How do mosquitoes get in when the windows are closed?

Even with every window shut, a home is never completely airtight. Mosquitoes are only 3–6 mm long — a gap of just a few millimetres is enough. They’re attracted to the CO₂ you exhale, body heat, and certain skin odours. They can detect these cues from several metres away and follow the trail to the smallest opening.

The most frequent entry points we see with our customers: doors (even if open for just a few seconds), vents and mechanical ventilation, roller shutter boxes, worn seals, and cable pass-throughs. Sometimes, mosquitoes were already indoors without you realising.

Mosquitoes indoors with windows closed: the 7 most common causes

1. The front door or patio door: the #1 entry point

This is the cause we encounter most often. You open the door for a few seconds (coming and going, deliveries, taking the rubbish out) and several mosquitoes take advantage. Drawn by escaping CO₂ and heat, they slip in instantly. Once inside, they hide (curtains, under furniture, ceiling corners) and only come out in the evening or at night.

MOSKILL tip: this is exactly why a magnetic door fly screen is the first line of protection to put in place. It closes automatically after each passage.

2. Air vents, extractor fans and ventilation outlets

Mosquitoes can come in through air vents and ventilation outlets, especially if the insect mesh is missing, damaged, or too wide (over 1.2 mm). This is common in older homes or buildings with natural ventilation.

3. Roller shutter boxes (invisible gaps)

Roller shutter boxes are often overlooked. Yet their corners and slat channels regularly leave tiny gaps. You might not see it with the naked eye, but it’s more than enough for a mosquito.

4. Worn seals, cracks and cable pass-throughs

Worn window/door seals, loose skirting boards, small openings for services (fibre, air conditioning, electrical cables), a hairline crack near a window… These are discreet pathways but more than enough. A seal that no longer sits flush over 2 cm is effectively an open door for insects.

5. Common areas, landings and rubbish rooms (apartments)

In flats, mosquitoes can come from the landing, stairwells, the rubbish room, or basements. They follow warm air and CO₂ flow and get in as soon as your door opens. If shared areas are damp or poorly ventilated, the issue is often recurring.

6. Stagnant water nearby (balcony, garden, gutter)

Mosquitoes lay eggs in stagnant water: plant saucers, a forgotten bucket, an uncovered rainwater tank, a clogged gutter, a child’s toy filled with rainwater… It only takes a few millilitres of still water for 7 days for a breeding site to establish. If it’s close to your openings, you’ll see regular arrivals.

7. Mosquitoes already inside

Sometimes the cause is simpler than you think: they got in the day before (or earlier) and stay out of sight during the day. Some species can also lay eggs in a damp spot inside your home (shower tray, laundry area, under the sink). Result: it feels like they appear “out of nowhere”.

How to find where mosquitoes are getting in (checklist)

Go through each point to identify where they’re coming from:

• Light test (in the evening): turn off all indoor lights. With a torch, inspect around windows, doors and vents: any visible gap = likely entry point.
• Under doors: run your hand along the bottom of the front door and patio doors. If you feel a draught, mosquitoes can get through too.
• Vents and ventilation: check that a fine, intact mesh is present on every vent.
• Roller shutter box: inspect corners and slat channels using a torch.
• Stagnant water: check the balcony, garden, and terrace. Empty any standing water (saucers, buckets, gutters, drains).
• Targeted observation: if mosquitoes show up mainly in the evening near one specific door or window, the entry point is very close by.

Effective solutions for mosquitoes indoors (immediate + long-lasting)

Immediate solutions (for tonight)

• Reduce light attraction: close shutters and curtains before dusk. Avoid bright lights near open doors and windows.
• Eliminate all stagnant water: even a small plant saucer can be an egg-laying site.
• Spot and remove: mosquitoes often rest on walls, curtains and ceilings during the day. Inspect these areas in the late afternoon.
• Aim a fan: mosquitoes struggle to fly in airflow. A fan directed towards your bed or sofa helps keep them away.

Long-lasting solutions to stop mosquitoes indoors

• Seal entry points: replace worn door and window seals, block gaps under doors (adhesive door sweep), and seal cable pass-through holes.
• Secure vents: install or replace insect mesh on ventilation grilles and outlets.
• Install a fly screen on your main openings: it’s the simplest and most durable solution. A magnetic fly screen lets you ventilate freely without letting a single mosquito in, and installs in 10 minutes with no drilling.