HomeSymptom

Symptom Guides · windows

Why do my double-pane windows fog up on the inside in winter?

Humidity checks, quick fixes, and what a real repair costs

If you can wipe the fog off with your sleeve, the window seal is fine. Warm indoor air is hitting cold glass and dropping below its dew point. The fix is lowering indoor humidity, moving air across the glass, or warming the glass surface. Most homeowners solve it for under $120.

9 min read · Updated July 27, 2026 · Careful DIY · DIY $20–$120 · Pro $200–$900

Stop if you notice this

  • Persistent condensation feeds mold on wood sills and drywall returns. Dry the sills daily until you fix the source, and treat any black or gray staining as a mold problem, not a cleaning problem.
  • If windows fog heavily in a house with unvented gas heaters, that water is a combustion byproduct and a carbon monoxide warning sign. Get a CO alarm on every level and have the appliance inspected.
  • Don't run a gas oven with the door open for heat. It dumps both moisture and combustion gases into the room.
  • Never vent a clothes dryer into a crawlspace, attic, or garage, even with a lint trap bucket on the end. It's a code violation and a rot and mold generator.
  • Wet window sills next to electrical outlets or baseboard heaters deserve attention. Water tracking down a wall cavity can reach wiring you can't see.
  • If you find mold covering more than roughly 10 square feet, stop cleaning and call a remediation contractor.

Ranked likely causes

6 causes

  • Why it happens

    At 70°F and 45% relative humidity, room air condenses on any surface below about 48°F. The center of a clear double-pane window sits near 43°F when it's 0°F outside, and the glass edge runs colder. The window fogs because the air met its dew point, not because anything failed.

    First check

    Put a $15 digital hygrometer in the room that fogs worst and read it at 6 a.m. If it's above 40% with cold weather outside, run bath fans longer, crack the kitchen hood while cooking, and drop any humidifier setting.

    Call a pro when

    If humidity stays above 50% after two weeks of source control, you likely have a hidden water source and need a pro with a moisture meter and thermal camera.

Seeing this same pattern at home?

See a sample ranked report →

The short version

Warm indoor air holds a lot of water. Cold glass cools that air below its dew point, and the water comes out of the air onto the pane. That's the whole mechanism.

At 70°F indoors and 45% relative humidity, your dew point is about 48°F. On a 0°F night, the center of a clear double-pane window sits near 43°F and the bottom inch of glass runs colder still. Water has nowhere to go but onto the glass.

So the window is telling you something true about your house. It's the coldest surface you own, which makes it the first place excess moisture shows up.

Fixing it means some combination of three moves: put less water into the air, get more of it out of the house, or warm up the glass. Most people get there with the first two.

Quick test: run a dry finger across the fog. If it leaves a clean streak, your seals are fine and this is a humidity issue you can solve yourself.

Three different fogs, three different problems

Interior surface fog is what this page covers. It wipes off, appears on cold mornings, and shows up worst on north-facing and curtained windows.

Fog between the panes means the insulated glass unit lost its seal and the desiccant in the spacer is saturated. You'll see haze, mineral streaking, or a permanent milky ring that no cloth can reach. That's a glass replacement, typically $250-600 per unit installed in 2026 for standard residential sizes.

Exterior fog on the outside surface, usually on clear autumn mornings, is a compliment. It means the glass is so well insulated that the outer pane cools to outdoor dew point overnight. Nothing to fix.

  • Wipes off from inside: indoor humidity, fixable by you
  • Trapped and permanent: failed IGU seal, needs new glass
  • Wipes off from outside: high-performance glass working correctly

See my most likely cause

Describe what's happening or add a photo, and get your most likely cause free.

See a sample ranked report →

Free to start · Guidance only, not a substitute for licensed trade work

How we actually diagnose it

First stop is a hygrometer, not the window. I want indoor temperature and relative humidity at 6 a.m., which is the coldest and wettest moment of the day, and the outdoor temperature for the same hour.

Then I walk the house and map which windows fogged and which didn't. A house-wide pattern points at total moisture load. A single-room pattern points at a source in that room or a cold spot in the duct system.

Next, an infrared thermometer on the glass. I read the center of the pane and then one inch above the bottom edge. On a decent low-e unit at 20°F outside, center glass should land around 58-62°F. Under 45°F tells me the unit is old, gas-depleted, or was never low-e to begin with.

Last, I go looking for the water. Dryer duct, bath fan terminations, kitchen hood (recirculating hoods vent nothing), crawlspace, sump pit, and any aquarium or houseplant collection that would qualify as a greenhouse.

Humidity targets by outdoor temperature

There's no single correct indoor humidity in winter. The number has to slide down as it gets colder outside, because the glass gets colder with it.

These are the numbers I give homeowners with reasonably modern double-pane windows. If your glass is 1990s clear double pane, shave 5 points off each one.

  • Outdoor 40°F: keep indoor RH at or below 45%
  • Outdoor 30°F: 40%
  • Outdoor 20°F: 35%
  • Outdoor 10°F: 30%
  • Outdoor 0°F: 25%
  • Outdoor -10°F and below: 20% or lower
Below about 25% indoor humidity, expect static shocks, dry sinuses, and gaps opening in hardwood floors. If you have to go that low to stop condensation, the real answer is warmer glass, not drier air.

DIY vs pro cost bands

DIY$20–$120
Pro$200–$900

Action checklist

0/6

What the fixes actually cost

The DIY path is cheap and it solves the majority of these calls. A hygrometer runs $12-25, a bath fan countdown timer switch is $25-45, and a tube of quality sealant for a drafty sill is under $15.

Mid-range work brings in a contractor. Replacing an undersized bath fan with a properly ducted 80-110 CFM unit runs $250-500 installed. Cleaning and rerouting a dryer duct is $200–$900. An HVAC visit to service or reset a whole-house humidifier is typically $200–$900.

Interior storm panels are the underrated option for cold glass. Magnetic or compression-fit acrylic inserts run $200–$900 per window and raise interior glass temperature enough to stop most condensation without touching the existing windows.

Bigger fixes: crawlspace vapor barrier and encapsulation lands anywhere from $2,500 to $9,000 depending on square footage and drainage work. A ducted HRV or ERV for a tight house is $2,000-5,000 installed. Whole-window replacement runs $700-1,500 per opening for mid-grade vinyl in 2026.

  • Hygrometer and testing: $20-40
  • Bath fan timer switch, DIY install: $25-45
  • Bath fan replacement, pro: $250-500
  • Dryer duct correction: $200–$900
  • Interior storm panels: $200–$900 per window
  • Basement or crawlspace dehumidifier: $900-2,200 installed

What you can handle and where to stop

Measuring humidity, adjusting a humidistat, swapping a fan switch to a timer, opening blinds, and tracing a dryer duct are all homeowner work. So is buying a portable dehumidifier for a damp basement and running a drain line to the floor drain.

Call a licensed pro when the work crosses into ducting through an exterior wall or roof, electrical circuit changes, gas appliance venting, or anything involving mold beyond a small surface patch.

Also call when the numbers don't add up. If you've cut every source you can find and January humidity still reads 50%, there's water entering the building somewhere. A tech with a thermal camera and a moisture meter will find it faster than a month of guessing.

One boundary worth naming: nobody should be selling you replacement windows as the first answer to interior condensation. If a salesperson does that without asking for a single humidity reading, get another opinion.

Can this wait until spring?

Light misting on the glass that's gone by 10 a.m. can wait. Wipe the sills, note the pattern, and start managing humidity when you have time.

Water running down the pane and standing on the sill can't wait. That's enough liquid water to break the paint film and get into the wood within a few weeks, and it means your indoor humidity is 10-15 points too high right now.

Anything with visible mold on the sill, frame, or drywall return needs action this week. Clean it, dry it, and fix the source before it colonizes the wall cavity.

Special case: if fogging appeared suddenly across the whole house with no change in habits, look at your heating system before you look at anything else. Blocked flues and unvented combustion produce a lot of water vapor along with gases you don't want to breathe.

Sudden heavy window condensation plus headaches, drowsiness, or a stuffy feeling in the house is a combustion safety issue until proven otherwise. Get outside, ventilate, and have the heating appliance checked.

How your house's era changes the answer

Homes built before 1980 usually leak enough air that winter humidity stays low on its own. When they fog, it's almost always a specific source: an open crawlspace, a dryer venting into the basement, or a humidifier someone added because the house felt dry.

Houses from 1985 to 2005 often have the original clear or first-generation low-e double panes with aluminum spacers. Those units run cold at the edges, so bottom-inch fogging shows up at humidity levels a newer window would shrug off.

Post-2010 construction, and any older house that's been weatherized, has the opposite problem. The envelope is tight and there's often no mechanical ventilation beyond bath fans nobody runs. Normal family moisture has nowhere to go.

Brand-new construction adds curing moisture. Framing lumber, concrete, drywall mud, and paint release a serious amount of water during the first heating season. Fogging in a house's first winter often quiets down by the second.

The moisture sources people miss

A family of four generates roughly 2-4 gallons of water vapor a day through breathing, showering, cooking, and laundry. That's the baseline before anything goes wrong.

The additions that surprise people: firewood stored indoors (a single cord can release dozens of gallons as it dries), a recirculating range hood that filters grease and vents nothing, unvented gas fireplaces and space heaters, and a sump pit with no lid.

Aquariums, indoor plant collections, and hanging laundry to dry inside all add up fast. So does a slow supply-line drip under a sink that keeps a cabinet damp year-round.

Check the crawlspace last but take it seriously. Exposed soil under a 1,500 square foot house can push more water into the living space than every shower combined.

When lowering humidity isn't the right fix

Some houses can't go drier without real discomfort. Homes with wood floors, antique furniture, or occupants with respiratory issues genuinely need 30-35% humidity, and if the windows fog at 30%, the glass is the problem.

The cheapest way to warm glass is airflow. An open supply register under the window and unblocked blinds can raise the interior surface temperature 4-6°F, which is often the entire margin you need.

Interior storm panels are the next step up. A well-fitted acrylic insert creates a dead air space that pushes interior glass temperature up by 8-12°F for $200–$900 per window, and it's reversible.

Full glass unit replacement makes sense when several units have failed anyway. Ask specifically for a warm-edge spacer, not just low-e coating, because most of the winter fogging happens at the perimeter.

Safety and mold, straight talk

Condensation itself won't hurt you. What grows in it can. Mold needs about 60% surface humidity to establish, and a wet painted sill hits that every single cold morning.

Small patches under a couple of square feet: detergent, water, a stiff brush, then dry it completely and repaint with a mildew-resistant enamel. Skip the bleach on porous wood, it doesn't penetrate and the water content makes things worse.

Larger growth, or growth that returns after you've dropped the humidity, means water is coming from somewhere other than the room air. Stop cleaning and get an inspection.

The one thing I'd never brush off: a house that starts fogging heavily right after someone installed a new heating appliance, blocked a chimney, or converted a fireplace. Water vapor and carbon monoxide are produced together, and only one of them is visible.

Putting it together

Start with a reading, not a purchase. Know your indoor humidity and the outdoor temperature before you spend a dollar.

Kill the sources you control: dryer venting, bath fan runtime, kitchen hood use, humidifier setting, indoor drying racks. Give it two weeks and re-measure.

If the number drops and the windows clear, you're done for well under $120. If it doesn't budge, there's water entering the house and that's a pro visit worth paying for.

Replace glass only after you've won the humidity argument. Otherwise you'll spend thousands and find the fog moved to your closet walls and attic sheathing instead.

Keep a cheap hygrometer permanently in your coldest room. Once you know your house's normal winter number, any jump above it tells you something changed before the damage starts.

Interactive decision tree

Is there an immediate safety risk (gas, sparks, flooding, CO)?

Step 1 of 3

Frequently asked questions

Does condensation on the inside of my window mean the seal failed?

No. Failed seals show up as haze, streaks, or a milky film trapped between the two panes, and no amount of wiping touches it. If you can clear the fog with a paper towel, the insulated glass unit is sealed and the moisture is coming from your indoor air.

What indoor humidity should I run in winter?

Match it to the outdoor temperature. Roughly 40% when it's 30-40°F outside, 35% at 20°F, 30% at 10°F, 25% at 0°F, and 20% or lower when it drops below -10°F. Most homes that fog are running 48-55% in January.

Why do only some of my windows fog?

Three usual reasons: those windows are on the north or windward side, they have closed blinds trapping still air, or they're older units without low-e coating. Bathroom and kitchen windows fog first because that's where water gets released.

Will new windows fix condensation?

They'll reduce it, sometimes dramatically, because a modern low-e argon unit with a warm-edge spacer runs 10-15°F warmer on the interior surface. New windows also tighten the house, which can raise indoor humidity and move the fogging to the next-coldest surface. Spending $8,000 on windows to solve a $40 humidity problem is a bad trade.

Should I run a dehumidifier in the winter?

It works, but it's the expensive answer. A dehumidifier pulls 300-600 watts continuously and fights heat loss instead of finding the water source. Try exhaust ventilation and source control first, and reserve the dehumidifier for basements and crawlspaces where the moisture originates.

Is the black stuff on my window sill mold?

Usually yes, and it's typically Cladosporium or Aspergillus growing on paint and wood dust. Clean small spots with detergent and water, dry thoroughly, and repaint bare wood. If it keeps returning after the humidity is under control, water is getting into that sill from outside.

Why did this start after we air-sealed the attic?

You didn't make more moisture, you removed the leaks that used to carry it out. A house that goes from 6 to 3 air changes per hour at 50 pascals holds onto interior water vapor much longer. Tight houses need mechanical ventilation, and the window fog is the first sign it's missing.

Does condensation only at the bottom edge mean something's wrong?

It means the spacer bar at the perimeter is conducting cold, which is normal on aluminum-spacer units from the 1980s and 1990s. A one-inch band of fog at the bottom on a 5°F morning is expected. Water pooling on the sill by 9 a.m. is not.

Get your causes ranked

Describe what's happening or add a photo, and get your most likely cause free. If anything involves gas, burning smells, sparking, sewage, or active flooding, stop DIY and call a licensed pro or emergency services.

See a sample ranked report →

Takes about a minute · No account needed for your free preview

HomeSymptom Editorial Team

Home repair writers

HomeSymptom Editorial Review

Trade-experienced editors

Checked for practical accuracy and safety before publish.

Updated July 27, 2026How we reviewEditorial standards

Related guides

Symptom Guides

Fog Between Double-Pane Window Glass

Foggy condensation getting worse between your window panes? Here's why the seal failed, what a fix costs in 2026, and when to replace the glass unit.

Symptom Guides

Old Window Won't Stay Open

Your older double-pane window won't stay open? It's usually worn sash balances or broken cords. Here's how to diagnose it and what a fix runs in 2026.

Symptom Guides

Fog Comes and Goes Between Window Panes

Fog that appears and disappears between double-pane glass points to a failing seal. Here's why it's intermittent, what repairs cost in 2026, and DIY vs pro.

Symptom Guides

Cold Air Leaking Around the Window Frame

Feeling a draft at the frame rather than the glass? Air is moving through the shim gap between window and wall. Here's how to find it and what a fix costs.

Symptom Guides

Hazy film that won't clean off double-pane glass

Haze on double-pane glass that no cleaner touches usually means a failed seal or mineral etching. Here's how to tell which, and what replacement runs.