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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:
- R-38 + R-3.5/inch = 10.8 inches: The R-value will be specified on the insulation package or on the facing sheet of most batt insulation.
- R-values can be added where insulation is already installed. For instance, if you
have R-8 insulation in your ceiling now, and want to increase it to R-38,
you can add insulation with an R-value of 30, provided the weight of the
insulation doesn't exceed the structural weight limits of the ceiling. New
insulation installed on top of old should be unfaced batts or loose fill.
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.
2. Loose fill insulation is poured or blown into the space to be
insulated (Fig 1.3). Loose fill is usually made of mineral fiber, cellulose
fiber, vermiculite, or perlite. It is a convenient insulation type to use
in unfinished attics, especially if some insulation is already installed.
3. Rigid board insulation (fiberglass, polystyrene, polyurethane) is
usually attached with adhesive mastic. This type of insulation is used for
crawl space perimeters, concrete walls and "exposed beam" ceilings.
TABLE A
R-Values Per Inch of Some Typical Insulating Materials |
|
| Mineral Wool Batts |
3.5 |
| Fiberglass Batts |
3.1-3.5 |
| Mineral Wool (Loose Fill) |
2.8 |
| Cellulose (Loose Fill) |
3.1-3.7 |
| Vermiculite (Loose) |
2.1-2.1 |
| Perlite (Loose) |
2.3-2.7 |
| Polystyrene Boards (Rigid) |
3.6-5.3 |
| Polyurethane Boards (Rigid) |
5.6-7.7 |
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| Note: Manufacturers' or Dealers' claims of higher R-values than those listed should be investigated. Some may not qualify for financing. Check with your utility. |
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.
TABLE B
Recommended R-values for Houses in the Pacific Northwest |
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| Ceiling |
R-38 |
| Crawl Space Under Floor |
R-19 |
| Crawl Space Perimeter (Exterior) |
R-10 |
| Crawl Space Perimeter (Interior) |
R-19 |
| Wall |
R-11 |
| Duct |
R-11 |
| Pipe |
R-3* |
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| * May need more in severe winter climates. |
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