Winter Prep for a Window AC Unit You're Leaving in the Wall
Not every window AC can come down before the cold sets in. Maybe it’s wedged into a wall sleeve, maybe the unit weighs 70 pounds and your window is on the second floor, maybe you just don’t have anywhere to store it. Whatever the reason, leaving it in place for winter is perfectly fine, as long as you seal it up, insulate it, and give it some basic protection. Here’s exactly how to winterize a window AC so it survives the cold months dry, draft-free, and ready to fire back up next June.
Why You Shouldn’t Just Leave It As-Is
A window unit was built to cool a room, not to act as a weatherproof seal against January wind. Ignore it all winter and you’re looking at a handful of problems:
- Cold drafts sneaking in through the gaps around the housing and window frame
- Higher heating bills, since all that warm air you’re paying for just leaks right out
- Condensation and moisture buildup, which eventually turns into mold or rust
- Pests (mice, ants, the occasional spider) squeezing through gaps you didn’t even know existed
- Frozen or cracked parts if water gets into the coils or housing and then freezes
Twenty to thirty minutes of prep now can save you a repair bill, or a January heating spike, later.
Step 1: Clean the Unit First
Don’t seal in a layer of summer grime. Before anything else:
- Pull the front filter and rinse it or vacuum it out
- Wipe down the grille and vents where dust tends to collect
- Check the outside housing for leaves, spider webs, or anything that looks like a nest (squirrels love these things)
- Let it dry completely before you cover it. Trapping moisture inside is honestly worse than leaving the unit exposed to the weather
Step 2: Seal Gaps Around the Unit
This is the step that actually matters most, both for your comfort and your utility bill. Even a quarter-inch gap can let in air that feels like it’s straight off a ski slope.
- Foam weatherstripping: Run it along the interior and exterior edges where the unit meets the window frame.
- Rope caulk or removable caulk: Go with a peelable, temporary caulk on the exterior seams. You’ll thank yourself in April when it’s time to pull it off.
- Foam insulation panels: Hardware stores sell pre-cut “AC blocker” kits shaped to fit around window units. They cut drafts dramatically, and they’re cheap, usually under $15.
- The accordion side panels: Those plastic or vinyl extensions on either side of the unit are notorious leak points. Foam strips or heavy-duty tape along the seams fix this fast.
Step 3: Cover the Exterior
Rain, snow, ice, UV rays, an outdoor cover handles all of it.
- Pick a breathable, water-resistant cover. Skip solid plastic; it traps condensation underneath and that’s basically a rust factory.
- Strap or tie it down tight. One good windstorm will rip off anything that isn’t secured.
- Make sure the cover hangs a few inches past the bottom of the unit so wind-driven rain and snow can’t sneak up underneath.
Step 4: Add an Interior Cover or Insulated Panel
If you live somewhere genuinely cold, don’t stop at the outside.
- A quilted or insulated interior cover that fits snugly over the front of the unit makes a real difference.
- No cover handy? Tape clear plastic sheeting, the same stuff sold in window insulation kits, over the indoor side to create a dead-air barrier.
- This one step alone can eliminate a lot of that “why is this corner of the room always cold” feeling.
Step 5: Turn Off and Unplug
- Flip the unit fully off at its own control panel. Don’t just nudge the thermostat dial and call it done.
- Pull the plug. No need to risk a phantom power draw or, worse, a fire hazard from a unit sitting idle for four months.
- If the outlet itself sits in a drafty spot, an outlet cover is worth the two dollars.
Common Mistakes to Avoid
- Wrapping it in solid plastic outdoors: Moisture gets trapped against the metal housing and rust shows up faster than you’d expect.
- Skipping the indoor seal: Most of the heat loss actually happens on the inside-facing gaps, not the exterior ones. People forget this constantly.
- Not cleaning first: Seal in dust or dampness and you might open it up in spring to find mold.
- Leaving it plugged in all winter: “Off” doesn’t mean zero power draw, and there’s no good reason to take the risk.
When Spring Comes
Peel off the caulk and tape before you touch anything else. Then pull the covers, check the coils for dust or rust, and only after that, plug it back in. Clean the filter one more time and run it on a mild day, just to confirm everything still works, before the first real heat wave shows up and you need it.
Final Thoughts
A window AC sitting in the wall all winter doesn’t have to mean cold drafts or a ruined unit come summer. Clean it, seal it properly, throw on a decent outdoor cover, and unplug it. Do that and the thing will still be humming along fine next July, while your heating bill stays reasonable and your living room stays actually warm in the meantime.