Horizontal vs Vertical U-Values: Why Testing Angle Matters

U-values are one of the most important metrics used to assess the thermal performance of glazing. Architects, specifiers, and building control officers rely on U-values to determine whether windows and rooflights meet regulatory and energy-efficiency requirements. However, one critical aspect of U-value performance is often misunderstood or overlooked entirely: the orientation at which the glazing is tested. 

A window installed vertically behaves very differently from a rooflight installed horizontally. Heat flow, air movement, and radiative losses all change with orientation, meaning a U-value obtained from vertical testing may not accurately represent real-world performance when that same glazing is installed flat. This is why horizontal U-value testing is so important for rooflights and skylights. 

In this article, we explore why testing angle matters, how skylight U-value calculation angle affects performance results, the role of BR 443 conventions, and why flat glass U-value correction is essential for realistic specification. Understanding these principles helps avoid overestimating performance, reduces the risk of condensation, and ensures compliance with modern building standards. Learn more about our solutions at Skylights & Roof Lanterns. 

What Is a U-Value? 

A U-value measures the rate of heat transfer through a building element. It is expressed in watts per square metre per degree Kelvin (W/m²K). In simple terms, it tells us how much heat is lost through a window, rooflight, or wall when there is a temperature difference between inside and outside. 

Lower U-values indicate better insulation and lower heat loss. Because U-values are easy to compare, they have become the standard metric for assessing thermal performance. However, U-values are not fixed constants. They are influenced by multiple factors, including glazing composition, frame design, installation detail, and, crucially, orientation. 

Why Orientation Affects Thermal Performance 

Orientation changes how heat moves through glazing. In vertical windows, warm indoor air rises along the surface of the glass and is replaced by cooler air, creating a relatively stable convective pattern. In horizontal glazing, warm air rises and remains in direct contact with the glass for longer periods. 

This difference increases convective heat transfer in rooflights. At the same time, horizontal glazing loses more heat through radiation to the night sky, especially during clear conditions. These combined effects mean that a rooflight typically loses more heat than a vertical window with the same construction. 

Because of this, a U-value measured in a vertical orientation does not accurately represent the performance of glazing installed horizontally. 

Horizontal U-Value Testing Explained 

Horizontal U-value testing measures thermal performance with the glazing positioned flat, replicating the conditions experienced by our range of high-performance roof glazing in real applications. This type of testing captures the increased convective and radiative heat losses that occur in horizontal applications. 

When glazing is tested only in the vertical position, the resulting U-value can be overly optimistic for horizontal applications. This has historically led to confusion, with manufacturers quoting attractive U-values that cannot be achieved in service. 

Horizontal testing provides more realistic data, allowing specifiers to understand how the glazing will actually perform once installed. 

Vertical Testing and Its Limitations 

Vertical U-value testing remains appropriate for windows and doors installed in walls. For these applications, vertical orientation reflects real-world conditions reasonably well. 

However, problems arise when vertical U-values are used to describe rooflights. Because vertical testing underestimates heat loss for horizontal glazing, it can lead to poor thermal performance once installed, higher energy consumption, and increased condensation risk. 

Understanding the limitations of vertical testing is essential when interpreting performance data for skylights and rooflights. 

Skylight U-Value Calculation Angle 

The skylight U-value calculation angle refers to the orientation assumed when determining thermal performance. Rooflights may be flat, pitched, or installed at varying angles, each of which affects heat loss. 

As the angle moves closer to horizontal, heat loss increases. Pitched rooflights fall somewhere between vertical windows and flat skylights in terms of performance. Accurate calculation must therefore reflect the intended installation angle rather than relying on a generic vertical value. 

Specifiers should always confirm whether quoted U-values relate to vertical, horizontal, or corrected orientations. 

BR 443 Conventions and Their Role 

The BR 443 conventions provide guidance on calculating and reporting U-values in a consistent and comparable way. They recognise that testing every glazing configuration in every orientation is not always practical. 

BR 443 allows for correction factors to be applied to vertical U-values to estimate horizontal performance. These corrections account for increased convection and radiation losses associated with horizontal glazing. 

While direct horizontal testing is preferred, BR 443 conventions provide a recognised framework for interpreting performance when testing data is limited. 

Flat Glass U-Value Correction Explained 

Flat glass U-value correction is used to adjust thermal performance figures when glass is installed horizontally rather than vertically. The correction increases the U-value to reflect the higher heat loss experienced by flat glazing. 

This correction is particularly important when comparing products. Without it, flat rooflights may appear to perform as well as vertical windows, which is not the case in practice. Applying flat glass corrections helps ensure fair and realistic comparisons. 

Comparing Vertical and Horizontal Performance 

The difference between vertical and horizontal performance can be significant, as illustrated in the table below. 

This comparison highlights why orientation must be considered when specifying glazing. 

Whole-Unit U-Values vs Centre-Pane Values 

Another common source of confusion is the difference between centre-pane and whole-unit U-values. Centre-pane values describe the performance of the glass only, excluding frames and edge effects. 

In rooflights, frames and edge details play a larger role in heat loss than in vertical windows. Whole-unit U-values that account for orientation provide the most accurate picture of performance. 

Why Testing Angle Matters for Condensation Risk 

Condensation risk is closely linked to surface temperature. Higher heat loss leads to colder internal surfaces, increasing the likelihood that the surface temperature will fall below the dew point. 

Because horizontal glazing loses more heat, rooflights tested vertically may appear adequate on paper but still experience condensation in use. Horizontal U-value testing helps predict and mitigate this risk. 

Implications for Specification and Compliance 

Building regulations increasingly focus on actual performance rather than theoretical values. Using vertical U-values for rooflights can result in non-compliance once the building is assessed in operation. 

Specifiers, designers, and homeowners benefit from demanding transparent, orientation-appropriate U-values when selecting rooflights. 

Conclusion 

U-values are only meaningful when they reflect real-world conditions. Orientation plays a crucial role in thermal performance, and ignoring it can lead to overestimated efficiency, higher energy use, and condensation problems. 

Horizontal U-value testing provides realistic performance data for rooflights and skylights, accounting for increased heat loss due to convection and radiation. By understanding skylight U-value calculation angle, applying BR 443 conventions, and using flat glass U-value correction where appropriate, specifiers can make informed decisions and avoid costly mistakes. 

Want accurate rooflight performance? Contact our experts to learn how U-value testing affects your installation.