What Is the Best Insulation for 2x4 and 2x6 Walls?

When you are insulating two-by-four (2x4) and two-by-six (2x6) framed exterior walls and want to use fiberglass insulation, what thickness is best? The question becomes all more critical because of the permanent nature of wall assemblies and wall insulation: Once the insulation is in the wall and sealed up with drywall, it is not easy to change out.

When you add too little insulation, you will have a colder house. But adding too much insulation—packing in more than is needed—can also result in a colder house than is necessary. Tiny air pockets created within the insulation are what help keep a home toasty and warm, not the actual strands of fiberglass 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.

R-value is a standard unit of measurement for determining, among many things, how effective your insulation will be. The R refers to absolute thermal resistance. Higher R-value numbers mean that the insulating material resists the cold or heat from the outside better. Thickness, density, and type of materials are some factors that contribute to R-value.

Insulation for 2x4 Walls

Most wall assemblies, especially those in older homes, are built with two-by-four (2x4) studs. Since modern two-by-fours are not 4 inches, the true depth of the wall cavity is 3 1/2 inches.

In most wall applications, you will use R-13 or R-15 kraft-faced fiberglass insulation rolls for these two-by-four stud walls. While rated differently, these two types of insulation are close enough in thickness that they can both fit into modern two-by-four wall systems.

Older homes, especially those pre-dating the 1950s, may employ two-by-fours that truly are 2 inches by 4 inches. In this case, use R-13 or R-15 fiberglass insulation. There is no 4-inch thick faced fiberglass insulation in batts or rolls on the common market.

Insulation for 2x6 Walls

Some newer homes may have walls built with 2x6 studs. Use R-19 or R-21 kraft-faced fiberglass insulation for two-by-six (2x6) walls. This combination ensures that the insulation is neither too loose nor too tightly packed within the walls.

Fiberglass 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.

This is one reason why sprayed or rigid foam insulation works well. Millions of tiny air pockets are basically pre-installed in the insulation and they cannot be forced away.

It can be difficult to keep your house warm when you live in a cold climate where your R-value needs exceed the space you have available in your wall cavity. R-19 insulation only works when it is installed in an appropriately-sized wall: one that allows the insulation to expand enough to create air pockets that trap warm air.

Blown-in cellulose wall insulation is generally considered to be a less effective way of insulating walls when compared to rolled fiberglass insulation installed between wall studs. Cellulose insulation does not fit the wall cavities as adequately as fiberglass insulation does.

Short of rebuilding your walls to thicker dimensions—a cumbersome, expensive project—look at alternative ways of preventing your costly artificial heat from escaping:

• - Add thick insulation batts to 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.
• - Seal up door and window cracks with caulk. Cold air seeping into your home has a detrimental effect on your home's heat envelope.
• - If you plan to install new siding on your house, add 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. Replacement of the entire window is the best way to fix this problem.