What are the differences?
Recently I was asked what is the difference of replacements windows verses replacing a window both in cost and other factors?
Well I'll give you my best on this. First of all, like a stated previously in my journal about replacement windows, as they do not remove your window frame siding or trim generally. They were design as retro fits to existing windows where cost savings could be created by utilizing good existing frames and upgrading just the sash components to the newer better technologies of window manufacturing.
A replacement of an entire window, replacing a window, requires gutting the opening of all window components down to the house frame and sheathing. In most cases this will require careful consideration on window sizing if you want to limit the impact to the siding, however most situations will impact the siding some.
I have on occasion been able to size a window to fit within the exterior trim and siding so that the 908 or other molding fits exactly the same as it did and align the interior molding and trim almost near perfect on the interior, but that depends on circumstances and window manufacturing methods that result in a positive combination of interior and exterior measurements and in some case good fortune. If care is taken on these measurements and the manufacturers design matches well with the old window frame, yes a window can be totally replaced and not disturb the siding or affect the interior paint or wall paper.
A complete window replacing, is a few hours longer to complete and the adding of exterior and interior trim, as unlike window replacements you are utilizing nothing from the old window. The good advantage here is you will be upgrading the way the window is attached to the house with a complete nail fin which results in a superior seal around the windows contact points to your house. Most older windows are weakly sealed at the sill with no overlapping to the house at that point but a small groove where the siding may or may not be integrated.
The window measurements, the trim size and the distance the siding is from the windows components are all a crucial part of the ordering of the window size that will best fit to that opening affecting each variable. If you were to just measure the rough opening and get what the manufacture deducted to make that fit, you may be disappointed at how much work you have created for yourself and be forced to remove siding to attach the nailing fin properly. A good craftsman tries to avoid unnecassary work, by accurately measuring and planning, thus saving you money if he can help it.
Three things are to be considered I think when deciding whether you want Replacement windows or to replace the windows completely.
Cost, with all factor's equal, replacement windows are less expensive and less of an impact to your home with just a small amount of trim removed and then replaced when inserting them. The same high quality window can be bought in either design from most manufacturers, so there is no lose of value, or advantage to be gained at the sash. Dollar for dollar replacement windows are the best value if all other factors are secured.
Sight-lines, with replacement windows you lose a little on your opening sizes because they are a frame within your existing frame. In other word the amount of glass area may be slightly reduced depending on how much space is lost to the window structure compared to what you had previously. It isn't usually much, just about 1/2 inch all around in some cases.
Need, If your window frames are not worth saving or would take more than $100 or more to repair you may be at the breaking point of were replacing the entire frame and window is better than patching and installing replacements. This has to be weighed against the whole house project. If all your frames are good and only one is bad, then replacements are still a valued choice with the expense of only one window being a loss in the overall project aim.
I think I would add one more consideration that only weighs in if you are suspicious of how your windows are attached to your house and that is; Energy and weather resistance. A new window, that replaces all your trim both inside and out can be guided to a better mechanical seal than perhaps the original intentions of the builder because of todays better materials and more energy conscious designs that start at where the window is attached to the house. Although in most case a replacement window equals a modern design a complete window replacing can insure the most modern attachment to your houses frame and sheathing insuring the best upgrading to your investment. It will add to the cost to replace windows over replacement windows, but that decision has to take in consideration the three factors I mentioned and which is the best application of those factors to your situation.
As always, develop a relationship with a trusted professional who can help guide you to the best solution for your budget and desires.