The fire is out, the crew has packed up, and your instinct is to throw open every window in the house. That instinct is right, and the execution is usually wrong.
Airing out is a physical procedure. You are replacing the smoky air inside with clean air outside, quickly, and in one direction only.
Done in order, you can pull most of the airborne smoke out in the first several days. Done at random, you push it into rooms that were fine.
This page covers ventilation and air exchange only. For the wider first-day checklist, our guide on what to do first after a house fire covers utilities, insurance, and documentation.
When is it actually safe to go in and open the house up?
Not until the fire department clears the building. Fires reignite hours after they look dead, and floors, stairs, and roof framing can look sound and not be sound.
Before you carry a single fan in, ask the officer in charge two things. Whether the structure is stable, and whether the power and gas are shut off or safe to use.
If the cause is still being determined, ask before you move anything. An origin and cause investigator may need the scene untouched, and disturbing it early can complicate your claim.
Wear an N95 respirator, gloves, and closed shoes even for a ten minute setup. You will be walking through settled soot and kicking it back into the air.
If you are not sure the place is habitable yet, read whether it is safe to live in a house after a fire before you plan on sleeping there tonight.
Why do you shut the HVAC off before you air out a house after a fire?
Because a running system is a smoke distribution network. It pulls smoky air into the returns and blows it out of every register in the house.
Switch the system to off at the thermostat, not to auto, and flip the furnace or air handler breaker if you are unsure the thermostat is controlling it.
Then cover the return grilles with plastic sheeting and painter's tape. Returns are where the ducts breathe in, so an open return keeps loading the system with soot.
Cover the supply registers in unaffected rooms too. Sealing them keeps the ductwork from quietly leaking odor back into bedrooms while you ventilate downstairs.
Change the filter before the system ever runs again, and use a MERV 13 rating if the equipment accepts it. That is the filtration the EPA points homeowners toward for smoke particles.
Why open windows on opposite sides instead of all of them?
Air moves because of a pressure difference. Wind pushes on one side of the house and pulls on the other, and that difference is the engine driving the air exchange.
Open every window and you flatten the pressure difference. Air drifts in and out of the same openings and mostly circles inside the house.
Open one inlet on the windward side and one outlet on the opposite side and you get a straight, fast path through the building instead of a swirl.
Make the outlet the smaller opening when you can. A wide inlet feeding a narrower outlet raises the air speed on the way out, which sweeps the room rather than stirring it.
Aim the path through the smoky rooms, not around them. The inlet should be in the cleanest area and the outlet in the room that burned, so clean air chases dirty air out.
Take the screens out of the outlet window. A dirty screen is a surprising amount of resistance, and soot cakes onto it within a day.
Where do the box fans go, and which way does each one point?
The core setup is one fan, in the outlet window, blowing out. That single choice does most of the work, and most people get it backwards.
A fan blowing outward turns the whole house into an exhaust duct. It lowers the pressure inside, so clean air is drawn in through the inlet window on the far side.
A fan blowing inward at the smoky end does the opposite. It pressurizes the smoke and pushes it deeper into hallways, closets, and the rooms that were still clean.
Seal around the fan. Stuff the gaps above and beside it with cardboard or a towel so the fan is not just recycling air that leaks straight back in around the frame.
- Outlet window: box fan facing out, in the room with the worst smoke, screen removed.
- Inlet window: open, no fan, on the opposite side of the house and ideally upwind.
- Interior doors: open along the path between the two, closed everywhere else.
- A second fan: on the floor mid-path, pointed toward the outlet, to keep air from stalling in a long hallway.
- Exhaust fans: run the kitchen hood and bathroom fans, since they already vent outdoors.
Two fans in one window is a common mistake. Pointing one in and one out of the same opening mostly makes noise, because the air short circuits between them.
How do you create negative pressure so smoke leaves instead of spreading?
Negative pressure just means more air leaving a space than entering it. When the burned room is the low pressure point, air flows toward it and out, never back into the house.
You build it by making the exhaust the only easy way out. Fan blowing out of the damaged room, that room's door closed to the rest of the house, and its window the sole large opening.
Restoration crews do this with a HEPA air scrubber ducted out a window, which filters the exhaust as it goes. A box fan in a window is the crude homeowner version of the same idea.
This is exactly why a small fire ends up smelling like a whole-house fire. Our page on why a whole house smells like smoke after a small fire explains how far the air carries it.
Keep the containment honest. If you leave the door to the burned room propped open while the fan runs, you have just made the entire house the intake and undone the point.
When should you not open the windows at all?
There are days when ventilating makes the house worse, not better, and the honest answer is to filter the indoor air and wait.
- Wildfire smoke or ash outside: if the outdoor air is worse than the indoor air, opening up imports the problem. Health agencies say to wait until the outdoor cleanup is done and the air improves.
- Windy days with loose ash: wind carries ash and debris in from neighboring properties, and it lands on everything you were about to clean.
- High humidity or blowing rain: damp air makes smoke odor read stronger and re-wets materials you are trying to dry out.
- Freezing temperatures: an unheated open house risks burst pipes, which turns one insurance claim into two.
- An active investigation: if the cause is still being worked, do not disturb the scene or the air until you are cleared.
- A house you cannot secure: open windows on an empty, unwatched property is an invitation. Board up first, ventilate second.
On those days, run filtration instead of ventilation. A closed house with a good air cleaner beats an open house pulling in smoke, ash, or freezing rain.
Waiting a day is not a loss. What you cannot afford to wait on is wet materials and settled soot, since soot residue carries real health risks and only gets more acidic with time.
House still open to weather, or you cannot tell whether it is safe to be inside setting up fans? That call comes before the ventilation does.
(844) 810-6096How many hours a day and how many days should you air out a house after a fire?
Plan on four to six hours a day for roughly a week, in the driest, breeziest part of the day, usually late morning through mid afternoon.
Longer sessions are fine when the weather is good and the house is secure. What matters more than total hours is that the air is actually moving in one direction.
Here is a workable day-by-day routine for a fire contained to one or two rooms.
- Day one: get clearance, shut the HVAC off, cover the returns, remove burned debris and the burnt item itself, then run the inlet and outlet setup for four to six hours.
- Day two: repeat the ventilation, add a second fan mid-path, and start extracting standing water or wet carpet if the crew used hoses.
- Day three: ventilate again, then close the house at night with a HEPA air cleaner running in the damaged room.
- Days four to seven: keep the daily block going while you clean surfaces top down, since cleaning throws residue back into the air.
- Week two: drop to shorter sessions on dry days and judge the result with a cold nose test after a few hours away.
Close the house at night. Leaving windows open around the clock invites humidity, animals, and theft, and delivers almost no extra benefit while the air outside is still.
Expect the smell to rebound each evening. Odor keeps off-gassing out of walls and fabric, so a house that smelled clean at 3 p.m. can smell smoky again by bedtime.
What does a HEPA air scrubber remove, and what does activated carbon add?
They do two different jobs, and confusing them is why people buy a purifier, run it for a week, and complain the smell never budged.
A HEPA filter traps particles, which is the soot and fine smoke you can see hanging in the light. It is rated to catch 99.97 percent of particles at 0.3 microns.
An activated carbon filter adsorbs gases and volatile organic compounds. Those gases are what your nose reads as smoke, so carbon is the part that touches odor.
HEPA does nothing for odor and carbon does nothing for dust. For a fire, you want a unit that has both, run on the highest setting in the worst room.
An ordinary box fan is not a filter. It only moves air, which is useful for exchange and useless for cleaning, since everything that goes through it comes out the other side.
You can change that for about the price of a filter. Taping a MERV 13 furnace filter to the intake side of a box fan turns it into a real, EPA-endorsed low-cost air cleaner.
Skip the ozone generator in an occupied house. No federal agency has approved ozone generators for use in occupied spaces, and EPA testing found some units running well above accepted health limits.
Change filters far sooner than the box says. After a fire a filter can load up in days, and a saturated carbon filter starts giving the smell back to the room.
How do you dry the house out after the firefighting water?
Ventilating and drying are the same job on the same clock. Hoses put hundreds of gallons into your floors and walls, and that water starts its own problem fast.
Get the standing water out first with a wet vacuum, then pull up soaked rugs and pad. Air movement does very little until the bulk water is gone.
Then run a dehumidifier with the windows closed, aiming for 30 to 50 percent relative humidity. A dehumidifier and open windows fight each other, so alternate rather than overlapping them.
That gives you a simple rhythm. Ventilate during the dry part of the day, close up and dehumidify overnight, and repeat until materials read dry.
Does airing out actually get rid of the smoke smell, and do you need to hire anyone?
No, and this is the part most advice skips. Ventilation removes airborne smoke and speeds drying. It does not remove odor that is bonded into surfaces.
Smoke leaves an oily, acidic residue on walls, ceilings, cabinets, and fabric. That film keeps releasing odor no matter how much fresh air passes over it.
Researchers who measured homes after a major fire found gases that were expected to clear in hours were still there weeks later, rebounding every time the windows shut.
So treat airing out as step one of five. The rest is cleaning, HVAC work, soft goods, and sealing, which our guide on getting smoke smell out of a house lays out in order.
For the calendar, our page on how long it takes to get smoke smell out of a house gives realistic windows by fire size, so you know when to stop waiting.
As for hiring help, usually you do not need to. Ventilation is the one part of fire recovery that is genuinely a homeowner job, and a fifty dollar fan does the same physics as a rented one.
If the fire was a burnt pan, a toaster, or one scorched room, and the structure is sound, set the windows and fans yourself and save the money for the cleaning.
Hire out when the situation gets past a fan. That means standing water, soaked insulation, smoke through the ductwork, or any fire that touched framing or wiring.
Also hire out when air exchange is not physically possible. High-rise units, homes with sealed windows, and houses in an active wildfire zone need filtered negative air machines instead.
One line worth holding onto. If the smell survives a week of correct ventilation, the problem is residue, not airflow, and no amount of extra fan time will fix it.
Unsure whether your house needs equipment or just a week of windows and a box fan? A short conversation will tell you which side of that line you are on.
(844) 810-6096Common questions
How long should I leave the windows open after a fire?
Aim for four to six hours a day during the driest, breeziest part of the day, for about a week. Close the house at night. Around the clock ventilation adds little once the outdoor air goes still, and it invites humidity, animals, and theft into an already damaged home.
Should the box fan face in or out of the window?
Out. A fan blowing outward lowers the pressure inside and pulls smoky air through the house and outside, while clean air enters through a window on the opposite side. A fan blowing inward at the smoky end pressurizes the smoke and pushes it into rooms that were still clean.
Can I run my air conditioning or furnace while airing out the house?
No. Shut the system off at the thermostat, cover the return grilles with plastic and tape, and cover supply registers in unaffected rooms. A running system pulls smoke into the ducts and redistributes it everywhere, which is the single most common way a one room fire becomes a whole house odor problem.
What if there is still wildfire smoke or ash outside?
Keep the windows closed and filter the indoor air instead. Public health guidance is to wait until outdoor debris cleanup is finished and the outdoor air improves before ventilating. Run a portable air cleaner with HEPA and carbon filters, or a box fan with a MERV 13 filter taped to it, in the meantime.
Does an air purifier remove smoke smell, or just smoke?
A HEPA filter removes the particles, meaning the visible soot and haze, but it does not touch odor. The smell comes from gases and volatile organic compounds, which need an activated carbon filter. For fire odor you want a unit with both, run on high, with filters changed far more often than usual.
Will airing out the house get rid of the smoke smell for good?
No. Ventilation clears airborne smoke and speeds drying, but the odor lives in an oily residue bonded to walls, ceilings, cabinets, and fabric. That film keeps releasing smell after every airing. Airing out is step one, and cleaning, HVAC work, soft goods, and sealing are what finish the job.
Sources
- Wildfires and Indoor Air Quality (IAQ) — U.S. Environmental Protection Agency
- Mold Course Chapter 2: Why and Where Mold Grows — U.S. Environmental Protection Agency
- Ozone Generators That Are Sold as Air Cleaners — U.S. Environmental Protection Agency
- Volatile Organic Compounds Inside Homes Impacted by Smoke from the Marshall Fire — ACS ES&T Air, Dresser et al., CIRES and University of Colorado Boulder
- Guide to Air Cleaners in the Home — U.S. Environmental Protection Agency
- After the Fire: Returning to Normal — U.S. Fire Administration, FEMA
- Post-Fire Cleanup Guidance — Washington State Department of Health
- Indoor Air Quality After a Fire — Colorado Department of Public Health and Environment
- ANSI/IICRC S700 Standard for Professional Fire and Smoke Damage Restoration — Institute of Inspection, Cleaning and Restoration Certification
OdorHow to Get the Smoke Smell Out of a House After a Fire
OdorHow Long It Takes to Get Smoke Smell Out of a House
OdorWhy Does My Whole House Smell Like Smoke After a Small Fire?
AssessmentIs It Safe to Live in Your House After a Fire?
HealthIs Soot Dangerous? What Fire Residue Does to Your Health
EmergencyWhat to Do First After a House Fire, Step by Step