Why PVC windows fog up and how to fix it

Why PVC windows fog up and how to fix it

Fogging on PVC windows is a common issue in apartments and houses. Condensation appears when warm, humid indoor air meets a cold window surface. In this guide we explain the main causes, quick fixes and long‑term solutions to prevent recurring fogging.

How condensation forms

• Warm moist air contacts a cooler surface and releases moisture as droplets.
• Windows and frames can become condensation points when room humidity is high or ventilation is poor.
• Persistent moisture can lead to mold growth and damage to window seals and finishes.

Main reasons windows fog up

1. High indoor humidity from cooking, drying laundry, showers and plants.
2. Poor ventilation or blocked air inlets and exhausts.
3. Incorrect window usage: windows kept fully closed for long periods or lack of micro‑ventilation.
4. Cold bridges around the frame due to insufficient insulation or poor installation.
5. Worn or damaged seals and joints that allow cold air ingress.
6. Moisture inside the glazing unit — a sign of broken seal or failed insulating glass unit.

Quick actions you can take

• Wipe the glass and frames with a dry cloth to remove water and prevent damage.
• Increase airing: open the window for 10–15 minutes to replace humid air with drier outside air.
• Use exhaust fans in the kitchen and bathroom during and after use.
• Place desiccant packs near problem windows for temporary moisture control.

Medium-term fixes

• Check and clean ventilation grills and ducts; ensure they are not blocked.
• Use micro‑ventilation modes on the window or install a trickle vent for controlled fresh air intake.
• Replace old seals and gaskets to restore proper sealing and reduce cold air infiltration.
• Improve insulation around the opening to remove cold bridges.

Long-term solutions

• Install or upgrade a mechanical ventilation system with heat recovery (MVHR) to control humidity and heat loss.
• If moisture is trapped between panes, replace the insulating glass unit — this requires a professional.
• Proper installation and sealing during window replacement reduce future condensation risks.

Practical tips for kitchen and bathroom

• Cover pots while cooking and run the extractor hood.
• Keep the bathroom extractor running for 20–30 minutes after showering.
• Allow air circulation under doors or add vents to maintain airflow.

Condensation between panes — what it means

Moisture between panes usually means the unit has lost its seal. This cannot be fixed by wiping and requires replacement or professional resealing.

When to call a professional

• Condensation appears permanently between panes.
• You notice mold that returns despite cleaning and airing.
• There is visible frost or significant icing on the frame and sill.
• Seals look damaged or the window is drafty.

FAQ

Why does only the bottom of the window fog up?

The bottom cools faster because of cold surfaces near the sill and possible thermal bridges. Insulating the sill and improving sealing helps.

Will a heater or radiator solve the problem?

Raising surface temperature helps locally, but without reducing humidity or improving ventilation the issue will persist. Combine heating with ventilation measures.

Is it OK to dry laundry next to windows?

Drying indoors near windows increases local humidity and encourages condensation. Dry clothes in well‑ventilated areas or use a dryer with external venting.

What if the glass looks cloudy inside the unit?

Clouding inside the insulating glass unit means the seal is compromised. Contact a glazing specialist for repair or replacement.

Do desiccants work long term?

They help in small spaces temporarily but do not solve the root cause. Use them alongside ventilation and humidity control.

Summary

• Fogging occurs when humid indoor air meets cold window surfaces.
• Quick fixes: wipe, ventilate briefly, use extractor fans and desiccants.
• Better ventilation and insulation remove the root causes.
• Moisture between panes indicates a failed sealed unit and needs professional repair.
• Proper seals and correct installation reduce thermal bridges.
• Mechanical ventilation with heat recovery is the most reliable long‑term solution.