The idea is similar to the cat doors you can install in a sliding glass door, but this method won't obstruct your back door and its much cheaper. Instead of using the back door, we will install the cat door in a first floor window. I must insist that you don't try to put a cat door in a second story or higher window.....
This set up is wonderful for people who are renting, people who are less than excited at the idea of tearing up a wall, and people who simply want something temporary to be able to change it out for the different seasons.
Step 1: Measuring and Cutting
First, you'll have to pop out the window screen to get a proper measurement, the screen needs to come out for this to work anyway. Measure the height of the window and the width at the widest the window will open. The panel doesn't need to be too wide, since you can close the window to the width you need.
*Make sure you measure the length of the window a little short so you will be able to lift it above any lip on the window frame you may have.*
Now that you have the window dimensions down, we just need to map out the cat door and glass panel on the board. After cutting the board down to the right length I simply centered the cat door and glass where it looked like it would work, nothing too precise. The cat door had a template for the size to cut the hole for the door. I traced around the pane of glass and then I measured 3/4'' in from the edge of those markings to create a lip for the glass to rest on.
Now you get to cut!
Using a drill, drill two holes (or until you can fit the jigsaw blade in). Drop in the jigsaw blade and follow the lines you made. Easy!
Step 2: Painting!
You'll notice that my pictures will appear a bit out of order because of this...
Step 3: Installing the Door and Window
The glass pane took a bit more creativity.
First I laid down a bead of caulking to help seal the window and add a little surface tension to hold it in place. The real part that will secure the window is the moulding cap. As you can see in the pictures, the cap has a lip that allows the glass to fit in the gap and be held in place while the moulding can be tightened down without putting pressure on the glass.
Measure out the lengths of moulding along the glass and use a miter box and cutting 45degree angles (be sure to compensate for the extra length needed in cutting at an angle). Assemble the pieces around the pane and make sure they fit together.
Since I was working so close to glass and the moulding is pretty thin, I pre-drilled holes before I started hammering in the nails. This avoids split wood and stops you from needing to hit the nails as hard. Just be sure to drill holes smaller than the width of the nail.
I ended up being able to hammer the nails in fairly forcefully without cracking the glass....but that could have just been luck :)
Step 4: Install in the Window
I popped the board into the window and pinned it tight by closing the window up against the board. I then filled in all of the gaps with some weather stripping (there will probably be a significant gap at the top as the board needs to be a bit short). I did two layers of the stripping and I have had no problems with air leaking through. For extra safety and security, wedge some boards to the far edge of the window (opposite the cat door) to prevent it from being opened any further. This will add stability and deter potential intruders.
Tah dah! The cats will enjoy their new independence and you can rest easy knowing you can simply uninstall it from the window whenever you need. Enjoy!
*not pictured: I do have a step on the other side of the cat door for them to walk out onto so they don't just fall to the ground. *