How to seal window gaps around an air conditioner against insects
Window air conditioners often leave wide gaps and openings that allow flying insects like horseflies and wasps to enter the room. Makeshifts like duct-taped fabric unstick over time, leaving tenants struggling to properly insulate irregular windows like jalousies or units that do not fit standard openings.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from October 2026.
- This comes up 22.0× more often in r/hvacadvice than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Using duct tape to attach a pillowcase or cardboard to the glass and wallDid this help you?
- 2Using gaffer tapeDid this help you?
- 3Using plumber tape or zip system tapeDid this help you?
- 4Rolling and folding cardboard to expand and fit in the gapDid this help you?
- 5Using Saran wrap on each side and filling the gap with expanding foamDid this help you?
- 6Installing commercial foam side panels or screen door mesh panelsDid this help you?
- 7Putting a piece of masking or painters tape over the gap to seal it.Did this help you?
- 8keeping windows closedDid this help you?
- 9sealing gaps with foam panels or weatherstrippingDid this help you?
- 10using caulk around the edgesDid this help you?
- 11using foam insulationDid this help you?
- 12using duct tape to seal gaps around the window unitDid this help you?
- 13Using fire-rated expanding foam or fire block foamDid this help you?
- 14Using neutral cure silicone instead of foamDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I noticed when putting in the new ones that there’s a very slight opening near the top.”source ↗
“im in an apartment and put it in just now but theres TONS of smokers around me, and I cannot stand the smell of cigarettes or weed”source ↗
“i dont know if this is weird but i can hear everything outside now like the bugs now that I installed it idk if thats normal or if i did it wrong”source ↗
“im mostly just concerned cause the smell thats coming through the bug vents at the top smells badand im hoping thats not the smell of the cigarettes inside coming in”source ↗
“Does anyone have advice on how I can insulate my jalousie windows while I run my AC?”source ↗
“I’m renting and wasn’t allowed to get a window unit.”source ↗
“I really want to put it in the window cause I like my room real cold but I'm scared of any secondhand cigarette or marijuana smoke since theres plenty of smokers next to me”source ↗
“Before I put flooring in, is it a bad idea to seal gaps around floor registers with expanding foam?”source ↗
“I have an AC unit that sits inside a much wider window, and to keep out flying insects I've duct taped a pillow case onto the glass and surrounding wall. However, every once in a while it'll unstick and horseflies/wasps will enter my room.”source ↗