You cannot use R30 insulation properly in a standard 2x4 wall and get R30 performance, and you should not try. R30 fiberglass batts are designed for a cavity that is about 9.5 inches deep, which corresponds to a 2x10 floor joist or an attic. A 2x4 wall cavity is only 3.5 inches deep. If you force an R30 batt into a 3.5-inch wall, you will compress it by nearly two-thirds. When fiberglass is heavily compressed, it loses a large portion of its thermal resistance because you squeeze out the air pockets that actually do the insulating. A compressed R30 batt in a 2x4 wall will only perform at about R12 to R14, which is no better than a standard R13 batt that costs half as much and fits correctly. Additionally, the overstuffed batt will push against drywall, causing bulges, making it hard to install electrical outlets, and creating voids around wires where air can leak. Building codes require insulation to be installed in full contact with the cavity without compression. If you have 2x4 walls and want higher than R13, the correct products are R15 high-density fiberglass batts made specifically for 2x4 walls, or closed-cell spray foam which delivers R21 to R24 in a 3.5-inch cavity, or adding 1 to 2 inches of continuous rigid foam on the outside of the sheathing.
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