Recommended Thickness of Insulation for Walls

The thickness of the insulation that you will use to insulate your walls will depend on the wall cavity’s depth. In this article, we will review what size insulation for:

  •        2×4 walls
  •        2×6 walls
  •        2×8 walls

Fiberglass Insulation for Your Walls

Fiberglass is made of fine glass fibers and is a standard method of insulation in most homes. Fiberglass comes in batts, rolls, loose-fill, or rigid boards to suit different needs.

What Size Insulation for 2×4 Walls

The size, thickness, and materials used to make insulation determine how well it resists heat and cold. Simply put, how effective insulation depends on its R-value. Most homes have the 2×4 wall studs, especially the houses built long ago. The depth of the wall cavities on 2×4 wall studs is about 3 and a half inches.

The best thickness of fiberglass insulation for the 2×4 is either R-13 or R-15. Even though these two fiberglass types have different ratings, either can work perfectly on 2×4 wall studs. The insulation should have a thickness of 3 ½ inches. It’s essential to follow the thickness requirements because putting too much or too little could affect the insulation’s effectiveness.

What Size Insulation for 2×6 Walls

Some modern homes are built with 2×6 wall studs. For these, you want to make sure that you use the R-19 faced fiberglass insulation. The R-19 fiberglass insulation should have a thickness of 6 ¼ inches.

What Size Insulation for 2×8 Walls

Some newer homes have the 2×8 wall studs that require insulation with R-21 faced fiberglass. The thickness of the insulation should be about 5 ½ inches.

Spray Foam Insulation for Thinner Walls 

What Size Insulation for 2×4 Walls

Closed cell spray foam is the best for using in between wall cavities because it fills all gaps preventing air and moisture infiltration. When using closed cell foam, you want to make sure you only fill about 3 inches out of the 3 ½ inches depth of the wall cavity. The R-value after installation of closed cell spray foam should be about 19 inches. 

If you’re limited on finances and you decide to use open cell spray foam, you want to make sure that you insulate to a thickness of about 3 ½ inches. This translates to an R-value of about 13. 

What Size Insulation for 2×6 Walls

If your house has the 2by6 walls, you need to insulate using the right thickness of insulation material to achieve maximum benefits. If you’re using closed cell foam, the recommended thickness is about 5 inches which translates to an R-value of 32.5 inches. Each inch of closed cell spray foam has an R-value of 6.5 to 7 inches. 

Open cell spray foam on the other hand, you want to make sure that the thickness of the insulation for each wall stud is 5.5 inches. Since each inch of the insulation has an R-value of about 3.7, this gives a total R-value of 20.3 inches. 

What Type of Insulation to Use for Thinner Walls 

When it comes to spray foam, we have the open cell and open cell spray foam. The insulation is packaged into two cans of chemical isocyanate and polyol resin. Usually, you’ll find the cans labelled as A and B. 

Spray foam is the best form of insulation for thinner walls. For the 2 by 8 walls, the stud cavities aren’t deep enough to hold the right amount of insulation. 

What Happens when You Over Insulate 

Fiberglass works by forming air pockets in between the insulation. These air pockets are what makes fiberglass an effective insulation. If you cram too much fiberglass into the walls, it affects its ability to offer thermal resistance. The reasoning behind this is that, when you squeeze too much fiberglass in a wall cavity, it seals the air pockets that facilitate thermal resistance. 

Spray foam on the other has pre installed air spaces that trap air making it an efficient method of insulation. Unlike fiberglass, if you use too much spray foam, it doesn’t affect thermal resistance. In any case, spray foam if used in excess will only lead to wastage of money and the insulation. 


The thickness of the insulation you use depends on the type of walls you have. Older homes have the 2by4 and the 2by6 walls. The modern homes have 2by8 walls that have thinner wall cavities. For wall insulation, you can either use fiberglass or spray foam insulation depending on your preference and budget. The above guide details how thick you should insulate your walls depending on your insulation of choice.