02 / WINDOWS
Sealing the gaps around a portable AC window kit
The same problem as a winter draft, running the other way: the panel that came in the box does not seal, and the cold air you paid for is leaving through the gap.
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The panel supplied with a portable AC seals its own width and nothing else. Close the remaining gaps with thick open-cell foam tape around the panel edges, foam or rope caulk between the sash and the sill, and a strip across the top where the raised sash meets the frame. Never use expanding foam and never block the unit's drainage.
This page is the summer inverse of the rest of this site, and it is the same physics. A gap is a gap: in January it lets cold in, in July it lets the cold you paid for out, and it also lets in humidity, which is the part that makes a room feel worse than the thermometer says.
The three gaps every portable AC install leaves
- 1Panel to sash. The plastic slider panel is never quite the width of the window, so there is a slot at one or both ends.
- 2Sash to frame. The lower sash is raised to hold the panel, so it no longer sits in its closed position and the meeting rail is wide open.
- 3Panel to panel. Multi-piece slider panels have a joint that is a straight air path from one side to the other.
Match the part to your gap
Measure at the widest point. Then read across. If your measurement sits on a boundary, take the larger band — a seal that is slightly too big compresses, and one that is too small does nothing at all.
| Your gap | Part type | Our pick | Price |
|---|---|---|---|
| 1/4 in to 3/4 in around the panel | Thick foam tape | fowong AC Window Seal Foam Tape, 1 in x 1 in, 6.5 ft x 2Published 1 in section in two 6.5 ft rolls — sized for exactly this job. | CHECK PRICE |
| 1/8 in to 3/8 in | Half-inch open-cell foam | fowong Open Cell Foam Weather Stripping, 1 in x 1/2 in x 13 ftFor the smaller gap between the panel and the sash, and for the sash-to-sill line. | CHECK PRICE |
| Irregular, changes along its length | Rope caulk | Frost King B2 Mortite Fingertip Caulking Cord, 1/8 in x 90 ftDeforms to an uneven gap and peels out when the AC comes out in the fall. | CHECK PRICE |
Quick picks
Ranked for fit, not for hype. Tap a row to jump to the full write-up, or go straight to the listing.
| # | Product | Best for | Gap range | Price |
|---|---|---|---|---|
| 1 | fowong AC Window Seal Foam Tape, 1 in x 1 in, 6.5 ft x 2Inch-thick foam for the gaps a window air conditioner leaves — the summer version of the same problem. | Portable AC and window-unit gaps | Published 1 in thick; fills gaps of roughly 1/4 in to 3/4 in | CHECK PRICE |
| 2 | fowong Open Cell Foam Weather Stripping, 1 in x 1/2 in x 13 ftHalf-inch-thick open-cell foam for the gap nothing else reaches — an AC surround, a loose sash, a warped frame. | Large, irregular gaps | Published 1/2 in thick; fills gaps of roughly 1/8 in to 3/8 in | CHECK PRICE |
| 3 | Frost King B2 Mortite Fingertip Caulking Cord, 1/8 in x 90 ftRope caulk: pliable putty you press into a gap in October and peel out in April, leaving no adhesive behind. | Seasonal window sealing | Published 1/8 in cord; fills gaps of roughly 1/16 in to 1/4 in when pressed | CHECK PRICE |
| 4 | Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ftEPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard. | Medium frame gaps | Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in | CHECK PRICE |
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Portable AC and window-unit gaps
fowong AC Window Seal Foam Tape, 1 in x 1 in, 6.5 ft x 2
Published gap range
Published 1 in thick; fills gaps of roughly 1/4 in to 3/4 in
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Inch-thick foam for the gaps a window air conditioner leaves — the summer version of the same problem.
Where the number comes fromfowong publishes 1" x 1" section in two 6.5 ft rolls. An inch of open-cell foam compresses a long way, which is what makes it the answer around an AC surround.
- Published section
- 1" x 1"
- Published length
- 6.5 ft x 2 rolls
- Mounting
- Strong adhesive
- Use
- AC surrounds, large door and window gaps
- Color
- Black
- Mounting
- Strong adhesive
What it does well
- Thick enough for gaps every other tape here ignores
- Two rolls covers a full AC surround with margin
- Also useful on a garage side jamb that has warped
Where it fails
- Open-cell foam is not a long-term exterior seal
- Black shows on a white sash
- Compresses permanently under a heavy unit
Who this is forA window AC or portable AC exhaust panel with daylight around it.
Large, irregular gaps
fowong Open Cell Foam Weather Stripping, 1 in x 1/2 in x 13 ft
Published gap range
Published 1/2 in thick; fills gaps of roughly 1/8 in to 3/8 in
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Half-inch-thick open-cell foam for the gap nothing else reaches — an AC surround, a loose sash, a warped frame.
Where the number comes fromfowong publishes 1" wide x 1/2" thick x 13 ft. Open-cell foam compresses much further than rubber, which is why it covers a wider range — and also why it is not a long-life exterior seal.
- Material
- PU foam, PE cover
- Published width
- 1 in
- Published thickness
- 1/2 in
- Published length
- 13 ft
- Mounting
- Strong adhesive
- Mounting
- Self-adhesive
What it does well
- Half an inch of squash covers gaps rubber sections cannot
- The PE facing sheds water better than bare foam
- Cheap enough to over-buy and cut down
Where it fails
- Open-cell foam holds water if the facing is cut or torn
- Shortest life of any seal here in direct weather
- Too thick for a door that latches tightly
Who this is forA portable-AC surround, a basement sash, or any gap you can push a finger into.
Seasonal window sealing
Frost King B2 Mortite Fingertip Caulking Cord, 1/8 in x 90 ft
Published gap range
Published 1/8 in cord; fills gaps of roughly 1/16 in to 1/4 in when pressed
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Rope caulk: pliable putty you press into a gap in October and peel out in April, leaving no adhesive behind.
Where the number comes fromFrost King publishes the cord diameter (1/8") and length (90 ft). Rope caulk deforms into the gap, so one cord covers a range — press two strands together for a wider gap.
- Published diameter
- 1/8 in
- Published length
- 90 ft
- Removal
- Peels out, no residue when fresh
- Tools
- None — fingertip pressure
- Color
- Gray
- Mounting
- Pressed in by hand, no tools
What it does well
- Genuinely removable, which is why renters and old-window owners keep buying it
- No adhesive to fail and no surface prep beyond wiping dust off
- 90 ft does a lot of window
Where it fails
- Left in through summer heat it gets harder to remove cleanly
- Seals a window shut — no ventilation until you pull it
- Looks like what it is: putty in a gap
Who this is forOld double-hung windows you want sealed for winter and openable in spring.
Medium frame gaps
Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ft
Published gap range
Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in
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EPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard.
Where the number comes fromFrost King publishes the section dimensions exactly: 5/16" wide by 1/4" thick, 17 ft long. A hollow D seals across roughly half to full its stated thickness, which is where the 1/8-1/4 in working figure comes from.
- Profile
- Hollow D
- Published width
- 5/16 in
- Published thickness
- 1/4 in
- Published length
- 17 ft
- Material
- EPDM rubber
- Mounting
- Self-adhesive onto the door stop
What it does well
- EPDM stays flexible in cold, which is the entire point of a winter seal
- Published dimensions to three fractions — you can actually match it to a measurement
- Compresses without taking a permanent set the way open-cell foam does
Where it fails
- 17 ft is one door frame; a house needs several rolls
- Adhesive still needs a clean, smooth surface
- Thicker than a V-strip, so it can stop a tight door from latching
Who this is forA door stop or window sash with a visible, consistent gap you have measured.
What we would skip, and why
Nobody in this category ever declines to recommend something. That is what makes universal praise worth ignoring. Here is what we would not buy.
Expanding spray foam anywhere near a window sash
It is permanent, it expands with enough force to bow a sash or a vinyl frame, and it has to be cut out with a knife when the AC comes out. Every gap on this page has a removable answer that works as well.
Duct tape as the seal
It is not a seal, it is a bridge over a hole, and its adhesive fails within weeks in summer heat and sun. It also leaves residue on a painted sash that outlasts the air conditioner. Foam under compression is the seal; tape is at best what holds the foam.
Sealing the AC's own drainage or airflow paths
A window unit needs its rear section outside and its drain path clear. Sealing around the whole casing rather than around the panel traps condensate inside the room. Seal the panel to the window; never seal the unit to itself.
How we picked
- Every gap around an AC install is measured and matched to a published foam section, exactly as with a winter draft.
- Prefer removable materials throughout: this whole assembly comes out in the fall.
- Never recommend anything that would block drainage or airflow the unit needs.
We have not tested these products and we do not claim to have. The full method, including what we refuse to claim, is on how we pick.
Complete the kit
The parts that decide whether the seal you just bought actually holds. Skip these and you will be doing the job again in March.
Removing old weatherstrip adhesive
Goo Gone Pro-Power Adhesive Remover, 8 oz
The step everybody skips: the new tape will not stick to the residue the old tape left behind.
Deep gaps before caulking
Frost King C21CP-C Caulk Saver Backer Rod, 3/8 in x 350 ft
Backer rod: the foam you push into a deep gap so the caulk over it is a seal rather than a bottomless hole.
Door and window frame sides
fowong V Seal Weather Stripping, 1/2 in x 19 ft
V-strip folds into the gap and springs back, so it seals a jamb that never closes to exactly the same place twice.
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Fitting it so it comes out again in October
- Compress the foam, do not stretch it. Foam works by being squashed between two surfaces, so cut it slightly long and press it in.
- Put the adhesive on the panel, not the window, wherever you have the choice. The panel is disposable; your sash is not.
- Use rope caulk on anything painted. It leaves nothing behind, which matters on a rented apartment window.
- Check the exhaust hose run while you are there. A hose that sags collects condensate, and a hot hose crossing the room adds back some of the heat you are removing.
When the unit comes out in the fall, the same window usually needs the winter treatment — and the sash you have just been propping open for three months is often the leakiest one in the house by then. How to seal windows for winter picks up from there.
Questions people actually ask
- How do I seal the gap around my portable AC window kit?
- Close the panel-to-sash slots with thick foam tape, run a strip along the top of the raised lower sash to seal the meeting rail, and fill any irregular gap with rope caulk. Do not use expanding foam and do not block the unit's drain path.
- Can I use window insulation film with a portable AC?
- Not over the same window — the panel and hose have to pass through. Filming the other windows in the room is worthwhile, because the same still-air layer that keeps heat in during winter keeps it out in summer.
- Is it worth sealing around a window air conditioner?
- Yes, and the gain is bigger than most people expect: an unsealed install leaks conditioned air out and humid air in continuously while the unit runs. Sealing costs a few dollars of foam and takes about fifteen minutes.
- What is the best foam for sealing around an AC unit?
- Thick open-cell foam tape, around an inch in section, so it can compress across a gap that is not uniform. Anything thinner will not span the slot at the ends of a slider panel.
Sources
Every specification on this page traces to a published manufacturer or retailer listing. Where a maker publishes no working gap range, we say so rather than estimating one.
Read next
The best weather stripping tape for windows
Tape, V-strip and rope caulk placed by gap and by where on the window they can survive.
How to seal windows for winter from the inside
The order of work that stops you filming over a leak you could have closed for free.
How to find drafts in your house
Five tests, no equipment, an hour — ordered so you find the big leaks before the small ones.