My Windows Fog Up or Have Condensation

Condensation on windows is common, but what it means depends entirely on where the moisture is. Interior condensation, condensation between the panes, and exterior condensation each have completely different causes — and one of them isn't a problem at all.

Quick Diagnosis (30-Second Checks)

1. Touch the moisture. Can you wipe it off from inside the house? That's interior condensation (humidity issue). Can you wipe it from outside? That's exterior condensation (normal). Can't reach it at all? It's between the panes (seal failure).
2. Which windows? All of them or just some? North-facing windows condense first because they're coldest. Bathroom and kitchen windows condense most because of moisture sources.
3. When does it happen? Cold mornings? After cooking or showering? All the time? Timing helps diagnose the cause.

Types of Window Condensation

Interior Condensation (Inside Surface)

What's happening: Warm, moist indoor air hits the cold glass surface and the moisture condenses — the same way a cold drink "sweats" on a warm day. This means your indoor humidity is too high for the temperature of the glass.

Common causes:

• Cooking, showering, running the dishwasher without ventilation
• New construction or fresh paint/concrete (off-gases moisture for months)
• Overuse of humidifiers
• Many houseplants
• Drying clothes indoors
• Large household (more people = more moisture from breathing, cooking, bathing)
• Poorly ventilated home (tight construction without mechanical ventilation)

Acceptable indoor humidity levels (winter):

| Outdoor Temp | Max Indoor Humidity | |-------------|-------------------| | 20F to 40F (-7C to 4C) | 35-40% | | 0F to 20F (-18C to -7C) | 25-35% | | Below 0F (-18C) | 15-25% |

Fix:

• Run bathroom exhaust fans during and 20 minutes after showers
• Use the kitchen range hood when cooking
• Turn down or off the humidifier
• Run a dehumidifier if needed
• Ensure dryer vents to the exterior (not the basement or attic)
• Open curtains/blinds during the day to warm glass surfaces
• Consider an HRV (Heat Recovery Ventilator) for tight homes — it exchanges indoor air without losing much heat. Cost: $1,500-$3,500 installed.

When to worry: Persistent heavy interior condensation in winter can cause mold growth on window sills and frames, paint peeling, and damage to wall cavities. Address the humidity source.

Between-Pane Condensation (Seal Failure)

What's happening: Double-pane and triple-pane windows have an insulating gas (argon or krypton) sealed between the panes. When the seal fails, the gas escapes and moist air enters the gap. You get a foggy, hazy appearance between the panes that you can't wipe away. It may come and go with temperature and humidity changes.

Why it happens: All window seals eventually fail. UV exposure, thermal cycling (expansion/contraction), and age degrade the sealant. Typical seal lifespan: 10-20 years. Some fail much sooner due to manufacturing defects.

Impact: The window loses a significant portion of its insulating value. Your energy costs increase. The cloudiness worsens over time.

Fix options:

1. Replace the insulated glass unit (IGU) — A glass company removes the failed pane assembly and installs a new sealed unit in the existing frame. Cost: $150-$400 per window. This is the most cost-effective option if the frame is in good condition.
2. Defog service — Some companies drill small holes in the glass, inject a cleaning solution, and install one-way vents. Cost: $75-$150 per window. Mixed long-term results — the insulating gas is gone permanently, so thermal performance remains reduced.
3. Replace the entire window — If the frame is also degraded or you want to upgrade to more efficient windows. Cost: $400-$1,200 per window installed.

Warranty note: Many window manufacturers offer 10-20 year seal warranties. Check your warranty before paying for repairs. Even if you're the second homeowner, some warranties transfer.

Exterior Condensation (Outside Surface)

What's happening: Condensation forms on the outside of the window when the glass surface is cooler than the outdoor dew point. This happens on cool mornings, especially with high-efficiency (low-E) windows that keep heat inside so well that the exterior glass stays cold.

This is a good sign. It means your windows are working as designed — they're keeping heat inside your home.

Fix: None needed. The condensation will evaporate as the sun warms the glass. If it bothers you, know that it's a confirmation of window quality, not a defect.

DIY Fixes

• Reduce indoor humidity (exhaust fans, dehumidifier, reduce humidifier use)
• Monitor humidity with a hygrometer ($10-$20)
• Open curtains during the day to warm glass surfaces
• Ensure exhaust fans are working and ducted to the exterior
• Check dryer vent termination (must go outside)
• Improve air circulation near windows (move furniture away from window sills)

When to Call a Pro

• Condensation between the panes — glass unit needs replacement
• Persistent heavy interior condensation despite humidity control — may indicate ventilation system issues
• Mold growing on window frames or sills — needs remediation and humidity source addressed
• Many windows have failed seals — consider whole-house window replacement
• Humidity stays above 50% despite exhaust fans — may need an HRV/ERV system

Pro Detail

Diagnostic Procedures

1. Hygrometer survey — Measure relative humidity in each room to identify moisture sources.
2. Window inspection — Check seal condition, measure U-value with a thermal transmittance meter if available.
3. Thermal imaging — Reveals which windows have failed seals (they show distinctly different thermal patterns than intact windows).
4. Ventilation assessment — Verify exhaust fan CFM ratings, check ductwork, and ensure adequate whole-house ventilation rate.

Prevention

• Maintain indoor humidity at 30-40% in winter (use a hygrometer to monitor)
• Always run exhaust fans when cooking and bathing
• Service exhaust fans annually — clean blades and check duct connections
• Open windows briefly on mild days to exchange moist air for dry air
• When replacing windows — choose quality manufacturers with strong seal warranties
• Don't apply window tint to sealed glass units — the added heat can accelerate seal failure

Cost Guide

| Service | Typical Cost | Notes | |---------|-------------|-------| | Hygrometer | $10-$20 | DIY monitoring | | Dehumidifier (portable) | $200-$400 | For high-humidity areas | | Exhaust fan upgrade | $150-$400 | Proper CFM and ducting | | Glass unit replacement (IGU) | $150-$400 per window | Seal failure repair | | Window defogging service | $75-$150 per window | Temporary solution | | Full window replacement | $400-$1,200 per window | Including installation | | HRV/ERV system | $1,500-$3,500 | Whole-house ventilation |

Shipshape Integration

SAM monitors the conditions that cause window condensation:

• Humidity monitoring tracks indoor levels room by room and alerts when they exceed recommended ranges
• Window inventory in the home profile tracks window age, type, and warranty status
• Seasonal reminders to adjust humidifier settings as outdoor temperatures change
• Energy correlation — SAM detects when failed window seals contribute to increased energy consumption
• Dealer coordination for window assessments, glass unit replacements, and ventilation upgrades