Spent the weekend on my Hisense install and learned a lot

I had a similar nightmare with my Midea U-shaped unit last summer. Those stock accordion panels are basicly useless for keeping out humidity, let alone smoke. I ended up using 1-inch thick rigid insulation from Home Depot and spray-painting it to match the casing so it didn't look like a construction site from the sidewalk.

One thing that relly helped me was focusing on the meeting rail where the top sash hits the bottom window. I found a massive gap there that I didn't see at first. I stuffed some open-cell foam in that 36-inch span and then taped over it with white HVAC tape. It stopped the draft immediatly. I also used a heavy-duty bracket from Ivation becasue my sill was too narrow for the standard feet. Getting that level right makes a huge difference in how the condensation drains, wich prevents that musty smell from creeping in. It’s alot of work up front, but not having to smell the neighbor's grill every night is a huge win for the air quality in the house.

I went a different route with my Frigidaire Gallery because I rent and needed something I could pull off easily in the fall without leaving a sticky mess. I swear by Mortite rope caulk for the tiny edges. It’s basically like gray modeling clay that you press into the seams where the side panels meet the window frame. It handles those awkward corners near the sash lock much better than duct tape or standard weatherstripping where air always seems to whistle through. For that big gap between the two window panes, I skipped the flimsy foam strip entirely. I bought a length of 2-inch diameter foam pipe insulation from the plumbing aisle and jammed it in there. It creates a much tighter seal against bugs and humidity than the flat stuff that comes in the box. I also draped a piece of white vinyl over the outside of the side panels to protect the foam from UV damage, which usually makes those cheap inserts crumble by September. It’s held up through three heatwaves so far without any air leaks.

I found that those black foam pipe insulation tubes from the plumbing aisle are perfect for the gap between the window panes. I just sliced one down the middle and wedged it over the top edge of the lower sash. It creates a much tighter seal than the flat strips they give you. Also, if your unit has a hole where the cord comes out of the side, I plugged mine with a bit of duct seal putty. It stays flexible and stops that tiny whistle of air that carries smells. It's cheap and you can just peel it off when the season ends.

Seen plenty of folks ruin a good sill by sealing it too tight without checking the pitch first. Back in the day, we’d use a bit of "Thumb Gum" — that old non-hardening putty — around the frame seams. If you’re worried about that baby breathing clean air, do the old match test. Light a candle and move it slow around your seals. If the flame flickers, you've still got a bypass. Also, make sure that unit is tilted back just a hair. I’ve seen many a nursery floor ruined becasue the condensation backed up into the house instead of dripping off the back rail like it's supposed to.