Window Condensation: Causes, Types, and How to Fix It
Condensation on windows is one of the most common homeowner complaints — and one of the most misunderstood. The appearance of moisture on or between panes triggers worry about mold, damaged seals, and energy loss. But not all condensation signals a problem. Some of it is actually a sign that your home is working correctly.
This guide explains the three types of window condensation — interior, exterior, and between panes — and tells you what to do about each one.
Why Condensation Forms
Water vapor is always present in air. When air containing water vapor contacts a surface cool enough to bring the air below its dew point, moisture condenses on that surface.
The dew point depends on relative humidity. Air at 60% relative humidity and 70°F has a dew point of around 55°F — meaning any surface below 55°F will collect condensation from that air.
Window glass is often the coldest surface in a room, especially at the edges and in corners. This is why windows show condensation before walls do — the same physics is at work, but walls are warmer.
Interior Window Condensation
What It Is
Interior condensation forms on the room-facing side of window glass. It’s the moisture you can wipe away with your hand. It typically appears overnight or in the morning, when indoor temperatures drop and relative humidity peaks.
What Causes It
Interior condensation means one of two things: the indoor humidity is too high, the window’s interior surface is too cold, or both.
High indoor humidity sources:
- Cooking without exhaust ventilation
- Long showers without exhaust fans running
- Houseplants (significant moisture transpiration in groups)
- Gas appliances (combustion produces water vapor)
- Humidifiers set too high for outdoor temperatures
- Poorly sealed crawl spaces or basements
- New construction (building materials release moisture for 1–2 years)
Cold interior glass surfaces:
- Single pane windows
- Low-quality double pane with high U-factor
- Damaged or compressed weather stripping allowing cold drafts
- Rooms with inadequate heat distribution
Is It a Problem?
Some interior condensation is normal. A bathroom mirror fogs after a shower. Kitchen windows may cloud when cooking. These are brief and clear as the room ventilates.
Persistent interior condensation — lasting hours, or present daily — signals that either humidity is chronically high or windows are inadequate for your climate. Both deserve attention.
Moisture risks from persistent condensation:
- Mold growth on window frames, sills, and surrounding drywall
- Wood frame rot
- Paint peeling at window surrounds
- Damp walls adjacent to windows
How to Fix Interior Condensation
Reduce indoor humidity first. This is the primary solution for most homes.
- Run bathroom exhaust fans during and 20–30 minutes after showers
- Use range hood exhaust when cooking
- Run a dehumidifier in chronically humid areas (basement, main floor in shoulder seasons)
- Ensure dryer vents exhaust outside, not into the crawl space or attic
- Check that crawl spaces have vapor barriers
Target relative humidity for indoor temperature:
| Outdoor Temperature | Recommended Indoor RH |
|---|---|
| Above 20°F | 35–40% |
| 10°F to 20°F | 30–35% |
| 0°F to 10°F | 25–30% |
| Below 0°F | 15–20% |
These numbers reflect the balance between comfort and condensation risk. At lower outdoor temperatures, even moderate indoor humidity will condense on cold glass.
Improve air circulation. Condensation is worst where air stagnates. Running ceiling fans, ensuring furniture doesn’t block window airflow, and keeping window treatments open during the day all help warm the glass surface and prevent moisture accumulation.
Upgrade windows. If condensation is appearing on quality double pane windows that are properly installed, humidity management is likely the issue. But if older single pane or low-quality windows are condensing despite reasonable indoor humidity, window replacement addresses the root cause.
Exterior Window Condensation
What It Is
Exterior condensation forms on the outside surface of the glass. It’s moisture you cannot wipe from inside — it’s on the exterior face of the outermost pane. It often appears on clear mornings and burns off within an hour of sunrise.
What Causes It
This is actually a sign of high window performance. High-efficiency windows insulate so well that the exterior glass surface stays cold overnight, below the outdoor dew point. Moisture then condenses from the outdoor air onto the cold exterior glass.
Exterior condensation is most common in spring and fall when nights are cold and humid. It’s also more prevalent on north-facing windows that don’t receive direct morning sun.
Is It a Problem?
No. Exterior condensation is harmless and actually indicates your windows are doing their job. The glass surface is cold because heat is not escaping through it — exactly what you want in an energy-efficient window.
The condensation evaporates as temperatures rise and causes no damage to the window or frame.
What You Can Do
Wait. Exterior condensation typically clears within 30–60 minutes of sunrise, often sooner. Nothing about the window is malfunctioning.
If you find it annoying, there’s no cost-effective fix — it’s a consequence of the window performing well. Running the AC or heating system to keep more interior heat in the glass won’t help and wastes energy. Using window treatments to block nighttime exterior cooling slightly reduces the effect but is generally not worth the bother.
Condensation Between Window Panes
What It Is
Condensation between panes is the most concerning type. It appears as fogging, cloudiness, or visible droplets in the sealed airspace between the glass layers of a double or triple pane window. You cannot wipe it away — it’s sealed inside.
This is sometimes called “failed seal” or “seal failure” and is a sign that the insulating unit has been compromised.
What Causes It
Double and triple pane windows are sealed units. The glass panes are bonded at their edges with a primary seal (butyl rubber) and a secondary sealant, creating a hermetic space filled with gas (usually argon). Desiccant — a moisture-absorbing material — is embedded in the spacer bar at the perimeter.
When this sealed unit fails, two things happen:
- The insulating gas slowly escapes
- Outdoor air — containing moisture — enters the space
- The desiccant, designed for a small amount of residual moisture, becomes saturated
- As temperatures cycle, moisture condenses inside the sealed space
Common causes of seal failure:
Age: Most window seals last 10–20 years. Natural material degradation eventually allows moisture infiltration. This is the most common cause of seal failure in older homes.
Poor installation: Windows installed out of level, or without proper shimming and fastening, can flex under wind and thermal load in ways that stress seals.
Thermal stress: Rapid, repeated temperature cycling degrades seals faster. South-facing windows in hot climates experience more thermal cycling and often fail sooner.
Improper cleaning products: Harsh chemicals, pressure washing directly at window edges, or petroleum-based products can degrade the rubber sealants.
Physical damage: Chips or cracks in the glass edge, damage to the spacer during installation, or impact near the perimeter can compromise seals.
Low-quality manufacturing: Budget windows often use lower-grade sealants and spacers. Aluminum spacers conduct temperature more readily and create more thermal cycling stress than warm-edge spacers.
Is It a Problem?
Yes — seal failure is a real problem, though not an emergency. The practical consequences:
Energy loss: The insulating gas has been replaced by air, and moisture in the air further degrades insulating performance. The window may drop from a U-factor of 0.28 back toward 0.45–0.55, closer to a single pane.
Permanent cloudiness: Once desiccant is saturated, condensation cycles will deposit mineral residue on the glass surfaces. Even after the fogging clears on warm days, a hazy residue often remains and worsens over time.
No self-repair: Seal failure doesn’t get better. Once the spacer desiccant saturates, the fogging will worsen through future temperature cycles.
How to Fix Condensation Between Panes
Option 1: Replace the insulating glass unit (IGU)
Most window frames allow replacing just the glass unit without replacing the frame. A glazier measures the unit, orders a replacement, and installs it in the existing frame.
Typical cost: $75–$200 per unit, depending on size and glass type. Labor adds $50–$150 per window.
This is the right fix when the frame is in good condition and the windows are otherwise functional. It restores full insulating performance and appearance.
Option 2: Replace the entire window
If the frame is damaged, deteriorated, or the windows are old enough that multiple units are failing or will soon fail, full window replacement makes more sense.
Full replacement costs $300–$800 per window installed for double pane, depending on size and frame material.
Option 3: “Defogging” services
Some contractors offer a defogging service: they drill holes in the glass unit, inject cleaning solution, then seal the holes and insert new desiccant.
This is controversial. It can temporarily reduce the visual fogging but doesn’t restore the original sealed unit or its energy performance. Many professionals don’t recommend it. The results are mixed and often temporary.
Option 4: Do nothing
If the visual effect is tolerable and the window is otherwise functional, deferred replacement is a legitimate choice. The energy impact of a single failed unit in a multi-window home is modest. Address it when the budget allows.
Identifying the Type You Have
Use this quick guide:
You can wipe it away from inside: Interior condensation. Address humidity and ventilation first.
It’s on the outside and clears by mid-morning: Exterior condensation. This is fine — your windows are performing well.
It’s inside the sealed unit and won’t wipe away: Seal failure. Budget for IGU replacement or full window replacement.
It appears at the frame rather than on the glass: Check weather stripping, caulk around the window exterior, and look for gaps at the window-wall interface. This is often an air sealing issue, not a glass issue.
Preventing Future Condensation Problems
For Interior Condensation
- Keep indoor humidity in the recommended range for your outdoor temperature
- Run exhaust fans consistently
- Ensure crawl spaces and basements are properly sealed and dehumidified
For Seal Failure Prevention
- Clean windows with mild soap and water, avoiding chemicals that degrade rubber
- Don’t pressure wash directly at window edges
- Inspect and touch up exterior caulk annually — cracked caulk lets water infiltrate the frame area and speeds seal degradation
- Choose quality windows with warm-edge spacers (foam or composite rather than aluminum) when replacing
When to Call a Professional
If you’re seeing widespread seal failure across multiple windows, interior condensation despite reasonable humidity management, or if you’re finding moisture damage at window frames or surrounding walls, a professional assessment is worthwhile. A window contractor or energy auditor can identify systemic problems — whether that’s window age, installation defects, or building envelope issues — and give you a repair or replacement plan.