Understanding Why Windows Fog Up
A cloudy or misted appearance inside your double-glazed windows is one of the most common problems Irish homeowners face. At first glance, it can look like ordinary condensation — but where that condensation appears tells you everything about the cause and the cure.
If the moisture is on the surface you can touch, it’s typically a ventilation or temperature issue within the room.
If the misting is inside the sealed unit between the two panes, that’s a sign of unit failure — and the only solution is to replace the glass.
Condensation on the Inside Surface (Touch-Side Moisture)
When condensation forms on the inner pane — the side you can touch from inside your home — it’s not a glazing fault. It’s simply warm, moisture-laden air in your room meeting a cooler surface.
Common causes:
- Poor ventilation – no extractor fan in kitchens or bathrooms.
- Drying clothes indoors – raises humidity quickly.
- Blocked air vents – reduces airflow and traps moisture.
- Low room temperature – cooler glass encourages condensation.
How to reduce it:
- Keep window trickle vents open.
- Run extractor fans or briefly open windows after showers or cooking.
- Use a dehumidifier during damp winter months.
- Maintain a consistent background temperature throughout the day.
These simple steps can significantly reduce visible surface moisture without touching the glass itself.
Condensation Between the Panes (The Real Problem)
If misting or fogging appears inside the glass unit, between the two panes, that means your double-glazed unit has failed.
What’s actually happening:
Each double-glazed unit is made of two panes of glass sealed around the edges with a spacer bar and airtight sealant.
Inside that sealed cavity is either dry air or argon gas, which provides insulation.
Over time, especially in older windows, the edge seals break down due to:
- Natural ageing and weathering,
- UV exposure,
- Flexing of the frame or minor impact,
- Poor installation or trapped moisture during manufacturing.
Once the seal fails, moist air from outside enters the cavity. When the temperature drops, that moisture condenses on the colder interior surfaces — creating the cloudy, misted look you can’t wipe away.
Why Fogged Windows Need Replacement
Unlike surface condensation, there’s no permanent fix for moisture trapped between panes.
Temporary “de-misting” services or drill-hole drying kits might offer short-term clarity, but they don’t restore the insulation or argon gas — so the window remains inefficient.
The only real solution is to replace the glass unit (called the DGU – double-glazed unit) while keeping your existing frames if they’re still in good condition.
This process is quick, clean, and far more cost-effective than full window replacement.
Upgrading When You Replace Fogged Units
A failed double-glazed unit is also an opportunity to upgrade your home’s comfort and efficiency.
Modern A-rated Low-E (low-emissivity) double glazing uses advanced coatings and argon-filled cavities that reflect heat back into your room while letting sunlight in.
Benefits of upgrading:
- Up to 70% more energy-efficient than older failed units.
- Noticeably warmer rooms and fewer cold spots.
- Reduced heating costs during winter.
- Clearer glass with less condensation risk on the inner pane.
- Improved sound insulation and UV protection for furnishings.
In most Irish homes built between 1995 – 2010, the original glazing is now well past its peak performance. Replacing just the glass — not the frame — with new A-rated units can transform how the house feels in both comfort and efficiency.
How Double Glazing Replacement Works
Professional glaziers can usually replace a fogged unit within an hour.
- The failed glass is removed carefully from the existing frame.
- A new, made-to-measure A-rated unit is fitted and sealed.
- Old glazing and spacers are recycled responsibly.
No plastering, painting, or frame work required — just immediate clarity and improved insulation.
Frequently Asked Questions
Can I fix a fogged window myself?
No. Once moisture gets between the panes, the airtight seal is broken and cannot be resealed without replacing the entire unit.
Will replacing the glass fix my drafts too?
If you also have worn seals or hinges, replacing them at the same time will restore full airtightness. Most glazing companies can check this during a site visit.
Is it expensive to replace just the glass?
Not compared to full window replacement. In most cases, glass-only replacement is 60–70% cheaper than fitting new frames.
Will my new glass match the rest of my windows?
Yes. Modern units can be made to match existing tint and frame profiles perfectly.
Preventing Future Failures
- Choose A-rated double-glazed units with edge-seal technology.
- Ensure correct installation and full compression in the frame.
- Maintain ventilation and balanced humidity indoors.
- Periodically clean frames and keep drainage slots clear.
A well-installed modern DGU can last 20 years or more with minimal maintenance.
Why Choose Professional Replacement
Replacing fogged or failed double glazing might look simple, but professional installation ensures:
- Perfect airtight sealing,
- Correct spacer alignment and glazing tape compression,
- Verified A-rated performance, and
- Long-term warranty coverage.
DIY or low-cost shortcuts often leave small gaps that let moisture back in, repeating the problem within a year or two.
Upgrade Once – Benefit for Decades
When your windows start fogging internally, take it as a sign — your old units are no longer insulating effectively.
By upgrading to new A-rated Low-E double glazing, you can restore clarity, reduce energy waste, and extend the life of your existing window frames.
Replacing only the glass keeps your home comfortable and your costs sensible.
For Expert Fogged-Glass Replacement in Dublin & Wicklow
If you’ve noticed condensation trapped between your panes, get it checked early.
P. Fahy Glazing Ltd provides professional fogged and misted double glazing replacement across Dublin and Wicklow — including same-day assessments and A-rated glass upgrades.
🔗 Visit P. Fahy Glazing Ltd to learn more or request a free quote.