Best Insulation for 2x4 and 2x6 Walls

When insulating exterior walls, what type of insulation and insulation thickness should you use? Adding too little insulation means wasted energy. Adding too much insulation—stuffing in more than is needed—can also waste energy.

The right type of insulation to use for your home's exterior walls comes down to the thickness of the walls.

• Walls built with 2x4s: Use R-13 or R-15 kraft-faced fiberglass insulation or mineral wool insulation when the house's exterior walls are built with 2x4s.
• Walls built with 2x6s: Use R-19 or R-21 kraft-faced fiberglass insulation or mineral wool insulation when the house's exterior walls are built with 2x6s.

How to Know the Thickness of Your Home's Walls

Measuring the size of exterior wall studs by looking at the edge of a window or door can be difficult. But it's easy to check if you have a drill, a stud finder, and something thin to probe with, like a kitchen skewer.

1. Turn off circuit breakers to wiring running through the wall.
2. Find two wall studs with a stud finder.
3. Drill a 1/8-inch into the drywall.
4. Stop after you have drilled about 1/2-inch.
5. Insert the probe until it stops.
6. Mark the probe where it touches the wall. Remove it.
7. Measure the distance. If it's 4 inches, you have 2x4 walls. If it's 6 inches, you have 2x6 walls.
8. Cover the hole with spackle.

Insulation For 2x4 Walls

Wall assemblies, especially those in older homes, are built with two-by-four (2x4) studs. The wall thickness will be 3-1/2 inches, not 4 inches. The width of modern 2x4s is 3-1/2 inches.

Use R-13 or R-15 kraft-faced fiberglass insulation or mineral wool insulation rolls for 2x4 stud walls. While rated differently, these two types of insulation are close enough in thickness that they can both fit in these wall systems.

Older homes, especially those predating the 1950s, may employ two-by-fours with a true measurement of 2 inches by 4 inches. In this case, use R-13 or R-15 fiberglass or mineral wool insulation. There is no 4-inch thick-faced fiberglass insulation in batts or rolls on the common market.

Insulation For 2x6 Walls

In homes that have exterior walls built with 2x6 studs, use R-19 or R-21 kraft-faced fiberglass insulation or mineral wool insulation.

This combination ensures that the insulation is neither too loose nor too tightly packed within the walls.

How Insulation Works

Fiberglass and mineral wool insulation works partly by trapping air pockets within the insulation. If you cram too much insulation into a wall that is too thin, you reduce the insulation's air pockets and thus reduce its ability to provide thermal resistance.

A thick down-filled jacket or sleeping bag works the same way. When the feathers fluff up and create air pockets, thermal resistance is at its greatest. Bags or jackets that are wet or have been rolled up for a long time do not retain body warmth because there are fewer and smaller air pockets.

Tiny air pockets created within the insulation are what help keep a home toasty and warm, not the actual strands of fiberglass, mineral wool, or paper facing.

Striking a perfect balance between too little insulation and too much insulation will keep you and your family warm throughout the winter or cool in the summer.

Other Insulation Types

Insulating the exterior walls is just a start. Use these other materials and methods to keep your home weathertight:

• Install thick insulation batts in the attic. Batts are long strips of unrolled and unfaced fiberglass insulation. Installing attic insulation is one of the most valuable ways to save energy and keep your home warmer.
• Add blown-in cellulose wall insulation. Cellulose insulation does not fit the wall cavities as adequately as fiberglass or mineral wool insulation, but it's a good option if the walls cannot be opened up.
• Seal up door and window cracks with caulk. Cold air seeping into your home has a detrimental effect on your home's heat envelope.
• Install exterior wall sheathing beneath new siding. Sheathing can help boost your walls by as much as an extra R-6 level.
• Add storm windows at the beginning of each cold weather season to the front of your existing windows.
• Replace your windows. Your current windows may have already lost the insulating gas between their panes. Replacement of the entire window is the best way to fix this problem.