What is the most cost-effective energy improvement?

Air sealing and attic insulation consistently offer the best return on investment. Sealing gaps and cracks with caulk and weatherstripping costs very little. Adding attic insulation to recommended levels costs $1,500 to $3,000 and can save 15 to 30 percent on heating and cooling costs. LED bulbs are the fastest payback, often recouping their cost in under six months. Both improvements pay for themselves quickly.