Any advice for installing a window unit on a really slanted old sill?

I've dealt with those steep 1920s sills plenty of times, and that outward pitch is meant for drainage, not for supporting a heavy compressor. The easiest way to fix this is to cut a **spacer block** or a shim out of pressure-treated lumber or even a piece of composite decking. You want to cut it to the width of the window opening and place it on the sill so the unit sits level. If you use standard pine, just hit it with some exterior primer and it'll last several seasons before rot even becomes a thougth. Since you have the green light to drill, you shoud screw that wooden spacer directly into the sill so it can't migrate. When you set the AC down, the bottom rail of the unit needs to sit firmly behind the interior stool of the window. If the unit's frame doesn't have a deep enought lip to catch on the inside, screw a small wooden cleat onto your spacer block. This creates a physical stop that makes it impossible for the unit to slide outward even if the pitch is aggressive.

For the top, don't just rely on the window sash weight. Those old weighted windows can be finicky and might creep up over time. You should cut two wooden dowels or 1x1 strips to size and wedge them vertically between the top of the lower window sash and the top of the window frame. This locks the window down onto the AC unit. It prevents the window from being moved and keeps the unit from vibrating out of place. One last thing: make sure you don't make it perfectly level. You acutally want a slight tilt—about a quarter inch—toward the outside so the condensation drains off the back properly. If it's tilted toward the room or even perfectly flat, you'll eventually have a mess on your floor or water damage inside the wall. Use the L-brackets that came with the unit to secure the top of the AC frame to the window sash, and it won't be going anywhere.

I'm relly worried about that 1/4 inch tilt for the drainage. If the sill is alredy slanted so much, how do you even measure that accurately while holding a heavy unit? Also, what do you guys do about the side panels in these old houses? Those flimsy accordion things feel like they're going to let all the cold air out and bugs in. I was thinking about using some extra weather stripping or foam board, but I don't want to make it look like a total mess from the street.

For the tilt, just grab a cheap torpedo level. Set the unit on your shim, and if the bubble is just touching the line on the house-side, you're golden. Don't stress the exact measurement; as long as the back is lower than the front, gravity does the work.

As for the side panels, those accordions are trash. Get a sheet of 1-inch rigid foam insulation and cut it to fit the gaps instead. It blocks heat way better and won't rattle. Tape the edges with foil tape or use foam backer rod to seal the cracks. It’s way more secure against bugs and keeps things much colder.

If you go with the rigid foam for the sides, grab a roll of real HVAC foil tape. It handles the sun and heat way better than standard duct tape and won't leave a gooey mess when you take it down in the fall. It's the best way to seal those tiny gaps where the bugs find thier way in.

Since it’s an old house, the vibration can be pretty loud. I usually put a strip of rubber shelf liner or a cut-up yoga mat between the AC and the wood shim. It realy helps dampen that humming sound that tends to echo through those thin 1920s walls.

Since those 1920s windows ussually have deep pockets where the sash sits, the unit's weight often puts too much pressure on the interior wood stool. I always slide a thin piece of scrap plywood under the front of the AC to distribute that load. It stops the unit from digging a permanent dent into the soft wood over the summer. Also, pick up some rope caulk. It's way better than foam for sealing those irregular gaps where the sash meets the casing. It stays air-tight and peels off clean in October without stripping any old paint.