Introduction

You fit new windows, everything is plumb and true… and the client texts you the next morning: “The glass is wet. They’re faulty.” Sound familiar? Condensation after window swaps is one of the most common (and frustrating) callbacks. The windows aren’t usually the problem—excess indoor moisture and cold surfaces are. In this guide, we’ll break down why it happens, what fixes actually work, and how to prevent the complaint before it starts. You’ll get practical steps you can take today, long‑term solutions that stick, and proposal language that protects your margin.

Table of Contents

• Key Takeaways
• The Condensation Problem: Why It Hits After Window Jobs
• Quick Wins: Immediate Steps That Calm Complaints
• Long-Term Fixes: Ventilation And Install Details
• Site Moisture Loads After Renovation
• Prevention And Proposal Language That Protects You
• Frequently Asked Questions
• Conclusion

Key Takeaways

• Condensation is a humidity and temperature issue, not usually a product defect. In general, at 20°C: 50% RH gives a dew point around 9–10°C; 60% RH around 12°C.
• Quick wins work: boost extract for 10–20 minutes after showers/cooking, open trickle vents, and use short‑term dehumidification during drying periods.
• According to UK Part F guidance, typical intermittent extract rates are around 15 l/s for bathrooms and 30 l/s for kitchens—undersized fans drive many complaints.
• Commonly, glass edges run 2–4°C cooler than the centre; this is where condensation starts first, especially with metal spacers or cold bridging.
• Contractors often report winter complaints peaking in the first 2–4 weeks post‑install due to residual site moisture—plan drying and set expectations.

The Condensation Problem: Why It Hits After Window Jobs

Many contractors find the complaint lands right after a clean install in cold weather. The physics is simple: warm air holds moisture; when it meets a surface at or below the dew point, water condenses.

What’s Really Happening

• In general, at 20°C indoor temperature: 50% RH gives a dew point around 9–10°C; 60% RH around 12°C. If any frame or glass edge sits near those temperatures, it will bead.
• Commonly, the glass edge is 2–4°C cooler than the centre because of spacer effects and perimeter heat loss.
• New windows improve airtightness, which is good for energy—but without adequate ventilation, indoor RH rises.

Why It Matters

• Callbacks eat hours, and refunds kill margin.
• Replacing “faulty” units rarely changes anything because the moisture driver is still there.

Field Example

You swap out single glazing for modern double glazing in mid‑winter. Next morning, internal glass edges are wet. You measure 21°C and 62% RH inside (a pocket hygrometer costs less than £15). That puts the dew point roughly around 13°C. Perimeter glass measured with an IR thermometer is 12–13°C. That’s a match: humidity, not a leak.

Quick Wins: Immediate Steps That Calm Complaints

When the phone rings, you need fast, defensible actions that show you’re on it.

Step 1: Verify With Simple Measurements

• Take indoor temperature and RH in the wettest room and near the window.
• Take surface temperature at the glass edge and mid‑pane with an IR thermometer.
• If surface temp is near or below the dew point, it’s a moisture issue. In general, documenting this reduces back‑and‑forth by half.

Step 2: Reduce Moisture Spikes Today

• Run bathroom and kitchen extract on boost for 10–20 minutes after use.
• Open trickle vents and crack internal doors to move air.
• Avoid drying clothes indoors; if unavoidable, run a dehumidifier on a timer.

Step 3: Heat And Vent Together

• Low, steady background heat plus ventilation clears moisture faster than “blast heat” alone.
• Many contractors report that within 24–72 hours of consistent extract and moderate heat, visible condensation falls dramatically.

Real‑World Example

A flat with no trickle vents and a tired 80 mm bathroom fan was fogging windows daily. You fit a 100 mm fan with a humidity/overrun controller and set a 15‑minute run‑on. Within two days, morning puddles on sills disappear, and the complaint stops.

Long-Term Fixes: Ventilation And Install Details

Short‑term steps calm things down. Long‑term, you need the right ventilation and careful install detailing.

Ventilation That Meets Guidance

• According to UK Building Regulations Part F, typical intermittent extract rates are around 15 l/s for bathrooms and 30 l/s for kitchens (higher if at the hob). Continuous systems use lower rates but run all day.
• Commonly, poorly ducted fans lose performance—long flexible ducts and multiple bends can cut effective airflow significantly.

Practical Upgrades

• Fit 100 mm fans with back‑draft shutters and set run‑on to 10–20 minutes.
• Keep ducts short and straight; use rigid ducting where possible.
• Consider continuous extract (MEV) where moisture loads are high.

Install Details That Raise Surface Temperatures

• Use warm‑edge spacers where the spec allows—edge temperatures improve and condensation risk drops.
• Insulate and air‑seal the perimeter—cavity closers, foam/rope backer, and a neat internal seal reduce cold bridges.
• Avoid over‑packing with metal shims at edges; create thermal breaks where needed.

Example: Edge Condensation Cured

A bay window with aluminium spacers showed edge fogging on cold mornings. You added insulated cavity closers and improved the internal air seal, then tweaked ventilation settings. After works, spot checks showed the edge 2°C warmer under the same weather—no more droplets.

Site Moisture Loads After Renovation

Even if you only changed windows, the home may be carrying moisture from other recent works.

Wet Trades = Elevated RH

• Commonly, fresh plaster/render can push indoor RH above 65–75% for 2–6 weeks, especially in winter.
• Timber, screeds, and paint all release moisture while curing.

Control The Drying Curve

• Heat gently and ventilate: trickle vents open, fans on timers, doors ajar.
• Short‑term dehumidifiers help; aim to keep indoor RH around 40–55% during drying to avoid condensation and mould risk.

Example: Post‑Plaster Plan

You finish a window swap in a kitchen that was replastered a week prior. You leave written guidance: run extract on boost after cooking, keep trickle vents open, and heat steadily. You book a 7‑day check‑in. By day 7, RH drops from 68% to 52% and windows are clear.

Prevention And Proposal Language That Protects You

Set expectations early and in writing. It avoids arguments and saves hours later.

Clear, Plain‑English Notes To Include

• “Condensation is caused by indoor humidity contacting cold surfaces. New windows improve airtightness; ventilation may need adjustment.”
• “Target indoor RH between 40–55% in winter. Use extract fans for 10–20 minutes after bathing/cooking and keep trickle vents open.”
• “Initial condensation can be higher for 2–4 weeks after wet trades. This is normal while the home dries.”

Symptom → Likely Cause → Fix

How Donizo Helps You Lock This In

When you’re on site and the client asks, “Will these windows mist up?”, capture the explanation on the spot. With Donizo, you can:

• Use voice, text and photos to create a clear, branded proposal that includes your moisture/ventilation notes.
• Send a professional PDF with client portal access and get legally binding acceptance via e‑signature.
• When they accept, convert the proposal to an invoice in one click—no re‑typing.

Many contractors find that including photos of trickle vents, fan settings, and a one‑paragraph RH note in the proposal reduces follow‑up queries significantly.

Frequently Asked Questions

Is Condensation A Window Defect?

Generally, no. Condensation is a sign the surface is at or below the dew point. In general, at 20°C and 50–60% RH, the dew point is around 9–12°C. If the glass edge is near that, you’ll see droplets even with perfect units. Product defects are rare and usually show as sealed‑unit failure (fogging inside the double‑glazed cavity), not on the room‑side surface.

How Do I Prove It’s Humidity And Not A Leak?

Measure and document. Record indoor temperature and RH, and take a glass edge surface temperature with an IR thermometer. If surface temp sits near the dew point, it’s a humidity issue. Add photos and a brief note. Many contractors report that a simple on‑site reading set resolves disputes quickly.

Should I Retrofit Trickle Vents?

If background ventilation is lacking, yes—especially in airtight homes. According to Part F principles, background ventilators are sized by dwelling and room type; trickle vents contribute the “always‑on” airflow that helps keep RH in check. Pair them with effective extract for best results.

What RH Should Clients Aim For In Winter?

Aim for 40–55% RH. Below 40% feels too dry; above 60% increases condensation and mould risk. Use extract fans after moisture events and keep trickle vents open. Short‑term dehumidification helps during drying periods.

When Do I Need A Specialist?

If you’ve implemented ventilation, checked installation details, and RH remains persistently high with mould growth, call in a ventilation or building‑physics specialist. Look for undersized fans, poor ducting, cold bridges at reveals, or unusual moisture sources.

Conclusion

Condensation callbacks aren’t a mystery—they’re physics. Verify with a quick set of measurements, calm things down with airflow and gentle heat, and fix the root causes with proper ventilation and warm edges. Set expectations in writing so you don’t pay for misunderstandings. If you want to capture those notes fast and send a professional, signable proposal before you drive off, use Donizo. Speak your scope, add photos, send for e‑signature, and convert to an invoice in one click. Less admin, fewer callbacks, stronger margin.