Is it okay to put R19 in a 2x4 wall?

It is not okay to put R19 in a standard 2x4 wall if you want it to perform as R19, and it can actually cause problems. R19 fiberglass batts are designed to fill a 6.25-inch cavity, which is the depth of a 2x6 wall. A 2x4 wall cavity is only 3.5 inches deep. If you stuff an R19 batt into a 3.5-inch space, you compress it by almost half. When fiberglass insulation is compressed, it loses much of its insulating value because the air pockets that provide insulation are squeezed out. A compressed R19 in a 2x4 wall will only perform at about R13 to R14, so you are paying more for R19 and getting R13 performance. Worse, the overstuffed batt will bow out drywall, make electrical boxes hard to install, and can create gaps and voids around wires and pipes that cause air leaks. Building inspectors will often flag it as a code violation because insulation must be installed to make full contact without compression. If you have a 2x4 wall and want better than R13, buy R15 high-density batts that are specifically engineered to deliver R15 at 3.5 inches, or use closed-cell spray foam which gives about R21 to R24 in a 2x4 cavity. If you already have R19 batts on hand, you can split them in half lengthwise, but it is messy and not recommended.