You can, but...
Here’s the thing. Old windows do not meet energy code. When you install a window, whether it’s in a new window opening or an existing one, that new window must be compliant with the minimum insulation values dictated by the current edition of the energy code. Your old windows likely do not meet the requirements of the energy code since the energy code is updated every three years to require windows to be more energy efficient.
Additionally old windows must be removed in order to be reused. The process of removing the window is really not that hard. A laborer could remove a window in 15 to 30 minutes. This assumes the laborer does not need to take any precaution to try not to damage the window during removal. If the laborer is supposed to maintain the quality of the window during its demolition, the process of removing that window will simply take longer. The laborer will need to ensure the window is more surgically removed to prevent any scratches, breaks, cracks, or any other defect from occurring during a demolition process that is typically not a very eloquent process. Once the window is removed, it would also need to be rehabilitated. This means the laborer would have to spend time scraping caulking and other adhesives off of the window frame to ensure it is clean and ready for re-installation later. The laborer would also need to make any repairs to the window to fix any inadvertent damage that may have happened during the removal. This prepping and repair will certainly take a good amount of time. I would estimate at least around four hours of time would be spent just to get the window ready for re-installation. Once the window is ready for re-installation, then it needs to have a strategy to make installation possible. New windows have more efficient hardware known as flanges that enable them to be installed more easily than older windows. Since older windows do not have a nail flange, they must be installed with a different method that involves shimming and caulking that also takes more time and results in a less watertight outcome. The installation of a new window, with its nail flange, is a quick and easy process. It usually takes half the time of an install of an old window. It also gives you a more watertight seal. With all things considered, it takes much longer to deal with the removal, prep, and installation of old windows when trying to reuse them. New windows simply take less time to install and result in a better outcome. And they actually look like new windows because they are new windows. The cost of purchasing the new window is typically easily offset by all the additional costs that it takes to attempt to reinstall old windows. Using new windows is a much better deal typically.
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