If the framing is that badly charred, you'd be better served replacing the framing. Hiding the damage will only lead to other problems in the future. Since you have to do the window anyway, I'd do it all now. The nailing flange is located on the outside, usually hidden by some exterior trim or siding. Depending on the kind of siding you have, and the trim detail, it shouldn't be too hard to use a new construction window. But I would be concerned about leaving badly charred framing to support my home. You should have the wall inspected by a professional to determine the extent of damage before doing anything else.

Hi everyone, Thanks for the responses. It's been more than four months since my original query so I thought I'd post an update. After talking with several other contractors and vendors, we decided that going with replacement windows was clearly the wrong choice, so we switched contractors to someone with more large-scale remodeling experience and went with new construction windows. In the end we chose Anderson 400s and used Azek trim on the outside. The old windows came out and we found some damage to the underlying sheathing and framing, but not too much. The new windows were installed and insulated properly and the difference is huge. While we were at it we also replaced most of the edge and corner trim and drip caps on the house as this was rotting off as well. The project is taking a lot longer (we're hopefully wrapping up this week) and is costing a lot more than we thought, but I'm glad we caught this when we did as the damage would have been a lot worse if we had waited. Interestingly, the cost of the new construction windows was actually less than the replacement windows. The labor was more, of course, but we were already being forced to replace the rotting exterior trim so that was all equal. We did have to replace the interior trim, which was not inexpensive, but in the end the project wound up costing the same because the windows were less expensive. The difference in the new trim is amazing- it's nice to have real window sills and stools, with a less generic casing around them. The question of new windows vs. restoration was never an issue- the old windows (only 18 years old) were of poor quality, poorly installed, and leaked cold air to the point that we had to install plastic sheeting over the openings in the winter. Also, many of the window sills, mullions, and jams were rotting, so the amount of work to restore the existing windows would have been huge, We were going with new windows one way or the other. Thanks again for your help, Josh

Not being "like for like" does not necessitate new construction. You are dealing with an empty opening whether you are installing into a buck frame or a rough opening. Replacement style is a nice choice for most people. New construction (full tear-out) is a better choice when you have existing water management issues, substantial structural frame damage/rot, or you are very concerned about the potential loss of glass area. The pros of replacement are cost ( typically a couple hundred + ), your interior woodwork is not disturbed, and a far less intensive installation. Did I mention that there is a MAJOR cost difference? ;)

Okay, the Construction co, putting our siding and roof on recommended 'their' windows manufacturer (BF Rich) that they use and said it would be better to replace windows now while the siding is being replaced. He said you can still get windows later but suggested that they would be new construction vs replacement windows. Is this a better reason to get them now rather than later? I was under the impression that regardless of the type of window you get, if you don't have a good installer, it won't matter how good of a window it is. How would one know if it's been installed correctly? It may take a year or so to find out, right? Half of my condo has the replacement windows by Vytex. I had them installed 4 mos ago. I need the rest replaced but was going to wait until next year. This company claims they are going to give us a good deal (smile) if we want new windows while they are doing the siding. They use BF Rich and I think their price is around $500 per window for their top of the line for us. Thoughts please. I want to do what's best as far as getting new windows but I don't want to rush into this if there is no really good reason to do it now.