Fiberglass Insulation in the Attic 1 - Preparation

A worker cutting glass-wool insulation with a utility knife.
  • 1 hours
  • Beginner
  • 3-5
What You'll Need
Flexi-vent strips

This is the first of a 3-part series on fiberglass attic insulation. Before installing your insulation, you must adequately prepare. Here is everything you need to do to lay the groundwork for your new insulation, starting with the different kinds of insulation available.

Sizes and Types

Fiberglass insulation commonly comes in 3.5-inch and 6-inch thick rolls. It can be applied in double layers to increase it past 6 inches. It is available in widths to fit between 16 inches or 24 inches on center framing members. It comes in two forms: batt and blanket. Batt insulation is available in 4-foot or 8-foot lengths. Blanket insulation is usually 56 feet long when 3.5 inches thick and 32 feet long when 6 inches thick. Blanket insulation is the better choice, as there are fewer gaps between pieces.

Vapor Barriers

Fiberglass is available in foil-backed, paper-backed, and un-faced batts and blankets. Both the foil and paper act as vapor barriers. The foil, however, is only of value when used in conjunction with a 0.75-inch air space. Un-faced fiberglass is used in conditions of potential fire hazards and as the top layer of a two-layer application.

Now that you know the different kinds of fiberglass insulation and vapor barriers, you can prepare your attic for this installation.

Look Around

Have a pencil and paper and work light with an extension cord before going up to your attic. Carefully examine your attic space before you begin.

Check Your Rafters

Look closely at the rafters, checking them for depth and uniformity. They must be adequate to house the depth of batting needed for your area. Furring out the rafters to the proper depth will also give you a point of attachment for the vapor barrier and a structure capable of supporting finishing material like drywall or paneling.

Some rafter systems are not deep enough to accept fiberglass batting or even a vapor barrier. Attaching scabbed-on boards to the existing rafters will fir them out to an even depth.

Add 2x4s

Nail up a set of reference strings on the rafters that protrude the most into the attic. Stretch the strings across the face of the rafters to show you the proper depth to align the scabbed on 2x4 boards.

Jot down the required lengths of each board. Measure and mark the boards with a steel tape measure and cut them to length with a circular saw. Cut all the boards at one time. Make the cuts in the attic, if possible, to avoid unnecessary trips up and down the stairs.

Position each board against the rafter and align it with the string. Nail it firmly into the existing rafter. Use 6d nails every 16 inches. Continue this process until the entire rafter system is scabbed out to accommodate the insulation.

Attach Moisture Barrier

Complete your preparation by attaching Flexi-vent material (a waffle-like strip of plastic) designed to allow air circulation and carry away moisture that would otherwise build up between the roof and the new insulation. Butt the strips right up next to each other and against the roof itself.

OK, now that you've prepared your attic for the new insulation, it's time to actually install it. For that, check out Fiberglass Insulation in the Attic 2 - Installation.