I agree, when you do a pocket window and leave the old frame and trim, you are relying on the fact the old installation was well insulated, which is unlikely.
If the window are 20 years old. Replacing them is probably a good thing. Be sure to get a quality Low E glazing for the glass. If you're going through the hassle of changing the window spend the extra for the higher efficient window, it's well worth it.
You have already gotten the two best answers. In fact I was going to suggest both of them. Adding the beams to reduce the span maybe the easiest solution, it most likely will give you the strongest floor also. If you can cut the span if half again you would end up with very little deflection on...
You need to use a sill pan under the door and make sure the rest of the installation is correct. You need drip cap on the top of the door also. Don't fix the floor until you fix the problem
I think trying to replace a new door in an old frame is one of the hardest DIY jobs. Even experienced builders can find it difficult. I would save my money and invest in a pre-hung with the sidelight already attached to the door. This replacement would also be easier and much more energy...
Here is another good link http://www.ezhangdoor.com/installing-house-wrap It's not just flashing you need to worry about, there is more to an installation than that. For doors, I would recommend sill protection instead of just caulking. This will channel out any water that makes its way in. I...
Here are some good tips and things to look for http://www.ezhangdoor.com/determine-energy-efficient-windows-and-doors Also as a few have mentioned, a good installation is a must, otherwise your wasting time and money.
Good point. Often times H clips are not required per code on 1/2" or thicker sheathing. That creates a problem when you don't get that proper spacing they create.