Attic Insulation 7 - Insulating Rafters

Hands installing a fiberglass batts in the rafters of an attic.

In part 6 of this series on attic insulation, we dealt with proper attic ventilation and how it goes hand-in-hand with proper insulation. In part 7, you'll learn how to insulate the attic rafters.

Working on your knees in an attic for long periods of time can be very hot and tiring. You will be covered from head to toe in clothing and gear.

Plan for a few breaks in your work schedule and try to work early in the morning, before it gets too hot. It may help to add a spray container of cold water to your toolbox—to spray yourself and your fogged-up goggles. You might even consider taking salt tablets or getting a tetanus booster. Check with your doctor to see if these might be necessary.

Rafter Baffles

You should only insulate your rafters if you have baffles installed to separate the actual rafters themselves from the insulation. Attic baffles, also known as rafter vents, go along the sides of your attic and keep your soffit vents from being blocked by insulation.

Cut Insulation

Once you have these in place, measure the length of the spaces between your rafters. Cut each piece of insulation an inch or two longer than needed, to assure a snug fit. Use your straightedge to compress the insulation before cutting, and use a very sharp utility knife to assure a good, straight cut across the fiberglass batt.

Attach Insulation

Staple it to the rafters or scabbed-on boards with a staple gun so that the paper backing is toward the interior of the room, to act as a vapor barrier. Use 3/8-inch heavy-duty staples every six inches. Attach the insulation to the rafter scab board by the paper flange, so as not to compress the fiberglass. Make sure the paper flange and the staples lie flat against the board to create an even surface for attaching the finished wall material.

A time- and work-saving aid here would be an air compressor with a staple-gun attachment. It can be rented, if you don't have access to one.

Where the spacing of the rafters is uneven, odd-angled, or not standard, you will need to cut the insulation to fit. Cut it 1 inch wider than the necessary width, and tuck the fiberglass in to create a flange for stapling. When installing insulation over wires, pipes, or fire blocking, it will be necessary to back cut the batt to fit over the obstacle, leaving the paper intact and the insulation uncompressed.

Attach Vapor Barrier

Once the insulation is in place between all the rafters, staple up a vapor barrier of 6 ml. visquine (a painter's drop cloth) over the insulation and across the face of the rafters. Although the paper or foil backing of the insulation is a vapor barrier, the added visquine covers completely, with no breaks, making sure no moisture forms in the cavity.

As you staple the vapor barrier to the rafters, draw it as airtight as possible, but be careful not to puncture the plastic unnecessarily as you put it in place. Should this occur, repair it with duct tape.

Your rafters are insulated. Now, there's only one area to go: the floor. Learn how to insulate your attic floor in part 8.