How to Insulate an Attic
The easiest place to insulate in most homes is the attic. Fortunately, the attic is also the place where proper insulation has the most pronounced effect. In the winter, most of the home's lost heat goes out through the attic. In the summer, the uninsulated attic acts as a heat collector, making the air conditioning system work harder than it should.
If you find that there is no insulation whatsoever in your attic, follow these steps for installing batts or blankets. You'll find that batts are generally easier to install than blankets in most attics. To determine how much insulation you need:
Step 1: Measure the length and width of the attic and multiply the length by the width to arrive at the total square footage.
Step 2: Measure the distance between the joists. Most are on 16-inch centers (16 inches from the center of one to the center of the next), but some are on 24-inch centers. Buy batts or blankets of the correct width to fit between joists.
Step 3: Calculate insulation requirements. For 16-inch centers, multiply the square footage by .90 to give you the number of square feet of insulation required. For 24-inch centers, multiply by .94.
You must install a vapor barrier in attics that have no insulation. The easiest way to lay down a vapor barrier is to install batts or blankets of insulation that already have a vapor barrier attached.
Before you begin installation, cut pieces of plywood to use as movable flooring, and carry up wide planks to serve as walkways. If you step onto the ceiling material, you probably would break right through it, but the joists will support your weight. If the attic is inadequately lighted, rig up a lighting system so you can see what you're doing. A drop light suspended from a nail or hook should be sufficient. If you are installing fiberglass or rock wool, you must protect yourself with gloves, safety goggles, and a breathing mask. A hard hat is also a good idea to protect your head from protruding nails and low rafters. To lay down the insulating material:
Step 1: Start under the eaves and push the end of the blanket or batt in place with a long stick. Be sure to put the vapor barrier side on the bottom. Press the insulation down firmly between the joists.
Step 2: Continue until you reach the center of the room. Then work from the opposite end of the joists out to the center.
Step 3: Use a sharp knife with a serrated edge to cut insulating material to fit around pipes and other obstructions. The material is easier to cut when compressed with a scrap piece of board. Trim the insulation to fit around any vents, recessed lighting fixtures, exhaust fan motors, or any heat-producing equipment that protrudes into the attic. Allow 3 inches of clearance. Do not pull on any electrical wiring to move it out of the way.
If you choose to insulate your uninsulated attic with loose fill, first staple the polyethylene between joists to serve as a vapor barrier. To calculate total material needs, measure the square footage of the attic and consult an insulation dealer. The dealer has a chart showing the maximum net coverage per bag at various thicknesses and the R-value for each thickness. The bags in which the loose fill is packaged also supply the same information. Be careful not to cover vents and heat-producing gadgets that stick up in the attic. Strips of insulating batts do a good job of guarding vents, while metal retainers made from tin cans will keep loose-fill insulation away from other problem areas. Pour the insulation into the spaces between the joists. Then spread and level the material with a garden rake. If you want the joists to be covered with the loose fill, work from all sides back toward the attic access hole. Finally, staple a batt of insulation material to the access cover.
You can use batts, blankets, or loose fill to add material to existing but inadequate insulation. Follow the same installation procedure outlined for an uninsulated attic but lay down unfaced batts or blankets instead of the versions with a vapor barrier attached. If you add loose fill, do not apply a vapor barrier over the existing insulation.
Hand-pack insulation around pipes and wires that come up through the floor of the attic, closing the holes around these elements.
If you decide to heat your attic, remove the existing insulation from between the joists before installing the finished attic floor. Add insulation under the eaves as necessary. Remember that insulation should only go between heated and unheated areas, and keep in mind that the vapor barrier always faces the heated area.
Staple insulation blankets between knee-wall studs and rafters before you cover them with paneling or wallboard. You might choose to use loose fill in sloping sections of the ceiling.
After the attic, the next most important places to insulate are the exterior walls. The process may be slightly more involved, but we give you simple step-by-step instructions in our next section.