How do mosquitoes get in when the windows are closed?

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

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

Mosquitoes indoors with windows closed: the 7 most common causes

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

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

MOSKILL tip: that’s exactly why a magnetic door fly screen is the first protection to put in place. It closes automatically after every pass-through.

2. Air vents, mechanical ventilation and ventilation grilles

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

3. Roller shutter boxes (invisible gaps)

Roller shutter boxes are often overlooked. Yet the corners and slat passages frequently leave tiny gaps. You may not see them with the naked eye, but they’re more than enough for a mosquito.

4. Worn seals, cracks and cable openings

Worn window seals, skirting boards coming away, service holes (fibre, air conditioning, electrical cables), micro-cracks near a window… These are discreet openings but more than sufficient. A seal that no longer sits flush over just 2 cm is an open door for insects.

5. Communal areas, landing and bin room (in apartment buildings)

In a flat, mosquitoes can come from the landing, stairwells, the bin room or cellars. They follow warm air and CO₂ flows and enter as soon as your door opens. If communal areas are damp or poorly ventilated, the issue is often recurring.

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

Mosquitoes lay eggs in standing water: plant saucers, a forgotten bucket, a rainwater collector without a lid, a blocked gutter, children’s toys filled with rainwater… It only takes a few millilitres of still water for 7 days for a breeding site to establish. If that site is 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 discreet during the day. Some species can also lay eggs in a damp area indoors (shower tray, utility room, under the sink). Result: it feels like they appear “out of nowhere”.

How to find the mosquitoes’ entry point (checklist)

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

• Light test (in the evening): switch off all indoor lights. With a torch, inspect the edges of windows, doors and vents: any visible gap = likely entry.
• Under doors: run your hand under the front door and patio doors. If you feel a draught, mosquitoes can get through too.
• Vents and ventilation: check for a fine, intact mesh on every vent.
• Roller shutter box: inspect corners and slat passages with a torch.
• Standing water: check your balcony, garden and terrace. Empty any still water (saucers, buckets, gutters, drains).
• Targeted observation: if mosquitoes mainly appear in the evening near a specific door or window, the entry point is very close.

Effective solutions against mosquitoes indoors (immediate + long-term)

Immediate solutions (for tonight)

• Reduce light attraction: close shutters and curtains before dusk. Avoid bright lights near open doors and windows.
• Remove all standing water: even a saucer with a small amount of water can be a breeding site.
• Spot and remove: mosquitoes often rest on walls, curtains and ceilings during the day. Check these areas in late afternoon.
• Directed fan: mosquitoes struggle in airflow. A fan aimed at your bed or sofa keeps them away effectively.

Long-term solutions to stop mosquitoes coming indoors

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