Condensation or Leak? How U-Values Affect Moisture on Glass

Few glazing issues cause more concern than water appearing on or around a rooflight. Drips forming on the glass or frame are often assumed to be the result of a leak, failed seal, or poor installation. However, in many cases the problem has nothing to do with water ingress at all. Instead, the cause is condensation driven by poor thermal performance. 

Understanding the difference between rooflight condensation vs leak is essential for homeowners, architects, and installers alike. Misdiagnosing condensation as a leak can lead to unnecessary repair work, replacement of perfectly sound pitched roof rooflights, and ongoing frustration when the real issue remains unresolved. One of the key factors behind condensation on glass is the U-value of the rooflight, which directly affects internal surface temperatures. 

This article explains how U-values influence moisture formation on glass, provides a clear dew point explanation for windows, explores the role of cold surface condensation, and shows how improving thermal performance can help stop rooflight dripping for good. 

Why Moisture Appears on Rooflight Glass 

Moisture on glass occurs when warm, humid indoor air meets a surface that is cold enough to trigger condensation. Rooflights are particularly vulnerable because they sit at ceiling level, where warm air naturally rises and collects. Activities such as cooking, showering, drying clothes, and even breathing increase the amount of moisture in the air. 

When that moisture-laden air comes into contact with a cold rooflight surface, water vapour changes into liquid droplets. This process can happen even in well-built homes, especially during colder months when indoor–outdoor temperature differences are greatest. 

The presence of moisture alone does not mean a rooflight is leaking. In many cases, the rooflight is performing exactly as installed, but its thermal performance is insufficient for the environment in which it is used. 

Rooflight Condensation vs Leak: Understanding the Difference 

Distinguishing between condensation and a leak is crucial. While both result in visible moisture, their causes and solutions are very different. 

Condensation often appears overnight or early in the morning and may clear as the day warms up. Leaks, on the other hand, usually worsen during or after rainfall and tend to show as localised damp patches or staining around the rooflight opening. 

How U-Values Affect Moisture on Glass 

A U-value measures how quickly heat passes through a building element such as a window or rooflight. The lower the U-value, the better the insulation and the warmer the internal surface temperature will be. 

Rooflights with high (poor) U-values allow heat to escape rapidly. As heat leaves the building, the internal glass surface becomes colder. Once this surface temperature drops below the dew point of the indoor air, condensation forms. 

This is why even a brand-new rooflight can drip if its thermal performance is inadequate. It is not the age of the rooflight that matters, but its ability to keep internal surfaces warm. 

A Clear Dew Point Explanation for Windows and Rooflights 

The dew point is the temperature at which air becomes fully saturated with moisture and can no longer hold water vapour. When air is cooled below this temperature, excess moisture is released as condensation. 

For example, indoor air at 20°C with a relative humidity of 60% has a dew point of around 12°C. If the internal surface of a rooflight falls below 12°C, condensation will occur regardless of how well the rooflight is sealed against rain. 

Improving U-values raises the internal surface temperature of the glass, keeping it above the dew point and reducing the likelihood of condensation. This is why thermal performance and energy efficiency are so closely linked to effective moisture control. 

Cold Surface Condensation in Rooflights 

Cold surface condensation is especially common in rooflights due to their orientation and exposure. Horizontal or near-horizontal glazing loses heat more quickly than vertical windows. Warm air rises and remains in contact with the glass for longer periods, increasing heat transfer. 

At night, rooflights also lose heat through radiation to the cold sky, further lowering glass temperatures. If the rooflight frame or glass has a poor U-value, internal surface temperatures can drop rapidly, triggering condensation even in rooms with moderate humidity. 

This effect explains why rooflights are more prone to condensation than vertical windows in the same space. 

Why Frames Matter as Much as Glass 

While glass often receives most of the attention, frames play a critical role in condensation risk. Frames with poor thermal performance can become colder than the glass itself, leading to moisture forming around the perimeter of the rooflight. 

In aluminium rooflights without thermal breaks, the frame acts as a cold bridge, drawing heat out of the building and encouraging condensation. Moisture on frames is often mistaken for water ingress, particularly when it drips onto internal finishes. 

Improving frame insulation can dramatically reduce condensation, even without changing the glazing specification. 

Why Condensation Is Often Mistaken for a Leak 

Condensation can appear suddenly and dramatically, especially during cold spells. Water droplets may run down the glass and collect at the base of the rooflight, creating the impression of a leak. 

However, the timing of the moisture often reveals the true cause. Condensation tends to be worse overnight and in the early morning, while leaks correlate with rainfall. In many cases, investigations find no fault with the roof covering or rooflight installation. 

Understanding rooflight condensation vs leak helps avoid unnecessary remedial work and focuses attention on improving thermal performance instead. 

How to Stop Skylight Dripping 

To stop skylight dripping, the underlying cause of condensation must be addressed. This usually involves improving thermal performance and managing indoor humidity. 

Upgrading to a rooflight with a lower U-value can significantly reduce condensation risk by keeping internal glass surfaces warmer. Ensuring frames are well insulated and thermally broken is equally important. 

Ventilation also plays a role. Reducing indoor humidity lowers the dew point, making condensation less likely even on colder surfaces. However, ventilation alone cannot compensate for a rooflight with poor thermal performance. 

The Impact of Rooflight Design and Orientation 

Rooflight size, orientation, and installation detail all influence condensation risk. Large rooflights have more surface area through which heat can escape, while north-facing installations receive less solar warming. 

Installation quality also matters. Poorly insulated upstands or gaps around the rooflight can create cold spots that encourage condensation. Even with good glazing, these thermal weaknesses can lead to moisture problems. 

Comparing Thermal Performance and Condensation Risk 

The relationship between U-values and condensation risk is illustrated in the table below. 

Lower U-values result in warmer internal surfaces and reduced condensation risk, particularly in humid environments. 

When Condensation Indicates a Specification Issue 

Condensation is not always a defect. In many cases, it indicates that the rooflight specification is not suitable for the room’s humidity levels or usage. Bathrooms, kitchens, and open-plan living spaces often require higher thermal performance than standard specifications provide. 

Recognising condensation as a design or specification issue allows for more effective long-term solutions. 

Conclusion 

Moisture on rooflight glass does not automatically mean there is a leak. In many cases, the true issue lies in the thermal performance of the rooflight and its ability to keep internal surfaces warm. Understanding rooflight condensation vs leak is essential for diagnosing problems accurately and avoiding unnecessary repairs. 

U-values play a central role in this process. Poor U-values lead to cold surfaces, cold surface condensation, and dripping glass. Improving thermal performance, insulating frames, and managing humidity are the most effective ways to stop skylight dripping and ensure long-term comfort. 

By considering both thermal performance and moisture behaviour, rooflights can deliver daylight and comfort without unwanted condensation. 

Experiencing condensation or dripping? Speak to our experts to identify the real cause and choose the right rooflight solution.