Insulation Guides for 2x4 and 2x6In terms of wall cavity thickness, proper insulation for 2x4 walls tends to fall in the R-13 or R-15 range. For larger-framed wall cavities built with 2" x 6" studs, you can fit in R-19 or R-21 fiberglass batt insulation.
It's deceptive, because you can physically fit R-19 batt insulation into a 2x4 wall cavity. The insulation will bow out a bit, but the drywall can easily push it back in. So, why not do this?
Insulation works partly by trapping air pockets within the insulation. If you cram too-thick insulation into a too-thin wall, you reduce the insulation's air pockets. Thus, it's pointless or even counter-productive to put R-19 into a 2x4 wall.
What to Do If Your Walls Are Too ThinYou run into problems if you live in a cold climate where your R-value need exceeds the space you have available in your wall cavity. You can't exactly rebuild your walls with 2x6 studs. The good thing--if you can call it good--is that you lose tremendous amounts of energy through two other places: the attic and through openings.
You can add thick insulation to the attic and you can seal up door and window openings.
In addition, if you plan to re-side your house, you can add exterior wall sheathing beneath the new siding. Sheathing can help boost your walls as much as an extra R-6.
None of these improvements are dependent on the thickness of the wall cavities.