Round your calculation up to a number evenly divisible by the coverage of one package, so that enough is left over for trimming.
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