A window or through-wall air conditioner left in place for winter is often a hole in the envelope. Cold air moves through the chassis, accordion side panels, and tired sash seals even when the unit is off. Indoor and outdoor AC covers (plus foam side-panel kits) are cheap seasonal tools to cut that draft before you jump to window replacement. Measure the unit and feel for leaks first. Sometimes pulling the AC out beats any cover; otherwise pick an indoor, outdoor or foam cover and fasten it so the wind can’t take it the first night.
What a cover can and cannot do
Covers help when the unit is still in the opening and you can feel cold at the face, side panels, or outdoor chassis on a windy day. The bill math works only if:
- Infiltration through that opening was actually costing heat (single-pane rooms, leaky accordion wings, through-wall sleeves with gaps).
- You match the cover to a unit you will not run under the cover.
- You’re not ignoring bigger envelope holes (attic hatch, garage door, exterior door sweeps).
Climate, rates, how leaky the install was, and whether anyone still cracks the window decide the outcome. A cover on a tight modern casement with a well-sealed sleeve may be comfort theater. A cover on a 1990s double-hung with floppy side panels often makes the room feel different the first cold night.
Check your home before you buy
Do this once before you cart a cover:
- Feel the opening on a windy day or after sunset — back of the hand at the indoor face, accordion wings, sash stops, and outdoor chassis. Note cold glass vs moving air.
- Decide remove vs leave — if you can safely lift, store, and reinstall the unit (and the window closes properly), removal usually beats plastic. If renters, through-wall sleeves, or heavy units make removal unrealistic, plan covers.
- Read the nameplate BTU — outdoor cover sizing often tracks cooling capacity bands (about ≤10k / 10–15k / 15k+). Photograph the sticker.
- Measure the chassis — indoor face width × height for quilted covers; outdoor width × height × depth of protrusion for outdoor bags. Kit “fits up to” numbers are not magic — compare to your tape measure.
- Inspect side panels — accordion plastic vs foam. If wings flex and whistle, a foam side-panel kit may out-earn a cover alone.
- Check mounting and sill support — loose brackets, rotten sills, and missing outdoor braces are safety fixes before weatherization shopping.
- Unplug and dry — covers trap moisture if you leave standing condensate; wipe the chassis and leave the unit powered off for the season.
- Pilot one room — cover the worst bedroom or living-room unit first, live through a cold snap, then kit the rest.
If the villain is a door gap or plain glass two windows over, fix those with weatherstripping or shrink film instead of blaming the AC alone.
What to do first
When covers is worth it
- Window or through-wall units stay installed all heating season and you feel drafts at the face or wings.
- Accordion side panels are floppy — pair foam panels with a seasonal cover.
- Bedrooms or living rooms with long occupied hours — comfort + runtime on the heating system matter more than a guest closet.
- You already did filters, setpoints, and obvious door sweeps and the AC opening is still a cold spot (HVAC filters, checklist).
- Renters who can’t replace windows — covers are reversible and cheap.
What not to do
- Units you still need to run — never operate a window AC under an indoor fabric cover or outdoor bag.
- Egress bedrooms — don’t block the only operable exit; prefer removal or a cover you can peel in seconds, plus thermal curtains.
- Combustion / exhaust paths — never tape over dryer vents, furnace intakes, or fireplace make-up air while “sealing the room.”
- Rotten sills or loose mounts — fix structure and support first; a bag won’t hold a falling chassis.
- Central outdoor condensers — different product and different job (debris shield per manufacturer), not this indoor-draft guide.
- Perfectly sealed modern sleeves with no felt draft — spend the afternoon on attic or door gaps instead.
- Space heaters inches from fabric covers — keep clearances; zone heat belongs on purpose-built gear like oil-filled heaters.
Cover vs remove vs film vs foam panels
| Job | Better tool |
|---|---|
| Unit can be stored and the window closes tightly | Remove and store the AC |
| Draft through accordion side wings (summer or winter) | Foam side-panel kit |
| Draft through the indoor face of a left-in unit | Indoor quilted cover + plastic liner |
| Wind/rain/snow on the outdoor chassis + outdoor infiltration | Outdoor polyethylene cover sized to BTU/depth |
| Plain glass with no AC in the opening | Shrink-film window kits |
| Door / sash edge gaps | Weatherstripping |
| Daily open/close layer over glass | Thermal curtains |
| Summer empty-home runtime | Smart AC controllers — not a winter cover |
Which one should you buy?
- Can you safely remove and store the unit so the window closes?
Yes → remove; add film/curtains only if the empty sash still drafts. No → continue. - Do accordion wings flex and leak when you push on them?
Yes → foam side-panel kit first (year-round seal). Still drafty at the face → add covers. - Is the main complaint cold air at the indoor grille?
Yes → indoor 2-piece quilted cover (measure face; AC9H-class standard vs AC11H-class large). - Is the unit exposed outdoors with wind-driven chill or weather risk?
Yes → outdoor cover sized by BTU + measured depth (AC2H / AC3H / AC5H-class bands). - Through-wall sleeve with gaps around the chassis?
Foam/caulk the sleeve perimeter carefully (see air sealing) plus indoor cover; don’t foam over weep paths the manufacturer requires. - Still unsure after measuring?
Start with one indoor cover on the worst room + fix door sweeps. Expand only after a cold night proves the leak.
Size cheat sheet (verify against your tape measure)
| Situation | Typical pick |
|---|---|
| Smaller indoor face (about up to 18″ × 25″ class) | Indoor standard quilted (AC9H-class) |
| Larger indoor face (about up to 20″ × 28″ class) | Indoor large quilted (AC11H-class) |
| Outdoor, up to ~10,000 BTU / shallower chassis | Outdoor AC2H-class |
| Outdoor, ~10,000–15,000 BTU | Outdoor AC3H-class |
| Outdoor, ~15,000 BTU and up / deeper chassis | Outdoor AC5H-class |
| Floppy accordion wings | Foam side-panel kit (trim to fit) |
When in doubt, measure twice — oversized outdoor bags flap in wind; undersized indoor covers won’t seat on the liner.
Our picks
We earn a commission if you buy through these Amazon links, at no extra cost to you. Check size and fit on the listing before you order.
Best indoor starter (standard face)Frost King AC9H quilted 2-piece
Best for: Typical bedroom/living-room window units where the indoor face fits the standard envelope and you want a washable fabric cover plus draft liner.
Watch-outs: Measure the face; elastic fit can be fussy around power cords. Use the plastic liner — fabric alone is thinner than marketing photos suggest. Unplug the unit.
- Frost King AC9H Indoor Quilted 2-Piece Standard Cover (about up to 17″ × 25″ class) Check price on Amazon
Best indoor large faceFrost King AC11H quilted 2-piece
Best for: Wider/taller indoor faces and many through-wall fronts that outgrow the standard cover.
Watch-outs: Still measure; “large” is not universal. Tape the liner on clean dry plastic; retape next season if the included tape is spent.
- Frost King AC11H Indoor Quilted 2-Piece Large Cover (about up to 20″ × 28″ class) Check price on Amazon
Best outdoor small/mediumFrost King AC2H
Best for: Many units up to about 10,000 BTU with a shallower outdoor chassis — weather protection plus outdoor draft blocking.
Watch-outs: Thin polyethylene; expect wind to work the strap — cinch fully, fold corners as the instructions show, and add a bungee if your exposure is brutal. Wrong size = sail.
- Frost King AC2H Outside Window AC Cover (18″ × 27″ × 16″ class, up to ~10k BTU) Check price on Amazon
Best outdoor large / deep chassisFrost King AC5H
Best for: Higher-BTU or deeper outdoor chassis (~15,000 BTU and up class).
Watch-outs: Bigger bag + more wind area. Secure the strap; inspect after the first storm.
- Frost King AC5H Outside Window AC Cover (20″ × 28″ × 30″ class) Check price on Amazon
Best year-round wing sealfoam side-panel kit
Best for: Replacing leaky accordion side covers so conditioned air stops leaking around the chassis — often higher bill leverage than a cover alone.
Watch-outs: Trim carefully; don’t block required drainage or force a panel that lifts the unit. Pair with winter covers if the face still drafts.
- M‑D Building Products air conditioner side panel kit (trimmable foam) Check price on Amazon
What we left out
- No-name covers with unclear dimensions and no size chart.
- “Universal” bags that skip BTU/depth guidance.
- Central condenser “furniture” covers marketed as heating-bill miracles for window units.
- Smart plugs as AC shutoff (wrong device class for compressor loads) — see smart plugs for peripherals only.
Install / seal troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| Indoor cover won’t stay on | Cord/knobs in the way; wrong size; elastic weak | Seat liner first; route cord; size up; light tape on clean plastic — never on peeling paint you care about |
| Still feel draft at sides | Accordion wings leaking | Foam side-panel kit; weatherstrip sash stops |
| Outdoor cover blows off | Strap loose; oversized bag; wind tunnel exposure | Cinch fully; fold corners; add bungee; correct size |
| Condensation / musty smell in spring | Covered a wet unit | Wipe dry before covering; crack cover on a mild day mid-winter if moisture builds; never run AC under cover |
| Room still cold after covering | Bigger envelope leak elsewhere | Door sweeps, attic hatch, film on other panes, filters |
| Tape pulls paint | Fragile latex / dirty trim | Clean landing; test a corner; prefer elastic covers over aggressive tape on historic trim |
| Unit shifted / sill soft | Mounting failure | Stop; support/brace; hire help — weatherization after safety |
What this won’t fix
- A failed furnace, heat-pump aux thrashing, or wide-open attic.
- A banana-shaped exterior door or missing door sweep.
- Single-pane glass across a whole wall — use film/curtains on non-AC openings.
- High bills driven by rate changes or strip heat with no infiltration problem.
- A window AC you still need as a heat pump in shoulder season — don’t cover a unit you must run.
After you set it up
- Unplug the window/wall unit for the season.
- Confirm indoor liner + cover seated; outdoor bag cinched if used.
- Foam or reseat side panels if wings still flex.
- Add door sweeps on rooms you just sealed so you’re not heating the porch (weatherstripping).
- Film or curtain other drafty panes in the same rooms.
- Replace a dirty HVAC filter so the heating system isn’t fighting a clogged return (filters).
- After the first cold windy night, re-check straps and feel for remaining drafts.
- On the next bill, compare kWh to the same month last year when you can — not to a marketing percentage.
Habits that help
- Remove units you can store; covers are plan B.
- Keep winter heat setbacks honest — sealing a room then cranking the thermostat wastes the win (smart thermostats).
- Don’t stack plug-in heat against a covered AC face.
- In spring, remove covers on a dry day, inspect the chassis, and store bags clean so next fall is a 10-minute job.
- For summer, reseal side panels before you automate with a controller.
FAQ
Do window AC covers lower heating bills?
They can when a window or through-wall unit is left installed and leaking cold air through the chassis, accordion side panels, or sash gaps. Savings depend on how leaky the install was, your climate, and your rates. A cover is a seasonal draft stop—not a substitute for attic air sealing, door sweeps, or a failed heating system—and we don’t guarantee dollar savings.
Should I use an indoor cover, an outdoor cover, or both?
Indoor quilted covers (with the plastic liner) cut drafts you feel in the room. Outdoor polyethylene covers protect the chassis from weather and block wind-driven infiltration from outside. Many households use both when the unit stays in the window. If you can safely remove and store the unit, removal usually beats any cover.
What size outdoor cover do I need for my BTU rating?
Match the manufacturer size chart to your unit’s rated cooling capacity and measured chassis depth—not just the marketing name. Typical Frost King outdoor sizing: AC2H up to about 10,000 BTU (shallower chassis), AC3H about 10,000–15,000 BTU, AC5H about 15,000 BTU and up. Measure width, height, and outdoor protrusion before you buy.
Are foam side-panel kits better than a seasonal cover?
Different jobs. Foam side panels replace flimsy accordion wings that leak year-round; they help in cooling season and still matter in winter if the unit stays installed. A winter cover wraps the face (and outdoors, the chassis). Fix side-panel leaks first if you can feel air at the wings, then add covers for the off season.
Is it safe to leave a window AC installed all winter with a cover?
Often yes if the unit is securely mounted, the sill/support is sound, and covers don’t block required combustion or dryer vents elsewhere in the room. Don’t cover an egress window you need as a bedroom exit. Unplug the unit, never run it under a cover, and keep space heaters away from fabric covers and outdoor plastic.
Should I cover a central outdoor condenser the same way?
Different product class. Window/through-wall units sit in the envelope and draft into living space; central condensers sit outdoors and mainly need debris/snow protection per the manufacturer—not a heating-bill draft stop inside the house. This guide is about window and wall units that punch through glass or masonry.
Related reading
- Window insulation kits for winter drafts — shrink film on glass without an AC in the opening
- Sliding glass / patio door draft sealing for winter — patio/slider drafts when the opening is a door, not an AC
- Whole-house fan winter covers — seal leaky ceiling shutters / attic fan louvers for heating season
- Window AC smart controllers & ceiling fans — summer/shoulder automation; foam panels overlap
- Best weatherstripping and door draft stoppers — sash and door edges covers won’t fix
- Thermal / blackout curtains — daily layer over drafty glass
- Air sealing with caulk, foam & outlet gaskets — sleeve and penetration leaks
- HVAC air filters: MERV & size — recurring win while you weatherize
- Oil-filled space heaters for zone heating — comfort after sealing; keep clear of covers
- Priority checklist — cheap envelope wins before gadgets
- Start Here — order of operations for new visitors
Bottom line
For US homes heading into heating season with window or through-wall ACs left in place, indoor and outdoor covers (plus foam side panels) are a low-cost, reversible draft stop — not a substitute for removal when removal is easy, or for attic and door sealing when those are the real holes. Measure BTU and chassis first, pick indoor vs outdoor vs foam by the leak you felt, unplug before you cover, and re-check straps after the first windy night. Pair with window film on other panes and weatherstripping on doors. For the broader order of operations, return to Start Here or this month’s priority checklist.
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