Exterior 'enclosed porch room' repair

Hi all!
I need advice on how to rebuild 2 exterior walls of an add on room to my house. The room is technically just an enclosed portion of a porch. Due to poor contruction, the walls are severely water damaged. The 'windows' appear to have been the problem. They were just plexiglass (unopenable) panels afixed with pretty wood molding to a shelf starting aout 3ft up from the floor and lining the 2 exterior walls.
Condensation slipped down and rotted the poor quality lumber (apparently they didnt use good stuff) and this allowed water to pour into the lower wall structure. The drywall rotted out and I pulled back the insulation to see misfitted 'blue board' (the styrofoamish like stuff they put over exterior walls before siding them). There is no true 'wall' (aka plywood) between the inner side and that waterproofing material. Obviously a bad construction job but we didnt know this when we bought the house.
Professional assessment is 38,000$ or more but it makes true walls and brings the room up to full specs as a bedroom code (rases floor and all sorts of things). This is beyond us financially. The good news is the studs were good quality and are fine as is the roof/ceiling. The porch it is off of is slab and the slab is fine with no cracks though raised about 1/2 inch above the porch level. The two 'interior walls' (one the old external of the house, the other a false wall off portion of the remaning porch) are fine except mildewed.
Our skill level is 'medium' with most things. Putting up drywall is easy although we might use simple panelling wood for the interior so we can get at the wall behind it easier to check it out in the future. Putting the insulation in is easy. We'd get an electrician to professionally rewire the area.
Advice on how to handle adding the missing exterior wood then the 'waterproofing layer' is what we need. We'll have our friend contractor help with removing then putting up the vinyl siding again. I think we just get the plywood-exterior-lumber and nail it to the existing studs and reinforce those studs wth an extra set along side each while they are easily gotten at. Reinforce a section with a 3rd set to support a horizonal piece that can then load-bear a window or 2. Then, cut out the plywood over where the 'window' will go and install simple ones. Get electrician to do their thing, then insulate and add interior paneling over it all. If we cant figure out hw to get a window in right, we can skip it for now and later go back to add it if we put the horizonal pieces in at the time when all is 'open' still. I'd have a room with no windows, but has an exterior door to the porch. It would do for now.
Oh, how did it get that bad? I'm Navy and was stationed overseas in Japan for 6.5 years while the place was rented out. Just got back 10 days ago. Carol
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This estimate for constructing a "bedroom to code" may be helpful in making up your mind what you really want. Do you need another bedroom (insulated, with electricity etc.) or would a screened porch meet your needs? This depends on where you live, temperatures and duration of summer and winter etc.
If you want a rule of thumb, it is that you must remove all water- damaged material (because any you leave behind is likely to infect what it touches, i.e. sabotage your reconstruction. Rule #2 is that you must decide whether (2a) you want to keep the weather out: in which case you need siding and caulk to do so, or (2b) you do not mind occasional damp, OK if the porch is so built that rainwater runs off fast and the structure dries out thoroughly (like my own winterized porch.) But you must first decide whether you want a genuine "indoors" room or a conveniently roofed outdoor space.
--
Don Phillipson
Carlsbad Springs
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Thanks for the reply!
What we have is what used to be a roughly 40ft wide, 11ft deep enclosed porch all along the back of the house (wall up 3ft then screened). About 10ft of it at the outer corner was later enclosed replacing the screen with plexiglass. The condensation on the plexiglass caused it to rot down the lintel in the enclosure. The rest of the remaining open porch is just fine.
I don't need an extra bedroom. My use for the space was a summer playroom and some extra bookshelves. In winter, we just closed the door and ignored the room except the things stored in it.

Yes, we take that as a given.

Ths seems what we need.

The porch does get wet in heavy rain but dries out fine. The enclosed part is raised about 1/2 inch higher and water never reaches that high or even near that corner due to the mild grading of the porch slab (designed to drain to the other side of the house, a level shows a very mild consistant grade).
I suspect we need to find out more about building codes and building permits before we can start any actual exterior work.
xxcarol
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