How to Seal a Window Air Conditioner for Winter
Seal a window air conditioner for winter with weatherstripping and caulk if it stays installed, insulation if you remove it, or a temporary draft plug if you need a short-term fix. Poor sealing leaves cold-air leaks, lets moisture collect, and wastes heating dollars. This guide shows how to choose, install, and remove the right seal for your setup.
This guide is part of our Winter Weatherproofing: How to Draft-Proof Your Home series.
What you’ll need
- Foam insulation with a compressible fit, cut to the width of the fixed gap
- Adhesive-backed weatherstripping for sash and jamb gaps
- Paintable latex or silicone caulk for fixed interior joints only
- Window insulation film if you need a temporary indoor seal
- Interior draft stopper for the sill or lower sash
- Exterior window AC cover sized to the unit shell
- Utility knife or scissors
- Measuring tape
- Rag and mild cleaner for dust and condensation
- Flashlight for checking light leaks around the frame
Window AC decision checklist
| Leave installed | Remove and store | Cover temporarily |
|---|---|---|
| Cold draft is moderate, the unit is reachable, and the room still needs a quick indoor seal. Best materials: weatherstripping, cut-to-fit foam, caulk on fixed joints, and a sill draft stopper. | Spring use is far off, the bracket is solid, and you can remove the unit without strain. Best materials: storage wrap, the original hardware, and foam or film for the empty opening after removal. | The unit must stay in place for access, rental rules, or a hard-to-reach window, and the main goal is exterior protection. Best materials: an exterior cover sized to the shell, plus interior sealing if gaps remain. |
| Look for visible side-panel gaps, a loose sash fit, or air leaking around the bottom support. | Choose this if the unit sags, tilts badly, or the support bracket cannot be trusted through winter. | Use this only when the cover will not trap moisture or block drainage paths. |
| Lowest condensation risk when the indoor side stays dry. | Lowest draft risk if the opening is sealed after removal. | Best for snow and debris protection, not for fixing indoor air leaks by itself. |
How do you seal the gap?
Start with the fixed indoor gap, then seal only the parts that do not move. Use foam insulation or weatherstripping for gaps that compress, and save caulk for stationary interior joints. If the unit still stays in the window, finish with a draft stopper or film where the fit allows it.
Check support and slope first
- Inspect the bottom support, side rails, and sash contact points.
- Confirm the unit still slopes slightly outward so water drains outside.
- Replace any loose support bracket before sealing, because sagging can open fresh gaps and strain the frame.
A window AC can leak cold air through side-panel gaps, a loose sash fit, and a sagging frame. The first fix is structural, not cosmetic. If the unit has shifted, sealant alone will not keep the opening closed for long.
Seal fixed gaps with foam or weatherstripping
- Measure the gap in inches and cut foam to match that width.
- Choose adhesive-backed weatherstripping for jamb or sash areas that compress when the window closes.
- Press the material into place only where the gap stays stationary.
- Add enough thickness to stop light and air, then close the window and recheck the fit.
Use foam insulation for uneven gaps that need a compressible seal. Choose weatherstripping when compression matters, such as along the sash or jamb. Damp surfaces can weaken adhesion, so clean and dry the area first. In cold weather, some adhesives hold better above the manufacturer’s minimum application temperature.
Use caulk only on stationary interior joints
- Apply paintable latex or silicone caulk to interior seams that will not move.
- Skip tracks, hinges, slider paths, and any joint that must open later.
- Smooth the bead so it fills the joint without bridging moving parts.
Caulk belongs on fixed interior gaps only. Do not put it on moving parts or anywhere that needs future movement. If the window sash must shift even a little, choose weatherstripping instead. That keeps the sash usable and avoids a stuck window in spring.
Add a draft stopper or film where it fits
- Place an interior draft stopper at the sill or lower sash if the draft comes from below the unit.
- Use window insulation film when you need a temporary plastic shrink seal over a stable opening.
- Heat-shrink the film after smoothing it to the frame.
Film works best for removable units or partial coverage where you can seal a clean frame around the opening. It fits poorly around active, awkward, or hard-to-remove units because the plastic needs a consistent perimeter to shrink against. A common first-timer mistake is trying to make film do the job of missing hardware. It will not replace a bad support or a warped side panel.
Say your 8,000 BTU window AC sits in a bedroom window with half-inch side gaps and a steady draft at the lower sash. Weatherstripping at the sash, foam at the fixed side panels, and a draft stopper under the sill address the leak without blocking the moving parts.
Should you remove the unit?
Remove the unit when spring use is far off, the opening can be sealed after removal, or the support hardware is weak. That choice often makes sense for renters with permission, for windows that are hard to reach, or when condensation around the unit keeps returning. Storage also reduces the chance of trapped moisture.
Remove and store it safely
- Unplug the unit and let it drain fully.
- Support the weight before loosening the fasteners.
- Lift the unit clear of the frame without twisting it.
- Store the unit upright in a dry place.
If the AC weighs about 76 pounds, plan for controlled handling and a second person if needed. Support brackets matter here because they prevent sagging while the unit is in the window and while you remove it. If the bracket feels loose, fix that before lifting the unit out.
Seal the opening after removal
- Measure the empty opening after the AC is out.
- Fill the opening with cut-to-fit foam or a temporary insulated panel.
- Use caulk only on the fixed interior seams of the new closure.
- Check for daylight at the edges and add foam where needed.
Once the unit is out, the goal changes from draft blocking around the appliance to sealing the full window opening. That is when foam, film, and caulk can work together more cleanly. If the opening is still uneven, start with foam and use caulk only on the stationary trim.
Suppose a renter has a window AC in a bedroom on the third floor and the lease allows removal but not exterior modifications. In that case, removing the unit and sealing the opening indoors is usually cleaner than adding a cover outside that may not fit the building rules.
Can you cover it outside?
Yes, but only as exterior protection for the unit shell. A window AC cover can block snow and debris, yet it should not trap indoor moisture or cover a poorly draining unit. If the inside still leaks air, a cover will not solve that by itself.
Match the cover to the unit dimensions
- Measure the exposed shell, not the whole window opening.
- Choose a cover that fits over the unit dimensions without crushing the side panels.
- Secure it so wind cannot pull it loose.
Some window ACs cool and dry air, then send it back into the room while pushing heat outside. Popular Mechanics notes that some window ACs also include dehumidifier modes. That matters in winter because trapped moisture can linger inside the cabinet if the cover fits poorly or seals too tightly.
Keep indoor moisture from getting trapped
- Dry the unit and nearby frame before covering it.
- Leave drainage paths and vents clear if the design needs them.
- Do not use an exterior cover as a substitute for indoor draft sealing.
Use an exterior cover only when the unit is staying in the window and the outside face needs protection from snow or debris. If condensation has been a problem, a closed cover can make it worse by holding moisture against the cabinet. The safest approach is an exterior cover plus an indoor seal at the frame.
Common mistakes
These are the failure points that cause most winter problems around a window AC.
- Using the wrong sealant on moving parts: Do not caulk tracks, sash edges that move, or any surface that must open later. Use weatherstripping instead.
- Blocking moisture with a bad cover: A cover that traps indoor moisture can create condensation inside the unit shell.
- Skipping support checks: If the bracket is weak or the unit sags, gaps reopen and removal becomes unsafe.
- Applying materials in the wrong temperature range: Adhesives and caulk can fail on cold, damp surfaces.
- Using foam where compression is needed: Weatherstripping works better at sash and jamb contacts that compress when closed.
Frequently asked questions
How to seal a window air conditioner for the winter
Start with the leak source, not the material. Check side panels, the lower sash, and the support below the unit, then choose foam, weatherstripping, or caulk based on whether the gap moves. A clean, dry surface gives adhesives the best chance to hold.
Should you cover a window air conditioner for winter?
Cover it only if the unit remains outside in the window and needs snow or debris protection. Use a cover sized to the unit shell, and do not rely on it to stop indoor drafts. If moisture has been a problem, inspect for condensation before covering.
Should you remove a window air conditioner in winter?
Remove it when you do not plan to use it until spring and the window can be sealed cleanly afterward. Removal also makes sense if the unit sags, the bracket is weak, or the window is hard to reach. Store it upright in a dry place after drainage.
What is the best way to stop cold air around a window AC?
Use weatherstripping where compression matters, foam for fixed side gaps, and caulk only on stationary interior joints. If the draft comes from the sill, add a removable draft stopper. If the unit is badly supported, fix the bracket first or the gap will return.
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