How to Winterize a Window AC Unit (Remove or Cover It Right)
Winterize a window AC unit before the first freeze: remove it and seal the window, or seal it in place with foam and a breathable cover. Under $15, under an hour.
Quick answer: The right move is to remove the window AC unit before temperatures drop below 32 degrees, seal the window with weatherstripping, and store the unit upright in a dry location. If removal is not practical, seal around the unit with closed-cell foam tape and cover the exterior with a breathable fabric cover. Either way, a bare unsealed unit in the window all winter costs money in drafts and heating bills, and sometimes damages the unit itself.
Window AC units are a warm-weather appliance. When nights start dropping into the 40s and you know October is close, there’s a short window to deal with yours correctly. Most people do one of three things: pull it out the right way, leave it in covered and sealed, or leave it in with nothing done. That third option bleeds conditioned air out and cold air in all winter. A small drafty window can add $50-100 to a heating season, depending on your climate and how the unit was installed.
Here’s how to do it right either way, without overthinking it.
What You’ll Need
If removing the unit:
- A second person (most window units weigh 40-90 lbs, and the weight is rear-heavy)
- Foam weatherstripping tape or a window insulation film kit ($8-12)
- Rigid foam board cut to fit the bracket slot, if there is one
- Plastic sheeting or an old blanket for covering the stored unit
- Scrap lumber or a pallet to keep the stored unit off concrete
If leaving it in place:
- Closed-cell foam weatherstripping tape (Frost King or 3M, $4-6 per roll)
- Window AC insulation kit or foam pieces cut to fit the accordion side panels ($8-12)
- Outdoor AC cover made of breathable fabric (Sturdy Covers or Classic Accessories, $12-20)
- Weatherproof tape for sealing foam edges
Step 1: Clean the Unit Before You Store or Seal It
Pull off the front panel and remove the air filter. Rinse it under warm water with a drop of dish soap, let it dry fully, and reinstall it. The mesh-type filters common on most window units clean up easily; a 10-minute job.
While the panel is off, shine a flashlight at the evaporator coil, the finned metal surface at the back of the unit. A heavy dust layer or any visible mold is worth treating before winter. A can of coil cleaner ($10) with a rinse and full dry-out handles it. Skipping this step means whatever is growing on the coil has five months in a sealed space to get worse.
Drain any water sitting in the unit by tilting it slightly toward the outside, if your mounting allows. Most window units drain continuously during operation, but some hold water in the tilt pan.
Step 2: Decide: Remove It or Leave It In
The default call is removal. The Department of Energy notes that window units installed in casement or slider windows may be harder to remove, but in standard double-hung windows, removal is the cleaner call. Frigidaire and LG both specify in their owner manuals that units should be removed or adequately covered when ambient temperatures drop below freezing, because ice forming in the drain pan and around the coil fins can crack the pan and damage the coil.
Remove the unit when:
- You have a second person available
- The unit is in a standard double-hung window
- You have a dry, indoor storage spot
- The unit is under 75 lbs
Leave it in when:
- The unit is mounted in a casement or slider window that requires disassembly
- You’re above the third floor with no balcony or easy access
- The unit exceeds 80 lbs and no help is available
If you’re leaving it in, the sealing work in Steps 5 and 6 is what matters. An uncovered, unsealed unit in the window is roughly equivalent to leaving a small vent hole open all winter.
Step 3: Remove the Unit Safely
Unplug the unit and turn the thermostat all the way down. Give it 15-20 minutes for residual moisture in the drain pan to settle before moving it.
Window units are rear-heavy because the compressor lives in the back half. Tilting the unit toward the outside, not inside, during the pull keeps the weight manageable. Lift straight up to disengage the mounting bracket, then slide the unit inward. The second person supports the back weight while you clear the window sash.
Set it down level. Remove the accordion side panels (they either pop out or have a single Phillips screw per side) and keep them with the unit for spring reinstallation. Pull any foam gaskets from the unit surround and decide whether they are still pliable enough to reuse or need replacing.
If your window AC install involved an interior mounting bracket or a support bracket outside, remove and store those too. The bracket hardware is easier to track if you tape it to the unit in a labeled bag.
Step 4: Seal the Window Opening After Removal
Close the lower sash fully. Run a strip of foam weatherstripping tape along the bottom rail where the sash meets the sill to seal the gap left by the bracket hardware. Press it down firmly with your thumb.
If there is a visible slot or gap where the unit housing sat, cut a piece of rigid foam board insulation to fit that space and press it in from the inside before closing the sash. Even a thin piece of foam in the gap drops infiltration significantly.
For older windows with loose sashes, a window insulation film kit covers the full lower sash in a layer of plastic film that you shrink tight with a hair dryer. These kits run $8-12 and the seal they create on a drafty window is noticeable. They peel off cleanly in spring.
Step 5: Seal Around the Unit If You’re Leaving It In
Most window AC units use accordion-style plastic panels to fill the gap between the unit and the window frame. Those panels are thin, uninsulated, and flexible. They let through more cold air than most people realize, especially after a few seasons of use.
Seal each accordion panel by pressing closed-cell foam tape along the inside perimeter where it contacts the window frame and the window sash. Frost King’s rope caulk works well for irregular gaps and uneven surfaces. Press it in with your thumb; it stays put all winter and peels off cleanly in spring without leaving residue.
Then inspect the unit housing itself. If there is any gap between the unit casing and the window sash, fill it with foam tape. A $4-6 roll of 3/8-inch closed-cell foam tape covers the typical unit perimeter.
Aim to eliminate drafts, not to vapor-seal the unit completely. Some air movement around the exterior is fine and preferable to trapping moisture against the coil.
Step 6: Cover the Exterior
Cover the exterior face of the unit with a purpose-made window AC cover. These are made from canvas or polyester with a water-resistant coating and are breathable enough to let moisture escape while keeping rain, snow, and debris off the unit. Frost King and Sturdy Covers both make models that fit standard window units from 24 to 42 inches wide. Measure the exterior width of the unit with the accordion panels extended before ordering.
Do not use a thin plastic tarp pulled tight against the coil fins. Plastic traps moisture against the metal surfaces, accelerates rust on the drain pan and coil, and creates exactly the mold environment you are trying to prevent. A breathable cover protects the unit without turning it into a terrarium.
Secure the cover with the built-in tie straps or add a bungee cord around the unit to prevent wind from pulling it loose mid-January.
The Energy Star program also recommends sealing around window units to reduce air leakage, which is a meaningful efficiency factor even in cooling season. Doing this in fall means the seals are in good shape when you reinstall in spring.
Common Mistakes
Leaving the unit in with nothing done. This is the most common one. Cold air infiltrates around the accordion panels, around the unit housing, and in some units directly through the vents. A small, poorly sealed window opening can cost more in heating loss than the cost of the unit itself over a few winters.
Using a plastic tarp or garbage bag. Moisture buildup under sealed plastic is worse for the unit than leaving it uncovered. The coils rust faster, and any mold in the drain pan will be well-established by spring.
Storing the unit on its side. AC compressors have oil inside, similar to how a car engine does. Storing a unit sideways lets the oil migrate. The standard guidance is to store window units upright and let them stand upright for 24 hours before running them after storage.
Forgetting to replace the foam tape in spring. The foam tape from last fall is usually compressed and less effective by May. New tape takes five minutes to apply when reinstalling, and it matters for both draft sealing in cooling season and energy efficiency.
FAQ
When should I winterize my window AC unit? When nighttime temperatures consistently drop below 40 degrees in your area, or before the first hard freeze. Mid-September to mid-October covers most of the continental US. The unit can stay in place a few extra weeks if weather stays mild, but do not wait until the night before a freeze.
Does leaving a window AC in over winter actually damage it? Sometimes. Ice forming in the drain pan and around the coil fins can crack the pan and deform the fins. Frigidaire and LG both specify in their product manuals that units should be removed or covered in climates with freezing temperatures. The damage is not guaranteed every winter, but it’s common enough that both companies exclude freeze damage from warranty coverage on units left in place uncovered.
Can I just use an interior foam panel instead of removing the unit? Yes. Window AC insulation kits, the kind sold at hardware stores specifically for this purpose, cover the interior face of the unit with a fitted foam piece that reduces drafts significantly. You still want the accordion panels sealed and an exterior cover on the unit. Interior panels alone do not address moisture against the coil from the outside.
How do I store a window AC unit in a small apartment? Upright in a closet, under a bed if the unit is shallow enough, or along a wall. The main requirements are upright, dry, and not on a bare concrete floor. A piece of scrap wood or cardboard underneath keeps the metal base pan from sitting directly on moisture-wicking concrete. A loose cover of plastic sheeting or an old blanket keeps dust off.
Will running the AC in cold weather damage it? Running a window unit when outdoor temperatures are below 60 degrees can cause the evaporator coil to ice over, which puts strain on the compressor. Most manufacturers specify a minimum outdoor operating temperature of 60-65 degrees. Below that, the coil does not get enough warm airflow to prevent icing.
My window unit is old and leaks around the frame in the rain. Should I still bother covering it? Fix the leak first. Seal the exterior perimeter of the unit with weatherproof foam rope caulk or clear silicone before installing the cover. Water that comes in during a rain event will also come in with winter wind and snow. That repair takes 20 minutes and improves both the winter seal and cooling-season performance.
What size outdoor cover do I need? Measure the exterior width of the unit with the accordion panels at full extension, and the depth from the window frame to the back of the unit casing. Standard window unit covers run from 24 to 42 inches wide. When the measurement falls between sizes, buy the larger size.
Conclusion
Removing the unit and storing it correctly is the cleaner call when removal is practical. If the unit stays in all winter, the combination of foam tape around the accordion panels and a breathable exterior cover keeps the cold out and the unit in workable condition for spring. Either approach costs under $15 in materials and takes less than an hour. The upside is a warmer house through January and a unit that reinstalls in May without needing coil repairs.
For the installation side of this project, setting up and mounting a window AC unit covers bracket positioning and accordion panel fitting for when you bring it back out. If window drafts are a broader issue in your house, sealing windows for better thermal performance addresses the full window perimeter beyond just the AC unit slot. And for anything HVAC-related that comes up once heating season starts, the HVAC troubleshooting guide covers furnaces, heat pumps, and common heating problems step by step.