Attic Insulation 5 - Attic Preparation

A background of pink, fiberglass insulation.

In part 4, you found out which tools you need to install attic insulation. Part 5 deals with how to prepare your attic for the new insulation.

The attic is probably the greatest single heat loser in the home. The lighter, heated air rises while the heavier, cool air, drops due to convection. That's why your feet are usually cold or why your much-taller friend says it's too hot when you want to turn up the heat.

Adequate insulation in attics is imperative. Newer homes are generally well-insulated, although it never hurts to check, many older homes have little or no insulation. Oftentimes what there is has deteriorated or compressed beyond even minimal worth.

Types of Insulation

The most widely used insulation in the attic is fiberglass. Fiberglass commonly comes in 3 1/2 inch- to 6 1/2 inch thick rolls. It can be applied in double layers to increase it past 6 inches. It is available in widths of 16 inches and 24 inches to fit between center framing members spaced at either interval. It comes in two forms: batt and blanket. Batt insulation is available in 4 foot or 8-foot lengths. Blanket insulation is available in lengths from 30 feet to 70 feet in standard manufacturer's thicknesses.

Types of Fiberglass Insulation

Fiberglass is available in foil-backed, paper-backed, and unfaced batts and blan

Fiberglass is available in foil-backed, paper-backed, and unfaced 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 3/4-inch air space. Unfaced fiberglass is used in conditions of potential fire hazards and as the top layer of a two-layer application. Otherwise, paper-backed is used.

Note: the rest of this series assumes you are installing blankets, as as opposed to any other form of fiberglass insulation.


You need to do some work prior to actually installing your fiberglass insulation. The first thing to do is decide how you want to use your attic space.

When you go up to the attic, have with you a pencil and paper and a trouble light with an extension cord. Carefully examine your attic space. Determine whether you will be using the space for living and therefore want to heat it, or whether you prefer to insulate the main part of the house below the attic. Locate your insulation in such a way that it encloses the heated areas only.

Preparation of Rafters

If you wish to finish and heat the attic space, look closely at the rafters, checking them for depth and uniformity. They must be deep enough to house the amount of batting needed for your area. Besides giving you enough room for the insulation, 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 for fiberglass batting, for an attached vapor barrier, or to support a finished wall of paneling or gypsum board. Attaching scabbed-on boards to the existing rafters will extend them and increase their depth.

Scabbing On Boards

If you are finishing the wall, you need to be sure these new boards, attached to the rafters, all come out to exactly the same level because you need to attach your finishing material to an even surface. To do this, nail up a set of reference strings on two scab boards, one on each end of the attic roof. Stretch the strings across the face of the rafters to show yourself the proper depth to align the scabbed-on 2x4 or 2x6 boards.

Jot down the required lengths of the boards. 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 them firmly into the rafter, using 16d nails every 16 inches. Continue this process until the entire rafter system is scabbed out to accommodate the insulation. Be sure all scab boards come right to the reference strings.

You may think you're all set to insulate your attic, now. You'd be wrong, though, because you need to make sure your attic is adequately ventilated first. That's what part 6 is all about.