Efficiency upgrade on old house - questions

I have an old rental property in Statesville NC.
I plan to put in central heat/AC. Trying to decide between propane gas pack and heat pump. The contractor recommended adding insulation and replacement windows.
I plan to get blow in insulation in enclosed walls. Have a bid for cellulose.
And battings and blow in for accessible attic areas.
The real question is windows. I could put in replacement windows at about $214 per window installed. But I don't see them paying off. The U factor is .37.
But the U. factor for a storm window over and old window is .6 according to this site:
http://www.servicemagic.com/article.show.The-U-factor-of-Thermal-Replacement-Windows.8839.html
I might get by replacing all or part of the storm windows. I am not sure how much a storm window will cost, its been hard to find pricing on the web.
I did some calculations and I just cannot see a replacement window saving more than $10 to $15 per year based on those U-factors. Probably less since I assumed a 50 degree F differential for 180 days per year which seems a bit high and I used $20 per million BTU of heating/cooling cost and 12 square foot window.
214 per window, vs unknown storm window cost and installation time.
The rental is about 3 hours drive so I do have to go to some trouble to travel down and install storm windows.
It will cost around $4000 if I go with replacment windows.
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Quality replacement windows will make a HUGE difference in comfort. I had half done this sppring and the house is much cooler than last year and wayyy quieter. I bet the savings will add up. If you dont live there do the cheap thing

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tom wrote:

http://www.servicemagic.com/article.show.The-U-factor-of-Thermal-Replacement-Windows.8839.html
There are many things other than just U factor that can make replacement windows improve energy efficiency and comfort. That, of course, depends upon your old windows.
New windows probably don't leak nearly as much as your old ones. Air leaks (in the winter) often cause a layer of cold air along the floor, which makes people feel cold even if the thermostat is set to something reasonable. New windows are usually "low-e". That is, they have a metal film in between the double panes that reflects infra-red and UV. That keeps the temperature down in the summer, reduces the sunlight fading of furnishings, and reduces the perceived (i.e. not indicated by a thermometer) cold of sitting near a window in the winter.
Compared to prices where I am $214 per window sounds like quite a bargain.
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storm windows add little to home resalve value and might be a negative where new windows will add home value, be more comfy and a nicer home is worth more rent money and likely easier to rent too.
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