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You wake up on a cold morning and your windows are covered in moisture. Should you be concerned? The answer depends on WHERE the condensation appears — on the inside surface, between the panes, or on the outside.
Interior Condensation (Inside Surface)
Condensation on the room-facing side of your windows is the most common type and usually not a window problem — it's a humidity problem. When warm, humid indoor air contacts the cold glass surface, moisture condenses. This is especially common in kitchens, bathrooms, and during cold snaps. Solutions include running exhaust fans, using a dehumidifier, and ensuring adequate air circulation.
Condensation Between Panes (Seal Failure)
This is the problematic one. Fog, moisture, or a milky appearance between the glass panes means the sealed insulating glass unit has failed. The argon or krypton gas has leaked out and been replaced by moist air. This permanently reduces the window's insulating performance by 30-50% and will only get worse over time.
Exterior Condensation (Outside Surface)
Moisture on the outside of your windows is actually a sign that your windows are working well. It means the glass is cold enough (because it's insulating so well) that dew forms on the exterior surface, just like it does on your car windshield. This is common with high-performance Low-E windows on humid mornings.
When to Replace vs. Repair
If you have condensation between panes, you have two options: replace just the insulated glass unit (IGU) or replace the entire window. IGU replacement costs $150-$400 per window and preserves the existing frame. However, if the frames are also showing wear, full replacement provides a better long-term solution and comes with a complete warranty.
Preventing Interior Condensation
Keep indoor humidity between 30-50% in winter (use a hygrometer to check). Run bathroom and kitchen exhaust fans during and 15 minutes after moisture-producing activities. Ensure your home has adequate air exchange — modern tight construction sometimes requires a heat recovery ventilator (HRV) to manage moisture without losing heat.