Is R15 worth the extra money over R13?

Whether R15 is worth the extra money over R13 depends on your climate zone, energy costs, and whether you are building new or retrofitting. R13 is the standard fiberglass batt designed for 2x4 walls and typically costs about $0.50 to $0.70 per square foot. R15 is a high-density batt that fits in the same 3.5-inch 2x4 cavity but uses more densely packed fiberglass to achieve about 15% higher thermal resistance. That extra R2 may not sound like much, but over the lifetime of a home it adds up. In a 1,500 square foot home in climate zones 4-6, upgrading from R13 to R15 can save roughly 4 to 7 percent on wall-related heating and cooling loss, which often translates to $20 to $40 per year in energy savings. The upgrade cost is usually only $100 to $300 more for an entire house when buying batts. If you are building new construction and the walls are open, R15 is absolutely worth it because the labor cost is identical and you get a better performing wall for a small premium. If you are doing a small retrofit and already have R13, the payback is much longer and may not be worth tearing out good insulation just to gain R2. For energy codes, many states now require R15 or R13 plus continuous foam in 2x4 walls, so R15 helps you meet code without adding exterior sheathing.