The door in question is between a fair sized, uninsulated entry area (about 10 x 10) which has the "real" door to the exterior (steel with a stained glass insert). The door that is being replaced leads from this area into my living room, so from an uninsulated to a living area. The current door is a nice wood stain (probably really wood) with an oval medallion made of glass. The glass medallion, which I have never liked, has been deteriorating for years and is now nearly ready to fall apart.

I priced getting the medallion replaced - $500. For the same cost, I might be able to have a new door installed. However, the current door is 35 x 79.5", not the standard 36 x 80".

I understand fiberglass offers the best insulation, and if I can do it fast enough, there is a small tax credit. However, can fiberglass be cut down by a whole inch?

Exterior wood doors are substantially more than fiberglass doors I have seen. However, they are not insulated. If I am going to get a wood door, how much difference in insulating value would there be between an exterior and an interior wood door, each with a glass insert?

A salvage place does have reasonable priced steel exterior doors with the frame. He says that if I buy the frame as well, it won't matter that the current door is slightly smaller than the standard. I understand the insulating value of steel is between fiberglass and wood - at least that's what Home Depot tells me about their doors. The salvage place just has the doors, with no technical info about them ( no warranties or anything).

The Home Depot guys seem very confident - but that doesn't always mean they know what they are talking about. The salvage guy had to ask someone else. So I thought I'd get some other advice. Thanks!

That's dependent on the rough opening. If it is framed for a 3'0" x 6'8" standard, then it should be no problem. If the existing door was cut down to 35" x 79.5" because of a smaller rough opening, then it's a little more involved re-framing the opening to accept a standard 3/0

Basically, rough openings for doors are 2" wider and 2" to 2&1/2" higher than the door itself.

To measure the rough opening, simply remove the interior trim and measure the width and height of the opening(stud to stud and floor to header(top stud).

If that opening is 37" wide, you will probably not be able to properly install a 36" door. If the RO is 37&1.2" wide, there is a good possibility a 36" door can be installed.

The reason for the wider RO is so shimming to get the door plumb and square can be done.

The height is less important. As long as the height of the RO is at least 3/4" higher than the door, there should be no problems.

Doors are actually simple to install. However, doing the install is very specific. If not done exactly as needed, the door will not work as designed. Having space between the building framing and the door framing is imperative to allow for those adjustments.