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Windows that have a .28 u factor or. Slightly lower, if installed correctly sealing the perimeter with foam are generally adequate for most homes. They will have low -e and argon normally in the csvity between the insulated glass. Now if you want to go for triple glazed glass and low e and Krypton glass, you can get down to .17 u. Generally the additional cost and added weight do not pay for the added expense. You may do well to also add insulation in the attic and make sure you ventilate the soffit and fascia. Heat tends to travel up. Make sure you get a good installer that know s how to insulate and do a quality install.
Like Jim said before, a proper attic insulation will help with lowering your electric bill as well as heat reflective roof shingles and exterior paint, ex. tex-cote. For windows we use Anlin, 100% american made using the newest heat reflective technology. GreenLiving specializes in exterior energy efficient home improvements!
There are many great door & window product lines to choose from. The most important consideration is the glass package and style. I recommend consulting with a few glass companies and door/window contractors to help with the decision. You need to choose your doors and windows based on location - cliamate zone. Marvin is top of the line, but it is extremely expensive, just like Renewal by Andersen. I recommend looking at Milgard, Jeld-Win and Andersen (not Renewal). Good Luck!
Murray Lampert Design, Build, Remodel
Pamela, there are a lot of window replacement options on the market. It's important to have a window which is designed for your region in the U.S. here is a chart by energy star which contains this info: https://www.energystar.gov/products/building_products/residential_windows_doors_and_skylights/key_product_criteria
If one is in your area, see if a local business manufactures windows tailored for your portion of the U.S. Avoid builder grade replacements, and try to work with companies which have been in business for 30+ years who can stand behind the warranties they provide. Always read the fine print, and make sure a remodeler will warranty the installation.
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