Thermal Transmittance in Windows: The Hidden Key to Comfort and Energy Efficiency

Thermal Transmittance in Windows: The Hidden Key to Comfort and Energy Efficiency

In the field of construction and architecture, windows play a much more significant role than one might think. They are not only aesthetic or functional elements that allow natural light to enter, but also one of the most critical points in a building’s envelope in terms of energy losses and gains.

Their thermal performance directly affects heating and cooling consumption, the building’s energy footprint, and, most importantly, the comfort of its occupants. To measure and understand this insulating capacity, an essential concept comes into play: thermal transmittance (U-value). This is a key point highlighted by Strugal in their blog.

What is Thermal Transmittance (U-value)?

Thermal transmittance, also known as U-value, measures the amount of heat that passes through a material or construction system. It is expressed in W/m²·K and represents the energy transferred through one square meter of surface for each degree of temperature difference between the interior and exterior.

The rule is simple: the lower the U-value, the higher the insulating capacity. This means less heat loss in winter, less overheating in summer, and consequently, lower energy consumption to maintain a comfortable indoor temperature.

In the case of windows, this indicator is especially relevant, since they are among the weakest points of a building’s thermal envelope.

How is the Thermal Transmittance of a Window Calculated?

A window is not a single element but a combination of glass, frame, and junctions. The overall U-value (Uw) is calculated by combining the contribution of each part, weighted according to its proportion of the total surface:

• Uw (Window): U-value of the entire window (glass + frame + junctions).
• Ug (Glass): thermal transmittance of the glazing at its center.
• Uf (Frame): thermal transmittance of the frame or profile.
• Ψ (Psi): linear thermal transmission coefficient of the spacer between panes in insulating glass units.

The calculation of Uw is regulated by the EN ISO 10077-1 standard, which defines the procedures to determine the actual thermal efficiency of a window.

Factors Influencing Thermal Efficiency

1. Glazing (Ug)

• Simple glazing has very high U-values.
• Double and triple glazing significantly reduce heat transfer.
• Low-E coatings reflect infrared radiation, reducing heat loss without compromising natural light.
• Inert gas fills (argon, krypton) further improve insulation.
• “Warm Edge” spacers reduce thermal bridges and condensation risks.

2. Frame or Profile (Uf)

• Multi-chamber designs (PVC or aluminum) improve insulation by interrupting thermal flows.
• Thermal break technology in metal profiles is essential to reduce conductivity and prevent condensation
  

3. Quality of Installation

Even the most efficient window loses effectiveness if poorly installed. Up to 85% of issues on site stem from installation errors, such as poorly sealed joints, unsuitable fixings, or material incompatibilities.

In Spain, the UNE 85219:2023 standard defines installation criteria to ensure long-term insulation, airtightness, and durability.

Standards and Certifications

• CE Marking: mandatory in the European market, certifying declared performance including Uw.
• Spanish Technical Building Code (CTE): establishes maximum thermal transmittance values for new builds and refurbishments.
• Building Energy Certifications: evaluate overall efficiency and recommend improvements, often including the replacement of inefficient windows.

Why is U-value so Important in Windows?

Windows can account for 15% to 35% of a home’s energy losses. Choosing a system with low thermal transmittance results in:

• Energy savings: reduced heating and cooling consumption.
• Increased comfort: more stable and uniform indoor temperatures.
• Sustainability: lower emissions linked to energy use.
• Higher property value: improved energy certification and market appeal.

At Dressler, we believe that a well-designed window, precisely manufactured and correctly installed according to standards, is not just a closing element. It is an investment in efficiency, comfort, and sustainability.