I am just starting to learn more about windows before replacing ours at the same time as our siding.  As a starting point, I need to determine whether we should get replacement windows or new construction windows.  The house is 18 years old and, going by the doors we already replaced, there may be "some" fiberglass between the house framing and the current window frames.  Opinions?

Also, I'm thinking of having an energy audit at some point.  Would a blower door test and infra red camera be helpful in deciding between these two types of windows? 

I really need some guidance here.    Thanks in advance for your help!

the biggest difference is that new construction windows can't really be installed without doing some damage to the exterior cladding, due to the nailing fins. If you're replacing siding at the same time as the windows, I'd go with new construction windows. Replacement windows CAN be more prone to leaks, depending on the installation.

Geome, now it the time to do it all and do it correctly.
Use fin type new construction windows since the siding is off anyway. Make sure they use window pans. Either copper or plastic ones are fine. All windows leak a bit eventually, pans keep it from causing damage. Make sure they don’t completely calk the bottom fin of the window so any water that does get in can drain
Think out carefully what glass to use to maximize beneficial solar energy and minimize summer gain. Basically use low solar gain windows east and west, high solar gain in the south side if you have reasonable overhang shading.
Use casement windows if possible. If you can’t afford all casements, use casements in the kitchen and bathroom where ventilation is more important A casement has twice the openable area as a double hung.
Now is also the time to consider adding foam outsulation. Because you are redoing the windows and siding there is very little labor penalty to do it.

Cheers,
Eric

geome;

replacement windows.................. but get your energy audit now, that way you will know where the most important needs are and some utility companies will assist financially with the improvements