Best Sized Insulation for 2x4 and 2x6 Walls

A man looking at construction insulation
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When you are insulating 2x4 and 2x6 framed exterior walls and want to use fiberglass insulation, what thickness should you use? This is a vital question because adding too little insulation means a colder house and adding too much can also mean a colder house.

Does this sound illogical? Not really. Thermodynamics rigorously follows the logic, and this logic says that the air pockets created within the insulation are what help keep a home toasty and warm.

Bottom Line

In most applications, you will use R-13 kraft-faced fiberglass insulation rolls for 2x4 stud walls and R-19 for 2x6 walls. This type of pairing ensures that the insulation is neither too loose nor too tightly packed within the walls.

Insulation TypeThickness of Insulation ItselfAppropriate for Wall Type:
R-133.50"2 x 4 stud walls
R-153.50"2 x 4 stud walls
R-196.25"2 x 6 stud walls
R-215.50"2 x 6 stud walls

When Too Much Becomes Too Little

As noted earlier, insulation works partly by trapping air pockets within the insulation. If you cram too much insulation into a too-thin wall, you reduce the insulation's air pockets.  

This is one of the misconceptions about fiberglass insulation.  Homeowners who have purchased the wrong insulation will often stuff the R-19 insulation into a thin wall rather than return the purchase to the store. This is also another reason why sprayed or rigid foam insulation works well: millions of tiny air pockets are "pre-installed" in the insulation, and they cannot be forced away.

Thus, it is pointless or even counter-productive to put R-19 into a 2x4 wall, for example.

What to Do If Your Walls Are Too Thin

You run into problems if you live in a cold climate where your R-value needs exceed the space you have available in your wall cavity.

As noted above, R-19 insulation only works when it is an appropriately sized wall; that is, one that allows the insulation to expand enough to create air pockets that trap warm air.

Because you cannot rebuild your walls with 2x6 studs, you need to look in other directions:

  • Add thick insulation batts to the attic. Installing attic insulation is one of the most valuable things you can do 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 re-side your house, then add exterior wall sheathing beneath the new siding. Sheathing can help boost your walls as much as an extra R-6.
  • Add storm windows.
  • Replace your windows. Your current windows may have already lost the insulating argon or krypton gas between its panes. Replacement of the entire window is the best way to fix this problem.

Or bite the bullet and take down the interior drywall to re-insulate. Often, you may have insulated walls, but the insulation has become moldy and damp over the years, greatly reducing its effectiveness. Remove and dispose of that old insulation and install new insulation. Do this in conjunction with fixing exterior wall problems that caused the dampness in the first place.