Green Building Library

Insulating Framed Walls With Fiberglass Batts

Insulation’s ability to slow heat is called its R-value. The higher the number, the better the insulation works to keep heat inside in winter and outside in summer.

Batts are carefully manufactured to have a specific insulating value. To keeps its rated R-value, a batt must not be compressed or crushed during installation. For example, a standard R-19 batt actually needs 6 1/4 in. to achieve its full insulating value. When that batt is installed in a 2x6 cavity, which is actually 5 1/2-in. deep, its R-value drops to less than R-18. Higher density batts are rated to deliver R-21 in 5 1/2 in.

Depending on the project, framing may be spaced 16 in. on center or 24 in. on center. Select batts to fit this framing “module.” For 2x4 framing, batts are cut 15 in. wide. You can choose from R-11, R-13, and R-15. For 2x6 framing, the batts are cut either 15 in. or 23 in. wide with R-value choices of 19 or 21. The batts are made about 1/2 in. wider than the space between studs so they’ll fit properly.

Batts should completely fill the stud cavity, fitting snugly against every piece of framing, the sheathing and the drywall. Crushing a batt dramatically reduces its R-value.

Even tiny voids can cause heat loss, because they allow air to move around inside the stud cavity. If a batt is stapled to the sides of the studs, crushed or cut short, air can rise up the warm side of the cavity and fall down the cool side. Heat rides on this convective loop, actually traveling around the insulation.

The drawings below show solutions to the most common batt installation problems.
–Mike O’Brien, O’Brien & Associates

Narrow Spots

narrow battsTo insulate narrow cavities, cut–not crush– the batt to get it to fit.

Batts and Wires

batts and wiresElectrical wires are one major obstacle to a good insulation job, because they run through just about every exterior wall. It’s tempting to shove the batt behind the wire, but that crushes much of the insulation in each cavity (right). There are two easy ways to fit batts around electrical wires.

batts and wiresOption 1
Cut the batt about half way through at the same height as the wire–typically 18 in. above the bottom plate.

batts and wiresOption 2
Split the batt by hand and drop it over the wire. Ask that the electrician help out by running the wires about 4 in. above the plate.

Electrical Boxes

batts and electrical boxesCutting is also needed around electrical boxes to prevent compression (right).

batts and electrical boxesFirst, work the batt behind the box. Then cut the batt around the box so that the fiberglass puffs up. When you finish, only the material behind the box is actually compressed.

Vapor retarders

One essential element of the moisture protection system is a vapor retarder. Vapor retarders help protect wall framing from decay caused by condensation. To be considered a vapor retarder, a material must have a “perm rating” of less than one. You have several types of vapor retarders to choose from. Be careful to install the vapor retarder in a way that doesn’t interfere with the wall insulation.

Face stapling

face staplingStaple the flanges of kraft or foil-faced batts to the surface of the stud facing the inside of the house. “Side stapling” compresses the insulation.

No stapling

no staplingSince the new high density batts are more rigid than older batts and won’t settle over time, there is no need to staple the flanges at all. This saves installation time and allows drywallers to use glue on the face of studs.

Plastic sheets

plastic sheetsSome insulators prefer to install unfaced batts and staple up clear 4- or 6-mil polyethylene sheets. Staple the plastic to the top and bottom plates. At seams, overlap the sheets three in. and staple both sheets to a stud.

If batts don’t fit

fitting battsSome places are just too tight for batt insulation to do a good job. Use foam boards to insulate behind electrical panels, bathroom cabinets and heating ducts located in outside walls.


paintVapor retarder paint is a good way to avoid conflicts with insulation. Unfaced batts are installed and covered with drywall. Select a primer that has a perm rating of less than one. About a dozen specially made vapor retarder paints are now available through paint suppliers.

This article appeared in Energy Source Builder Vol. III No. 1, February 1991,
©Copyright 2008 Iris Communications, Inc.