I appreciate the conflict associated with having a million dollar view and low performing windows. The good news is you can have a great view and high performance windows. Window technology has improved dramatically in the past few years.
Even though I seldom recommend replacing existing windows I do suggest that if your windows need to be replaced for reasons other than energy efficiency that you select the highest performing window your budget will allow.
Putting in smaller windows may be less beneficial than you might imagine, providing you select really superior windows.
When considering replacement windows you need to look at two values: u-value, and solar heat gain coefficient.
U-value should be based on the overall window assembly, not center of glass. The lower the u-value the more energy efficient. An Energy Star window in climate zone 5 would have a U 0.35 (good but not great).
I would recommend installing a window at or below U 0.27.
- Basically U-value is the reciprocal of R-value.
- A window with a U-value of 0.33 would have an R-value of 3. A U-value of 0.25 is an R-4.
- Windows in this range actually perform as well as a 2x4 wall stud.
Solar Heat-Gain Coefficient
SHGC refers to the window's ability to keep solar gain in the home down. Again, the lower the SHGC the better the window performs.
If your windows face direct sun you might consider adding a heat reflective coating to further reduce solar heat gain. If your windows do not get direct sun (north or solar shaded) the SHGC is unimportant.
Over the years I have audited thousands of homes for energy efficiency. During that time I have become acquainted with two companies offering high performance windows that I think stand out against the rest.
- Strassburger (http://www.strassburger.net/windows/) and
- Serious Windows (http://www.seriouswindows.com/).
Both companies could provide you with a replacement window that would improve thermal performance and maintain the view that you love.
Enjoy the view,
For more information:
Read "Can you recommend suppliers of low U-value solid wood windows?" a Q&A answered by Florian Speier.