Loading
Window condensation causes mould, timber rot, and health problems. Learn why period windows suffer most and how to solve the problem permanently.
You wake on a winter morning to find your beautiful sash windows streaming with water. The glass is fogged, droplets run down onto the timber, and there's that telltale musty smell. Condensation on period windows isn't just unsightly—it's actively damaging your home and potentially your health.
Understanding why condensation forms and how to prevent it can save your original windows from decay, protect your family from mould-related illness, and transform your home's comfort.
Condensation occurs when warm, moist air meets a cold surface. The air can no longer hold its moisture, which deposits as water droplets—exactly like a cold drink "sweating" on a summer day.
In homes, this happens because:
Period properties are particularly vulnerable because single-glazed windows get very cold—often approaching outdoor temperatures—while the rest of the house is heated.
Water running down glass collects on timber frames, particularly at the bottom rail of sash windows. Over months and years, this moisture:
Repairing rotten sash windows costs £500-£1,500 per window. In listed buildings, where replacement isn't permitted, the original fabric is irreplaceable at any price.
Damp conditions around windows are perfect for mould. Black mould (Stachybotrys) and other species thrive where condensation is persistent. You'll often see:
Mould exposure is linked to serious health problems:
The NHS estimates that damp and mould affect 4-6 million UK homes and contribute to thousands of respiratory deaths annually.
For expert secondary glazing advice and free consultations for listed buildings in London, contact Secondary Glazing Specialist on 020 7060 1572.
James Whitfield
Conservation Planning Advisor
You wake on a winter morning to find your beautiful sash windows streaming with water. The glass is fogged, droplets run down onto the timber, and there's that telltale musty smell. Condensation on period windows isn't just unsightly—it's actively damaging your home and potentially your health.
Understanding why condensation forms and how to prevent it can save your original windows from decay, protect your family from mould-related illness, and transform your home's comfort.
Condensation occurs when warm, moist air meets a cold surface. The air can no longer hold its moisture, which deposits as water droplets—exactly like a cold drink "sweating" on a summer day.
In homes, this happens because:
Period properties are particularly vulnerable because single-glazed windows get very cold—often approaching outdoor temperatures—while the rest of the house is heated.
Water running down glass collects on timber frames, particularly at the bottom rail of sash windows. Over months and years, this moisture:
Repairing rotten sash windows costs £500-£1,500 per window. In listed buildings, where replacement isn't permitted, the original fabric is irreplaceable at any price.
Damp conditions around windows are perfect for mould. Black mould (Stachybotrys) and other species thrive where condensation is persistent. You'll often see:
Mould exposure is linked to serious health problems:
The NHS estimates that damp and mould affect 4-6 million UK homes and contribute to thousands of respiratory deaths annually.
Victorian and Edwardian windows are particularly prone to condensation:
Original period windows have single panes of glass with a U-value around 5.8 W/m²K. This means heat passes through very quickly, and the inner glass surface gets cold. Modern double glazing (U-value 1.4-1.8) keeps the inner surface much warmer.
The elegant thin glazing bars of Georgian and Victorian sashes have minimal thermal mass—they get cold quickly and are often the first place condensation forms.
Ironically, draughty windows can make condensation worse in specific areas. Cold air entering through gaps chills the surrounding glass and timber, creating localised condensation hotspots.
Modern upgrades—loft insulation, wall insulation, draught-proofing doors—often leave windows as the coldest element in the building envelope. Moisture that previously escaped through general air leakage now concentrates on windows.
Opening windows in winter does reduce humidity—but it also loses heat, increases energy bills, and makes the house uncomfortable. It's not a sustainable solution.
Portable dehumidifiers help but don't address the root cause. They're expensive to run, noisy, and require constant emptying. They're a symptom treatment, not a cure.
These products add slight insulation to the painted surface but make negligible difference to condensation on glass—which is where the problem manifests.
Secondary glazing solves condensation by addressing the root cause: cold glass surfaces. Here's how:
The secondary glazing panel, installed inside the window reveal, stays close to room temperature. Warm, moist air contacts this warm surface—no condensation forms.
The air gap between original and secondary glazing acts as a thermal buffer. Your original single-glazed windows don't get as cold because they're no longer exposed directly to warm room air.
Any moisture in the sealed cavity is limited, and the original glass (now effectively outside the heated envelope) typically stays dry because the warm moist room air can't reach it.
With secondary glazing, you may occasionally see light condensation on the outer (original) glass during extreme cold. This is harmless because:
We can include discrete ventilation in the secondary glazing frame to ensure any cavity moisture escapes.
While solving condensation, secondary glazing simultaneously delivers:
Consider the economics:
Secondary glazing isn't just a comfort upgrade—it's preventive maintenance that protects your property and health.
If condensation is affecting your period windows, act before winter causes more damage. Request a free survey and we'll assess your situation, explain your options, and provide a detailed quotation.
Call 020 7060 1572 to discuss condensation solutions for your home.
London's leading secondary glazing specialists for Grade I, Grade II, and Conservation Area properties. Every project begins with a complimentary heritage survey.
