You could be eligible for a tax credit of up to $1500 for residential, or up to $1.80 per square foot, for commercial. Don't let the cost of installing a retrofit product keep you from reducing energy costs. Funding is available through legislation such as the American Recovery and Reinvestment Act.

Visit the links listed for the most updated information. 

Alliance to Save Energy www.ase.org 
National Association of State Energy Offices www.naseo.org 
National Association of Energy Service Companies www.naesco.org 
Energy Information Administration www.eia.gov 
Energy Loan www.energyloan.com 
National Fenestration Rating Council www.nfrc.org 
US Department of Energy, www.doe.gov 
Energy Starwww.energystar.gov/taxcredits 
Green Building Certification Institute, www.gbci.org 
Efficient Buildingswww.efficientbuildings.org 
Database of State Incentives for Renewables & Efficiency www.dsireusa.org