During a house fire, managing openings (windows, doors, ventilation grilles) directly affects the speed of flame propagation and the concentration of toxic smoke in each room. The answer to the question “should we open or close the windows” depends on parameters that are rarely considered together: the location of the fire, the actual airtightness of the home, and the state of mechanical ventilation.
Airtightness of the home against smoke: the underestimated factor
Most fire safety guidelines boil down to “close doors and windows.” This reflex is relevant, but it is based on an assumption: that the home, once closed, acts as an effective barrier against smoke.
A closed house is never completely airtight. Fine particles continue to infiltrate through ventilation grilles, roller shutter boxes, joinery defects, and electrical conduit passages. Closing the windows massively reduces smoke entry, but does not eliminate it.
The pertinent question to ask about opening or closing windows during a fire is therefore not binary. It involves simultaneously managing three air entry points: glazed openings, interior doors, and the controlled mechanical ventilation (CMV) system.
Recent health recommendations emphasize a complementary action: turn off the CMV and block air inlets with damp cloths. Without this action, mechanical ventilation actively draws in polluted outside air and distributes it throughout all rooms in the home, including those far from the fire.
Interior fire or external smoke: two scenarios, two opposite responses
The guidelines vary radically depending on the source of the danger. The table below summarizes the most common configurations.
| Scenario | Windows | Interior Doors | CMV | Main Objective |
|---|---|---|---|---|
| Fire in a room of the home | Closed | Closed (isolate the room on fire) | Off | Deprive the fire of oxygen, confine the smoke |
| Fire in the building (outside the home) | Closed | Closed (especially the landing door) | Off | Prevent smoke from entering through common areas |
| Forest fire or vegetation fire nearby | Closed | Closed | Off | Confine until help arrives |
| Smoke episode without direct fire on the house | Closed, then re-ventilate when air quality becomes acceptable | Open | Temporarily off | Limit inhalation of fine particles |
The common point in these four cases: windows remain closed as long as the danger persists. The only nuance concerns the timing of re-ventilation, which should only occur after confirmation that the outside air has become breathable again.
The particular case of the room on fire with a closed window
Closing the window in a room where a fire has broken out deprives the fire of fresh oxygen. Combustion slows down, allowing more time for evacuation. However, this configuration accumulates heat and pressure behind the walls. If the glass breaks due to thermal effects, the sudden influx of air can cause a rapid flashover.
Opening the window in a room on fire accelerates combustion due to natural draft effect. Smoke partially evacuates outside, but the flames increase in intensity. This action is not recommended by any emergency service, except to allow for an emergency evacuation when all other exits are blocked.
Managing the CMV and air inlets during a fire
Controlled mechanical ventilation is rarely mentioned in public reflexes, even though it plays a direct role in the circulation of smoke within a home. Under normal operation, a single-flow CMV extracts stale air from humid rooms and creates a depression that draws in outside air through the air inlets of dry rooms (bedrooms, living room).
During a smoke episode, this mechanism becomes a vector of contamination. Cutting off the power supply to the CMV is the first action to take, even before checking the windows.
Complementary measures to apply simultaneously:
- Seal the air inlet grilles of the windows with wide adhesive tape or damp cloths, starting with the rooms where occupants are present
- Place a damp cloth at the bottom of the landing door if the fire is located in the common areas of a building, to slow the infiltration of smoke through the threshold
- Check the roller shutter boxes, which are often poorly insulated, and cover them if smoke is visibly entering
The damp cloth remains a supplementary measure. It slows the passage of smoke but does not filter the finest particles. Its effectiveness decreases rapidly as it dries.
Access for emergency services: do not lock everything
Closing openings for protection does not mean barricading the home. Official recommendations remind us that access routes must remain clear for emergency services. A locked gate, a cluttered hallway, or a reinforced door without accessible keys from the outside can delay firefighters’ intervention by several minutes.
In a single-family home, the garden gate and the main entrance door must be able to be opened quickly by responders. If evacuation has already taken place, leaving the entrance door unlocked (but closed) facilitates access without compromising confinement.
Re-ventilation after the smoke episode
Once the fire is controlled or the external smoke has dissipated, the temptation to open all the windows wide is natural. Health authorities recommend waiting for confirmation that the quality of the outside air has become acceptable again before ventilating the home. In the case of a forest fire or vegetation fire, smoke episodes can last several hours or even several days.
- Re-ventilate intermittently for short periods (a few minutes) rather than leaving the windows open continuously while the air remains polluted
- Prioritize openings located on the side opposite to the wind direction carrying the smoke
- Restart the CMV only after re-ventilation, to avoid redistributing trapped pollutants in the ducts
Managing openings during a fire is not limited to the reflex of “closing everything.” The actual airtightness of the home, the state of the CMV, and the type of fire modify the appropriate response. The most protective action remains to close windows and doors while turning off mechanical ventilation, then only reopen once the outside air has been verified.