Why PVC windows fog up and how to prevent it

Why PVC windows fog up and how to prevent it

Fogged windows are a common issue in apartments and houses. Condensation on glass not only spoils the view but can also lead to mold, damaged sills, and poorer indoor air quality. This guide explains the causes of window fogging and gives practical prevention tips.

What causes condensation?

• Temperature difference: cold outside glass and warm indoor air create condensation on the glass surface.
• High indoor humidity: cooking, drying clothes indoors, showers without ventilation, many houseplants.
• Poor ventilation: modern PVC windows are airtight, so without proper air exchange humidity builds up.
• Installation issues or worn seals: drafts and cold spots form when seals are damaged or the frame is improperly installed.
• Failed insulating glass unit (IGU): fogging between panes indicates a broken seal and moisture inside the unit.
• Thermal bridges: cold sections in the wall, sill, or window reveal areas where glass cools down faster.

How to prevent fogging — practical tips

• Ventilate regularly: short, intense airing (5–10 minutes) several times a day is more effective than leaving a window ajar.
• Use trickle ventilation sparingly: it’s convenient but often insufficient to remove excess moisture.
• Control humidity: keep relative humidity around 40–60%. Use a dehumidifier when needed.
• Install supply vents or a balanced ventilation system with heat recovery to maintain fresh air without heat loss.
• Avoid placing furniture flush against exterior walls or the window — allow air circulation near the glass.
• Check and replace seals and adjust hardware: worn gaskets reduce performance and create cold drafts.
• Clean drainage channels at the bottom of the frame to ensure moisture is evacuated.
• If condensation appears between panes, contact a specialist — the IGU may need replacement.
• Choose energy-efficient IGUs and warm-edge spacer bars to reduce cold edges and condensation risk.
• Window coverings: heavy curtains that press against the glass can trap moisture; keep coverings slightly away from the pane.

Quick care and handling

• Wipe condensation with a microfiber cloth and then ventilate the room.
• Treat any mold with an appropriate cleaner and address the root cause of humidity.
• Avoid harsh chemicals or abrasive cleaners on seals and frames.

When to call professionals

• Condensation between panes almost always means the insulating unit has failed and should be replaced.
• If you notice frame deformation, persistent drafts, or water pooling, call an installation/service team.
• If simple measures (ventilation, dehumidifier) don’t help, consider installing mechanical ventilation or supply vents.

Frequently Asked Questions (FAQ)

Is it normal for windows to fog in winter?

A small amount of condensation in very cold weather can be normal, but frequent or heavy fogging shows excessive indoor humidity or ventilation issues.

Will replacing the IGU eliminate the problem?

A high-quality IGU with a warm-edge spacer lowers condensation risk but won’t fix issues caused by poor ventilation or high humidity.

Can I drill holes to ventilate the room?

No — drilling reduces thermal performance and can cause other problems. Install adjustable supply vents instead.

How often should I ventilate?

Aim for 2–4 short airings per day (5–10 minutes), and more often during cooking or showering.

How do I know the IGU has lost its seal?

Fogging or moisture trapped between glass panes and visible dirt or streaking inside the unit are signs of a failed seal.

How to avoid mold near windows?

Keep humidity under control, ventilate regularly, and repair leaks or damaged sills promptly.