Best Insulation for 2x4 and 2x6 Exterior Walls

The type of insulation and its thickness are the most critical factors when insulating exterior walls. Adding too little or too much insulation means wasted energy.

Learn the best insulation for exterior walls, whether 2x4 or 2x6 wall systems.

How to Measure Wall Thickness

Measure the thickness of exterior wall studs with a drill, stud finder, and something thin to probe with, like a kitchen skewer.

1. Turn off circuit breakers to wiring running through the wall.
2. Locate and mark two wall studs with a stud finder.
3. Drill a 1/8-inch into the drywall between the studs.
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.
7. Remove the probe.
8. Measure the distance. If it's 4 inches, you have 2x4 walls. If it's 6 inches, you have 2x6 walls.
9. Cover the hole with spackle.

Types of Exterior Insulation

Choose fiberglass, mineral wool, or foam insulation for your exterior walls:

• Fiberglass insulation: Fiberglass insulation is made from thin glass fibers. It's the most commonly used exterior insulation since it is inexpensive and energy-efficient.
• Mineral wool insulation: Mineral wool insulation is spun from recycled minerals like basalt. Unlike fiberglass insulation, it is not itchy or scratchy to the skin.
• Foam insulation: Spray foam insulation seals all areas of the cavity, including walls, floors, and ceilings, as well as structural members, holes, cracks, and seams. Spray foam cannot lose its shape except under extreme pressure and will not sag or settle over time.

Insulation For 2x4 Walls

Use R-13 or R-15 kraft-faced fiberglass or mineral wool insulation rolls for 2x4 stud 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.

Older homes may use 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.

Insulation For 2x6 Walls

Use R-19 or R-21 kraft-faced fiberglass or mineral wool insulation for exterior walls built with 2x6 studs,

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.

Other Ways to Insulate

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 the 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. The best solution is to replace the entire window.