How to Insulate Roof Rafters

If you have roof rafters in your attic that aren’t insulated, you’re throwing money away. Because heat escapes easily through the roof, uninsulated rafters can make your energy bills go up when you turn your heating system on. While attics, especially in older homes, are rarely insulated, you can easily insulate yours with the help of this guide.

Step 1 – Measuring the Space

First, climb into the attic and use measuring tape to determine the square footage of the space. Multiply the length and width of the attic to find this number. Don’t worry about precision; use square footage as an estimate about how much insulation and plastic sheeting you will need to purchase.

Step 2 – Cutting and Fitting the Insulation

Warning: Insulation materials can irritate your skin and airways. Always wear the appropriate safety gear when handling insulation, like a safety mask, clothing that covers your entire body, and gloves. Do not track insulation through the rest of your house.

If you're using blown insulation around your roof rafters, you can skip this step. If you’re using rolls, begin by measuring the distance between the roof rafters in the attic, and cut the rolled insulation, or batts, to these measurements. It is better to have insulation that is cut wider and slightly longer than it is to have them too small.

Cutting the Plastic Sheeting

Roll out the plastic sheeting over the attic floor as well, and trim it so that there is enough to climb up the walls by several inches. It’s okay to have multiple pieces of plastic. Use duct tape to secure.

Step 3 - Installing the Insulation

Using Loose Insulation

If you are using loose or blown insulation, you should follow the instructions on the bag to install it. It’s common for different blown insulation to have different instructions associated with it.

Typically, you will fill the insulation blower with the insulation, and blow it between the attic floor joists. Then, blow another layer on top of the roof rafters. If you have a solid floor covering the roof rafters, use the claw end of a hammer to pull the floor up in several locations, and blow the insulation under the boards. Replace the floorboards, and then spray more insulation on top of it.

Using Rolled Insulation

For rolled insulation, place it between roof rafters. Make sure it is flush against the wood, as even a 1-inch gap can cause a 20 percent heating loss.

Then, feed the rolled insulation under wires and pipe, if either is present. Place the plastic sheeting on top of the rolled insulation. If the plastic sheeting is in pieces, overlap the pieces by 8 inches, and staple it in place directly to the roof rafters and the wall. If there is a floor, you will need to remove it to properly insulate the roof rafters. Do not use plastic sheeting in this case.