Bay windows are beautiful architectural features, but they’re also one of the worst offenders for heat loss, condensation, and mould growth. Their shape and multiple external surfaces make them especially prone to cold spots, damp, and even structural issues if left untreated.

If your bay window area always feels cold in winter, has black spot mould around the edges, or suffers from peeling paint and damp patches – we can fix it.

Solve Cold, Noisy & Damp Timber Frame Bay Windows – For Good 

Oriel-style projected bay windows may look attractive from the outside, but behind the tiles and timber frame lies a hidden problem: little to no insulation and no proper draught seal. This makes them a magnet for cold air, condensation, and black spot mould – especially in winter.

Without insulation, these bays act like a weak spot in your home’s thermal envelope. Heat escapes easily, while cold air and moisture find their way in, creating the perfect conditions for mould to grow. On top of that, the lack of mass in the structure means outside noise – from traffic to neighbours – can pass straight through.

Many homeowners try to fix the problem themselves with weather strips, sealants, or fleece-lined wallpaper. While these can slightly reduce draughts, they do nothing to significantly improve thermal resistance, so the cold, damp, and mould problems keep coming back.

Our professional bay window insulation service tackles the issue at the source. We install high-performance insulation materials inside the bay’s structure, completely sealing gaps and eliminating cold bridges. The result? A warmer, quieter, healthier home – with no more condensation or mould worries.

Solid Brick & Render Bay Windows – Stop Heat Loss, Damp & Damage 

Older homes with solid brick walls and cracked render are especially vulnerable to cold and damp. Without insulation, solid walls – including brick-built bay windows – offer very little resistance to heat loss, meaning warmth escapes quickly and your home feels colder and less energy-efficient.

Just like timber-and-tile bays, the cold interior surfaces of solid brick bays can cause condensation to form in winter. This creates the perfect breeding ground for damp and mould, impacting both comfort and indoor air quality.

Cracked or damaged render makes matters worse by letting rainwater seep into the walls. Over time, this trapped moisture not only increases damp but can also weaken the bay’s structure, leading to costly repairs.

The solution is straightforward but essential: repair damaged render, install high-performance internal wall insulation, and improve ventilation to reduce condensation. These steps will make your home warmer, drier, healthier – and significantly cut heating bills.

Mould & Damp in Bay Windows – Why It Happens and How to Stop It 

1. Condensation
Older bay windows often have far less insulation than other walls, meaning the surfaces inside are much colder. When warm, humid air hits these cold surfaces, condensation forms – creating the perfect damp conditions for mould growth.

2. Poor Ventilation
Bay areas with heavy curtains, furniture placed close to the windows, or blocked vents trap moisture-laden air. Without airflow, this moisture lingers and mould can quickly take hold.

3. Penetrating Damp
Gaps in window frames, worn seals, deteriorated mortar, cracked render, or high ground levels can allow rainwater to seep inside. Timber window frames that absorb water only make the problem worse.

4. Insufficient Insulation
Most older bays were built without modern insulation, leading to persistent cold spots where mould thrives. Thermal bridging – where heat escapes through certain building materials – adds to the problem.

5. Exterior Issues
Leaking or blocked gutters and downpipes, along with cracks in exterior brickwork or render, can let water penetrate the bay walls.

6. High Indoor Humidity
Everyday activities such as cooking, showering, and drying clothes indoors add moisture to the air. Without good extraction or ventilation, that moisture heads straight for the coldest part of the room – usually the bay window.

Our Solution
The image below shows a recent project in Derbyshire where we installed PIR insulation to a cold, damp bay window. Thermal imaging before and after showed a 50% thermal performance improvement – completely transforming the comfort of the room and eliminating the condensation problem.