I need to replace old windows that were installed in the 1980's (double hung wood windows) and 1990's (double hung vinyl). My home was built in 1900. I know that the lower the U-factor is the better to keep heat in but in a home that doesn't have great insulation would it really be worth it to get u-factor .17 instead of u-factor around .28? The windows I have right now are .6 u-factor. I can't get a straight answer from window companies. The cost is a big difference. I live in MA. Help!