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Insulating an attic

Older homes in particular can be poorly insulated, and when they are, both heat and heating bills go through the roof. Even homes in the mildest climates should have about 9 inches of fiberglass insulation. Homes in northern areas— Buffalo, Des Moines, and Duluth, for example—should have more than 12 inches of fiberglass, according to the United States Department of Energy.

Insulating an attic is relatively easy if it has no floor: You simply roll in the insulation. If your attic has a floor, you’ll want to apply blow-in insulation. Blow-in is available as either fiberglass or loose-fill cellulose. The manufacturers of each type trumpet the advantages of their product, but in short, the tune goes like this: Blow-in cellulose provides more insulation than fiberglass. Fiberglass, however, is less prone to settling, which reduces the efficiency of the insulation over time. Cellulose turns into a gooey mess when wet; fiberglass temporarily loses its insulating ability, but it does recover once it’s dry.

The amount of insulation you need may determine which kind you get. Find your location on the map below to determine which zone you’re in. The chart tells you the R-value your insulation should have. The R-value is printed on insulation packages—the higher the R-value, the greater the insulation. Incidentally, if you have a whole-house fan, slip some insulation over the louvers during the winter.

VAPOR BARRIER SAVVY - When the warm, moist air hits the cold, outdoor air, water vapor condenses and collects in the wall or ceiling, where it causes all sorts of problems. Because of this, roll or batt insulation comes with a facing that acts as a vapor barrier. In most parts of the country, you install the facing toward the occupied part of the house. In some areas of the South, however, you should install the barrier facing the home's exterior. Check local codes. Two vapor barriers are actually worse than one, so if you’re adding insulation on top of insulation that already has a barrier, use insulation without a facing.

MATERIALS: Baffles, insulation

TOOLS: Tape measure, utility knife and extra blades, straightedge, particle-resistant dust mask, safety glasses, gloves

How much insulation is enough?

 

Ceilings below ventilated attics

Floors over unheated crawl

Exterior walls (a) (wood frame]

Crawl space walls (b)

Insulation zone

Gas, oil, or heat pump

Electric resistance

Gas, oil, or heat pump

Electric resistance

Gas, oil, or heat pump

Electric resistance

Gas, oil, or heat pump

Electric resistance

1

R-49

R-49

R-19

R-19

R-1

R-11

R-19

R-19

2

R-38

R-49

R-19

R-19

R-1

R-11

R-19

R-19

3n

R-38

R-38

R-19

R-19

R-11

R-11

R-19

R-19

4

R-38

R-38

R-19

R-19

R-1

R-1

R-19

R-19

5

R-30

R-38

R-19

R-19

R-11

R-1

R-19

R-19

6

R-30

R-38

(c)

R-19

R-1

R-1

R-19

R-19

7

R-30

R-30

(c)

(c)

R-1

R-1

R-19

R-19

8

R-19

R-30

(c)

(c)

(c)

R-11

R-11

R-11

(a) For new construction, R-19 is recommended for exterior walls. Jamming a R-19 batt into a 3 1/2-inch cavity will not yield R-19 because compression reduces the R-value.

(b) Insulate crawl space walls only if the crawl space is dry all year, the floor above is not insulated, and all ventilation to the crawl space is blocked.

(c) Thermal response of existing space for cooling benefits does not suggest additional insulation.

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