Can flies come through a window air conditioner?

Flies rarely travel through the AC unit itself. In most Montreal apartments, they enter through small gaps around the side panels, weatherstripping, or window frame created during installation — not through the cooling mechanism.

Why do flies appear only after I install the window AC?

Installing an AC often involves adjusting the window frame and side panels, which can leave small gaps that weren't there before. If your window faces a high-fly area — like rear-alley garbage bins common in Montreal — those gaps give outside flies a direct path inside.

How do I seal gaps around a window AC unit?

Check both side panels, the top and bottom of the unit, and all weatherstripping. Fill any gap where you can see daylight with foam insulation strips, adjustable side panels, or exterior-grade caulk where appropriate. The goal is an airtight seal around the entire installation.

When should I call a pest professional instead of sealing it myself?

Call a professional if you've sealed every gap and flies keep appearing, if you're unsure what type of fly you're dealing with, or if the source may be inside the home — such as drain flies from a plumbing issue. Correct identification determines the right fix.

Could these be fruit flies or drain flies, not house flies?

Yes. Fruit flies hover near kitchens and ripening produce. Drain flies come from slow or clogged drains. House flies typically enter from outside through gaps around windows and doors. Identifying the species tells you whether sealing the AC, cleaning drains, or removing a food source is the real solution.

Are Flies Coming Into Your Montreal Apartment After Installing Your Window AC?

Emma Yagi

July 14, 2026

Are Flies Coming Into Your Montreal Apartment After Installing Your Window AC?

TL;DR

If flies suddenly start appearing after you install your window air conditioner in a Montreal apartment, don't blame the AC just yet. In most cases, the unit isn't attracting or producing flies. The real problem is usually a small gap around the installation that lets flies already outside — often near back-alley garbage bins common in Plateau, Saint-Henri, and NDG walk-ups — find their way inside.

We Solved This One in Less Than 10 Minutes

A Montreal homeowner told us that every summer, as soon as the window AC went in, flies started appearing inside the apartment. His first thought? "The flies must be coming through the air conditioner." It sounded possible. Until we started asking questions.

Is It One AC or Both?

The first thing we asked was simple: does this happen with every air conditioner, or only one? The answer: only one. That immediately changed the investigation. If the AC itself was the problem, you would expect every similar installation to produce the same result. Instead, only one room was affected.

The Window Told the Story

We walked outside. The AC was installed on the back of the building — the kind of rear-facing window common in Montreal triplexes and mid-rises. Directly beside the window? Garbage bins. Food waste. Rotting organic material. And plenty of flies. Now everything made sense. The flies weren't coming from the air conditioner. They were already outside. The AC installation simply gave them a way inside.

The Smallest Gap Is Enough

Many people expect to find a huge opening. That's usually not the case. A tiny gap around the side panels, weatherstripping, or window frame is often enough for flies to enter. The installation may look perfectly fine from inside the apartment. Outside tells a different story — especially after Montreal's first real heat wave, when fly activity spikes behind buildings.

Why Only This Apartment?

Here's what made this case interesting. Nobody else in the building had an AC installed on the back side facing the garbage area. Everyone else either had their windows closed or their AC facing another direction. Only this apartment created an easy path between a high-fly area and the inside of the home. In Montreal's dense residential blocks, that rear-alley setup is more common than you'd think.

What You Should Check

If you're seeing flies after installing a window AC in your Montreal apartment, inspect these first:

Gaps around both sides of the AC
Missing or damaged weatherstripping
Openings above or below the unit
Poorly fitted insulation panels
Any place where you can see daylight through the installation

Then step outside. If your window overlooks garbage bins, compost, pet waste, or another area attracting flies — standard behind many Montreal walk-ups — you've likely found the source. In Quebec rentals, window AC units are typically tenant-installed, so sealing gaps is usually on you, not the landlord.

The Solution

The fix is usually straightforward: seal every gap around the AC installation properly. The flies outside are no longer your problem if they have no way to get inside. If your installation is airtight and flies are still appearing, identify the type of fly before assuming the AC is responsible. Different species have different sources — fruit flies, drain flies, and cluster flies each point to a different fix. That's where professional identification helps. EliteXterm can help you confirm what you're dealing with and, when it's outside our bed-bug specialty, connect you with a trusted specialist in our network. No hard sell — just the right next step.

Conclusion

Your window air conditioner isn't usually bringing flies into your Montreal apartment. An improperly sealed installation is. Before replacing the AC or spraying insecticides around your home, inspect the installation and look outside the window. Sometimes the answer isn't inside your apartment at all. It's sitting right outside your window.