I am not sure if you qualify, but the USDA gives interest free loans and EVEN SOME GRANTS (don't have to pay them back) for home improvements that make your home more energy efficient.
My family does exterior home improvements like this through the USDA regularly. they will replace your windows with new vinyl replacement windows that are energy star qualified. they put new roofs and insulation and siding on some houses, too.
This is the number for the TN office, but he should be able to tell you who to call in your area.