A. Let me start by saying that I have not had such bad luck with double-pane windows. Yes, they do occasionally fail, but the advantages far outweigh any risk. If your luck continues to hold its course, rest assured that you will not need to remove the entire window frame to make the repair. All window glass is stopped in (no more glazer's putty needed in a modern window). In order to replace a faulty pane, just remove the stops and the bad pane, clean off any caulk or adhesive that was used to seal the glass into its spot, and install a new double pane unit. (You know, the one that you measured for and ordered from your favorite glass or hardware store before you took out the bad one.)

But let’s explore your question about removing a window without having to cut back the siding. One possibility is to trim out the window before installing the siding. My favorite way to do this is to start by attaching a false sill just under the window. I cut this from a clear cedar 2” x 3”. I run it through my table saw to cut a bevel of about 12 to 15 degrees on one edge. This piece can then be nailed up under the window with the beveled edge against the house, and extending past each side of the window an amount equal to the width of the trim boards that you will use on the sides.

Next, cut the side trim pieces with a bevel on the bottom to match the angle you cut on the sill. These should be cut to a length that will bring the tops in line with the top of the window. The final piece looks best if you use a thicker board than on the sides. For example, use 1” x 4” material on the sides and 5/4” x 6” on the top. Be sure to finish the top with some “Z-metal” flashing, and caulk between the trim boards and the window. Now you can run your siding up to the trim, and if you feel the need to remove the window, you only have to remove the trim to get at the nail fin.

If you use this technique, you may want to add some additional blocking inside the sheathing, since the ends of the siding you install will only be attached to plywood. The framing around the window opening will most likely not catch the siding, because it will end about the same place as the trim boards. I have done it without additional blocking though, and it seems to work ok.

Alternatively, you could trim the window more simply with a “picture frame” style. This is frequently done on tract housing where the builder is trying to give the feeling of trim without the expense of the method I described above. I’m not too thrilled with the way it looks, but on a barn it might be ok.

One last thing, that is a little nit-picky, but something I would definitely recommend. Take some scrap lumber and cut some long strips the same thickness as the nail fin on the windows. Don’t forget to include enough to make up for the nail heads that are holding the window in place. When you nail up the trim, just slip these under the edge of the trim that is away from the window, so that the face of the trim boards is flat (parallel with the exterior of the structure). Don’t forget to caulk between the siding and the trim.