How To Cover a Window Air Conditioner | Winter Prep That Works

Covering a window air conditioner properly requires a breathable fabric cover, foam insulation for side gaps, and secure straps — but the best protection is removing the unit and storing it indoors for winter.

If you live where temperatures drop below freezing, winterizing a window AC isn’t optional. Left uncovered and unsealed, cold air pours in around the unit, and trapped moisture rusts coils, breeds mold, and can wreck the appliance you’ll need next summer. Skip the contractor bags — a non-breathable cover traps condensation and does more harm than good. Here’s the step order that actually protects your window AC, from the fastest fix to the best long-term move.

What You Need to Winterize a Window AC

Gather these materials before you start so the job flows in one go. The critical pair is a breathable cover plus closed-cell foam for the gaps — neither works alone.

• Breathable fabric cover (polyester mesh or a specialized breathable material) that fits your unit’s height, width, and depth
• 1-inch insulating foam covered in white flexible vinyl, cut to fit the side panels
• Foam weather stripping or cut-to-length foam sheets for the gap between upper and lower sashes
• Duct tape to secure foam around exposed pipes
• Construction adhesive that won’t peel house paint (only if you’re building a custom styrofoam box)
• Velcro straps, drawstrings, or buckles to hold the cover against wind

Step-by-Step: How to Cover a Window AC for Winter

This sequence seals the gaps that leak cold air and moisture, then wraps the unit in a layer that breathes. Work through it in this order, not backwards.

1. Turn off and unplug the unit. Press any breaker for the window line if it has one.
2. Clean thoroughly. Rinse the outer coils with a high-pressure hose, wash the accessible surfaces with warm soapy water, rinse again with plain water, and let everything dry completely. A wet unit under a cover grows mold in days.
3. Seal every gap. Cut foam insulation strips to the window’s full width and wedge them between the upper and lower sashes. Surround any exposed refrigerant or drain pipes with foam sheets and secure them with duct tape. Add foam strips around the unit where it meets the window frame — this is the spot most people miss, and it’s where drafts leak worst.
4. Install the cover. Measure the unit’s height, width, and depth. Place the breathable cover over the unit — don’t wrap it tight. Secure the cover with Velcro straps, drawstrings, or buckles.
5. Check monthly. Remove snow and ice buildup from the cover. Inspect straps and seams for damage. If the cover tears, replace it immediately.

When you install the cover, you’ll see the outer fabric pull into a smooth shape and the straps hold firm against the frame — that’s the success cue that everything is sealed.

Common Mistakes That Ruin a Window AC Cover Job

The mistakes below are the ones that turn a winter cover into a liability. Skip them, and the unit will start next summer like it just left the store.

• Using non-breathable plastic or wrapping too tightly. Trapped moisture causes rust and mold, and kills the unit faster than no cover at all.
• Leaving gaps unsealed. The space between sashes and around pipes is where freezing air and moisture enter. Foam fills those paths.
• Leaving the cover on too long. Remove it as soon as the frost risk ends in spring — a covered unit in warm weather bakes the trapped condensation into damage.
• Working alone. Lifting and fitting a heavy window AC cover or custom foam box is genuinely a two-person job. The cost of a second person is worth the back injury you avoid.
• Skipping the drainage angle.

The Best Protection: Remove and Store Indoors

The most effective winter strategy isn’t a cover at all. If you can’t remove the unit — because of a permanent sill mount or a rental situation — a breathable custom-fit cover with proper foam sealing is the only alternative that won’t backfire. In that case, invest the extra 15 minutes in the foam work; the cover alone does not seal well enough on a frozen night.