Considering replacing this window myself...

This window is leaking buckets of water when we get a good rain. I've did quite a bit of research and it seems that I can certainly tackle replacing this myself.

It appears to be two windows mulled together. Should I just measure the opening, and then subtract to come up with a close or best fit for replacements, or is it better to determine the exact size of the existing windows?

In looking at the existing window, I'm not surprised there's a problem. I was able to just pull back the flashing by hand and see exposed wood. Any thoughts on finishing trim for replacements? Should I just match the existing finishing so it matches that smaller kitchen window?

Also, was this trim supposed to have been silicone caulked or something to prevent the problems I'm having?

Below are some photos:

 

If you've determined that the leaking is caused by no flashing you could properly flash what's there and not have to replace the window. If you want to replace the window think if you want to do it using a replacement type window or new construction. New construction could be the best but entails more work and you would want to measure the rough opening to determine the size.

 

Well, you can replace the windows if you want, but the windows aren't necessarily the problem, it starts with the siding and it's how the windows and trim were installed and how the exterior trim was flashed. (Or not flashed) And whether or not there is housewrap behind the siding plays a role.

The cladding that you are pulling back with your hand is kind of like pearls on a swine because it can cover up rotten wood and look pretty when it might be all rotten underneath. It doesn't always have to be caulked on the outside perimeter but in some cases it probably should be. It should always be caulked to the window itself. The cladding looks like it was done by an amateur.

You didn't say where the water is coming in... I assume it's coming in at the top of the window, puddling on bottom.

Problems like this often start because no one was willing to peel back the siding and ensure the top of the window is flashed correctly, and the WRB is sealed. Sometimes a window directly above on the 2nd story will cause a leak on the window below, because vinyl siding is not waterproof. That's why a WRB is so important behind vinyl. No WRB almost guarantees windows will leak.

IMO you could probably solve this problem by just unzipping the siding (Malco Sideswiper) and replacing exterior cladding (replace the trim, clad it with aluminum trim coil and rent a metal brake to bend it) and a quality sealant, like Vulkem, OSI Quad or OSI Quad Max.

And clean any dirty stains on your windows with a rag and some Soft Scrub.. Lube the vinyl up with silicone spray and make sure all tracks are clean and polished.

 

Yeah I could probably reseal everything, but am not sure if that's the best solution since I am guessing there's quite a bit of water damage already.

I am guessing it's not wise to remove the window to assess/repair damage and re-install the same window? I guess that assumes you can remove it without damaging it.

 

You've already got sheetrock damage so I'd remove that since you'll have to repair it anyhow. Removing it will allow you to inspect the insulation and framing around the window.

 

Those windows are likely mounted to the house with a nailing fin.... 99.9% sure. Remove the cladding. Remove the rotten exterior trim. See whats rotten once you get the exterior trim off. You likely don't need to do anything to the window itself. But you (or I) won't know for sure until you take the trim off.

I'm not suggesting you simply seal things up as it is, so if that is what you were thinking, you misunderstood what I wrote.