Condensation on windows becomes more common in winter but did you know where the moisture appears can tell you a lot about the quality of your insulating glass units (IGUs)? In this FAQ, we answer common questions about foggy windows and show how Sparklike tools help glass manufacturers ensure airtight sealing and long-lasting energy performance.
Why do windows fog up in winter?
There are two common types of foggy windows:
Inside condensation (on the room-side surface):
This usually happens when indoor humidity is high from cooking, showering, drying laundry, etc. and the warm, moist air hits a cold window surface. This doesn’t automatically reflect a problem with the IGU itself, but it can indicate excess humidity and a need for better ventilation.
Outside condensation (on the exterior surface):
This shows that the cold stays outside and the inside stays warm a sign that your insulating glass unit is working well thermally. Good IGUs often show this behavior when outdoor temperatures drop.
What does fogging tell us about the condition of an IGU?
Different types of fogging help distinguish between normal environmental behavior and potential issues inside an insulating glass unit. While surface condensation is usually related to temperature and humidity, condensation inside the IGU cavity is directly linked to seal performance.
A properly sealed IGU prevents moisture from entering the space between the panes and keeps the insulating gas trapped inside. When fogging appears between the panes, it indicates that the seal has lost its airtightness. This allows moisture to penetrate the unit and often coincides with insulating gas loss — even if the IGU still looks intact from the outside.
This is why fogging is not just a comfort or visual issue. It can be an early indicator of seal failure and declining thermal performance, making seal integrity a critical factor for long-term IGU quality.
Why is seal integrity important for IGUs?
Airtight sealing in an IGU ensures the insulating gas, like argon, remains in the cavity between the panes. That gas is critical for:
- Thermal efficiency — better insulation keeps heat inside.
- Energy savings — less heat loss means lower heating costs.
- Long-term performance — gas retention maintains performance over time.
If the seal loses integrity and gas escapes, the insulation drops, leading to condensation issues, reduced comfort, and potential product failures.
How can manufacturers check if an IGU seal is airtight without damaging the unit?
Sparklike devices use non-invasive gas concentration measurement to determine if the insulating gas inside an IGU has been retained. This method lets quality control teams verify gas fill levels without breaking the glass or affecting the unit’s performance. Tools include:
- Sparklike Handheld™ – portable for double-glazed IGUs
- Sparklike Laser Portable™ – for double and triple glazed units with coatings and lamination
- Sparklike Laser Integrated™ – built into production lines for automated quality control of gas press
Can you give a real example of this in action?
Yes — In our blog post Weihai Blue Star Special Glass used the Sparklike Handheld™ device to prove their IGU seals were airtight:
In a demonstration video, the company submerged a finished IGU in water and then measured the insulating gas concentration afterward. Even after immersion, the gas level stayed stable, confirming both the seal and the fill process were reliable. This fun but technically meaningful test gave the manufacturer confidence in their production quality and helped reinforce customer trust in their product’s performance.
What benefits does non-invasive gas testing offer?
- Improved quality assurance — confirms gas fill and seal performance without destruction
- Operational efficiency — fast checks save time and reduce waste
- Greater customer confidence — manufacturers can prove long-lasting insulation performance.
How does this help with product competitiveness?
By verifying airtight seals and proper gas retention, glass producers can market IGUs with quantifiable performance benefits. This reassures architects, installers, and end-users that their windows will deliver on energy efficiency and durability even under demanding conditions.
If you want to integrate non-invasive gas measurement into your quality control process, Sparklike devices can help you test faster, measure better, and deliver superior IGUs worldwide. Contact Sparklike or your local distributor to explore solutions tailored to your production line.