Windows and Wildfire Smoke: What They Can and Cannot Do for Indoor Air

Wildfire smoke is now a regular part of late summer and fall across Northern California, including in places far from any fire. The fine particles in smoke are small enough to pass deep into the lungs, and they get indoors. This page looks at one part of the defense, the windows, and is honest about how large that part is. It belongs to our California window guide.

This is not a page about protecting the house from fire. Ember and flame resistance, and the state's Wildland-Urban Interface Code, are covered at wildfire-resistant exteriors.

How smoke gets inside

The U.S. Environmental Protection Agency (EPA) describes three routes:

1. Natural ventilation: open windows and doors.
2. Mechanical ventilation: fans and heating or cooling systems that bring in outside air.
3. Infiltration: in the EPA's words, smoke enters "through small openings, joints, cracks, and around closed windows and doors."

Windows are involved in the first and the third. Closing them deals with the first. The third is the reason a closed house still smells of smoke.

How much does closing up help?

More than nothing and less than people hope. EPA's training material on wildfire smoke says that with doors and windows closed and no air cleaner running, indoor fine particle levels are typically around 55 to 60 percent of outdoor levels during smoke events, and that the figure ranges from as low as 30 percent to as much as 100 percent depending on the house. With doors and windows left open, indoor and outdoor levels will be similar.

The EPA also notes that newer homes and homes with air conditioning tend to keep outdoor pollution out more effectively than older homes. A study of sensor data from California wildfire events, published in the Proceedings of the National Academy of Sciences in 2021, found the same pattern: lower infiltration in newer buildings and in those using air conditioning or filtration.

Two conclusions follow. A tight house matters. And even a tight house lets a good deal in, so the air inside has to be cleaned as well.

What a window's air leakage rating means

Windows are tested for how much air passes through the assembled unit when closed and locked. The result is the air leakage rating, in cubic feet of air per minute per square foot of window area.

• The NFRC label shows air leakage as a value of 0.3 or less; lower means tighter. Our guide to reading an NFRC label shows where it sits.
• California's Energy Code (Title 24, Part 6, Section 110.6(a)1) requires manufactured windows to have air infiltration rates not exceeding 0.3 cfm per square foot.
• ENERGY STAR's current criteria use the same 0.3 ceiling for windows.

So 0.3 is the pass mark, not a mark of distinction. Many good windows test well below it. Air leakage is not always printed on the label, so if airtightness is a priority, ask for the tested figure for the exact product, and compare numbers rather than adjectives.

Style makes a difference

How a window closes affects how tightly it can seal.

• Fixed windows have no moving joint at all.
• Casement and awning windows pull the sash against a compression gasket when locked, like a refrigerator door.
• Sliders and single- or double-hung windows rely on brush or fin weatherstripping that the sash slides along. It has to be loose enough to let the sash move, and it wears.

That is a tendency, not a law: a well-made slider can outperform a poorly made casement. But if you are replacing windows in a bedroom you intend to use as a clean room, it is a reason to consider a casement or to check the slider's tested figure.

The rating is for the window, not the hole in the wall

A window's air leakage rating is measured in a laboratory frame. It says nothing about the joint between the window and your house. A tight window set in an opening that was never sealed will leak around its perimeter. On a retrofit installation into an existing frame, the seal between the new frame and the old one, and the condition of the old frame's joint with the wall, decide the result. Ask how the perimeter will be sealed, with what, and whether the gap will be insulated.

When old windows really are the smoke pathway

For some houses the windows are a minor leak among many. For others they are the main one. Yours are likely to matter if:

• They are original aluminum sliders from the 1960s to 1980s. The pile weatherstripping on these has usually flattened or fallen out, and the sashes rattle in their tracks.
• They are old wood double-hungs with loose sashes, missing glazing putty or gaps at the meeting rail.
• They are jalousie (louvered) windows. Glass slats overlapping each other cannot seal.
• You can feel moving air at a closed window on a windy day, or see a curtain stir.
• Soot or fine ash collects on the sill or in the track after a smoke event.
• The lock no longer pulls the sash tight, or the window does not fully close.

If several of those are true, replacing the windows will make a real difference to smoke, drafts, dust and noise together. Our pages on moving from single to dual-pane windows and how often windows should be replaced cover the wider case.

If none of them is true, your windows are probably not where the smoke is coming from, and money is better spent on filtration and on sealing elsewhere.

Cheaper fixes first

• Replace the weatherstripping. On many windows it is a replaceable part.
• Adjust or replace the locks so the sash pulls in tight.
• Caulk the interior joint between the window frame and the wall where it has cracked.
• Clean the tracks so sliders close fully. Do not block the weep holes on the outside; they are drains, as explained in parts of a window.
• Consider an interior storm panel on a leaky window you want to keep; see our page on storm windows.

The rest of the house leaks too

Windows get the blame because you can see them. In most houses a large share of air leakage is elsewhere: exterior doors and their thresholds, the attic hatch, recessed lights, plumbing and wiring holes, the fireplace damper, the dryer vent, bath and kitchen exhaust ducts, and leaky ductwork in an attic or crawlspace. A door with a visible strip of daylight under it leaks more than several tight windows.

Two pieces of equipment common in California deserve a specific warning. The California Air Resources Board (CARB) says not to run swamp coolers or whole-house fans during smoke, because both work by pulling large volumes of outdoor air through the house.

What to do during a smoke event

The EPA and CARB give consistent guidance.

1. Keep windows and doors closed while outdoor air is bad.
2. Run the central system on recirculate. If your system has a fresh-air intake, close it or set it to recirculate, and set the fan to "on" so that air is filtered continuously rather than only when heating or cooling cycles.
3. Upgrade the filter. The EPA recommends a filter with a MERV 13 rating, or as high a rating as your system fan and filter slot can accommodate. CARB says MERV 13 or higher. Check with an HVAC technician if you are unsure what your system can take, and keep spares, because filters load up quickly in smoke.
4. Use a portable air cleaner sized for the room, and make sure it does not produce ozone. CARB certifies air cleaning devices sold in California and publishes the list.
5. Set up a clean room. Pick a room that fits everyone, close its windows and doors, and run an air cleaner there. The EPA adds: do not do anything that makes it hard to get out.
6. Do not add to the load. Avoid frying or broiling, burning candles or incense, smoking, and vacuuming without a HEPA filter.
7. Air out when it clears. When outdoor air quality improves, even temporarily, open the windows and flush the house.

Window air conditioners need a note of their own. The EPA says to close the unit's outdoor air damper, to avoid using it if the damper cannot be closed, and to seal the gap between the unit and the window as tightly as possible.

Smoke and heat at the same time

California's worst smoke often arrives during its worst heat, and the two pieces of advice pull against each other: close up against smoke, open up against heat. The EPA's guidance on this is direct. For most people, heat is more immediately dangerous.

That has consequences for windows:

• If you have air conditioning, closing up is sustainable. Glass with a low solar heat gain coefficient keeps the house cooler with the windows shut, which reduces the temptation to open them; see the best windows for cooling.
• If you do not, a closed house can become dangerous. The EPA advises seeking shelter elsewhere if it is too hot to stay inside with the windows closed. Its suggestion for homes without air conditioning is to open windows at night when it is cooler and close them once the house has cooled, watching the air quality as you do.
• Exterior shade on west and south glass helps in either case.

A tight house still needs fresh air

There is a trade-off in making a house airtight. Leaks are how older houses get their fresh air, unplanned and unfiltered. Tighten the house and you remove the smoke pathway, and also some of the dilution of moisture, cooking fumes and other indoor pollutants. On an ordinary day the answer is to open the windows and use the kitchen and bath fans. Many newer homes have mechanical ventilation designed in for this reason.

If new windows leave you with more condensation on the glass in winter than before, that is the house telling you it is tighter. Our page on fighting condensation on the inside of windows covers what to do.

If you are replacing windows with smoke in mind

• Ask for the air leakage figure of each product you are comparing.
• Favor compression-seal styles (casement, awning, fixed) where the room and the look allow, particularly for the room you would use as a clean room.
• Ask how the perimeter is sealed between the new window and the opening.
• Do not expect multi-pane glass to help with smoke. Dual or triple glazing is about heat and noise. Smoke comes through gaps, not through glass.
• Check the screens afterward. Ash clogs insect screens; rinse them once the air clears.
• If the house is in a fire hazard severity zone, the wildfire code governs what you may install. Start at wildfire-resistant exteriors.

The figures your windows must meet for energy are on our Title 24 page.

Sources

• U.S. EPA: Wildfires and Indoor Air Quality
• U.S. EPA, Wildfire Smoke and Your Patients' Health: Strategies to Reduce Exposure Indoors
• U.S. EPA: Create a Clean Room to Protect Indoor Air Quality During a Wildfire
• U.S. EPA: Heat and Smoke fact sheet (June 2026, PDF)
• California Air Resources Board: Protecting Yourself from Wildfire Smoke
• California Air Resources Board: Smoke Ready California
• California Air Resources Board: List of CARB-Certified Air Cleaning Devices
• Liang et al., Proceedings of the National Academy of Sciences, 2021: wildfire smoke and indoor air quality in California buildings
• National Fenestration Rating Council: The Energy Performance Label
• California Energy Commission: 2025 Building Energy Efficiency Standards (Title 24, Part 6), Section 110.6(a)1
• AirNow: current air quality and the Fire and Smoke Map

Working with Heritage Exteriors

We will tell you honestly whether your windows are a real smoke pathway or whether weatherstripping and a better furnace filter would do more for less. When we do replace, we seal the perimeter of every unit and can give you the tested air leakage figure for the product we quote. Licensed in California, CSLB #1055919. Call (916) 546-2991.