Window Condensation: What Causes It and How to Fix It

Not all window condensation is a problem — but some of it definitely is. Understanding where the condensation is forming tells you exactly what is wrong and what to do about it. Here is a clear explanation.

What This Guide Covers

• Internal condensation: caused by high humidity — improve ventilation, not the windows
• Between the panes: sealed unit has failed — unit replacement needed, not full window
• External condensation: a sign your glazing is working well (low U-value) — not a problem
• Sealed unit replacement cost: £60–£250 per unit — much cheaper than full window replacement
• Persistent internal condensation can cause mould — address the source, not just the symptom

Introduction

London homeowners face unique challenges when it comes to glazing — from period properties with sash windows in conservation areas, to modern developments requiring structural glass installations. Getting the right advice before you commit can save you significant time and money.

At BuilderFix Glazing, we have completed thousands of glazing jobs across Greater London and Surrey — from emergency repairs in Kensington and Chelsea, to bespoke walk-on glass installations in Mayfair and Belgravia. This guide is based on real experience and the questions our customers ask us most often.

What Does This Typically Cost in London?

Costs vary depending on the type and size of work, the glass specification, frame material, and access. As a general guide, always obtain a free written quote before committing — this protects you from unexpected charges.

At BuilderFix Glazing, we provide free, no-obligation written quotes for all work. Our prices are transparent and fixed before we start — no surprises on the invoice.

What to Ask When Getting a Quote

• Is the price fixed, or could it change once work starts?
• What glass specification is included?
• Are all fixings, sealants and finishing included?
• What compliance documentation will I receive?
• What happens if there is an issue after completion?