Passive House Window Selection: Uw Value Guide for European Projects

Passive House (Passivhaus) is the world’s most demanding voluntary energy efficiency standard for buildings. Windows are the single most critical component — a poorly selected window can eliminate a project’s Passive House eligibility entirely. For European importers, architects, and developers, understanding Uw value, triple glazing specifications, and thermal break frame engineering is essential for project approval. This guide covers practical selection criteria for windows designed to meet the Passive House Uw≤0.8 W/(m²·K) threshold.

What Is Uw and Why 0.8 W/(m²·K)?

Uw (Wärmedurchgangskoeffizient Fenster) is the thermal transmittance of the entire window assembly — frame, glazing, and edge seal combined into one value. The lower the Uw value, the better the window insulates. A Uw of 0.8 W/(m²·K) means that for every square meter of window surface, only 0.8 watts of heat transfer occurs per degree Kelvin of temperature difference between inside and outside.

The Passive House Institute (PHI) sets the Uw≤0.8 W/(m²·K) threshold for transparent components (windows and glazed doors) in cool-temperate climate zones. At this level, the window contributes minimal heat loss relative to the opaque wall, allowing the building to maintain comfort without conventional heating systems. Projects in colder climates (Scandinavia, alpine regions) may require even lower values, while warmer climate zones allow slightly higher thresholds.

• Uw ≤ 0.8 W/(m²·K): PHI certified component threshold (cool-temperate)
• Uw 0.9–1.2 W/(m²·K): Low-energy building standard (EnEV / EU EPBD)
• Uw 1.3–1.6 W/(m²·K): Standard double-glazed aluminum window
• Uw > 1.6 W/(m²·K): Older buildings, not suitable for energy-efficient construction

Uw Calculation: Uf + Ug + ψ — The Three Variables

The Uw value is not simply the glass performance — it is a weighted average of three components calculated according to ISO 10077-1. Understanding this formula is essential because it explains why two windows with identical glazing can have very different Uw values depending on frame quality and spacer type.

The formula: Uw = (Ug × Ag + Uf × Af + ψ × l) / (Ag + Af)

Where Ag is glass area, Af is frame area, and l is the perimeter length of the edge seal. The three variables that determine Uw are:

The frame typically accounts for 25–35% of the window area but contributes disproportionately to heat loss. This is why a high-quality thermal break aluminum frame is just as important as the glazing — a Ug of 0.6 with a poor frame (Uf 1.5) will not achieve Uw≤0.8.

Triple Glazing Specs That Actually Achieve Uw≤0.8

Double glazing cannot achieve Uw≤0.8 — this is a physical limitation, not a matter of product quality. Even the best double-glazed units (Ug 1.1) combined with premium frames result in Uw values of 1.0 or higher. Only triple glazing with the right configuration can reliably reach the Passive House threshold.

The following table shows the minimum triple glazing specifications required to achieve Ug≤0.6, which is necessary (but not alone sufficient) for Uw≤0.8:

Gas Fill: Argon vs Krypton

Argon is the standard gas fill — it reduces convection in the cavity and is widely available at low cost. Krypton offers 15–20% better thermal performance than argon but at significantly higher cost. For Passive House projects, krypton is recommended in narrow 12mm cavities where argon’s performance degrades, while argon is sufficient in 16mm cavities. StarveilBuild offers both options and provides the full IGU specification sheet for PHI documentation.

Thermal Break Aluminum: The Frame Variable

Aluminum is an excellent structural material for windows — strong, durable, and dimensionally stable — but it is also an excellent thermal conductor. Without a thermal break, an aluminum frame transmits heat at roughly 160 W/(m·K), making it impossible to achieve energy-efficient Uw values. The thermal break is a polyamide (PA66) strut inserted between the interior and exterior aluminum profiles, interrupting the thermal bridge.

The depth and quality of the thermal break directly determines the frame U-value (Uf). For Passive House projects, the thermal break strut must be at least 30mm deep, with multi-chamber profiles that trap insulating air pockets:

StarveilBuild’s Passive House window system uses a 40mm polyamide thermal break with a 5-chamber profile design, achieving Uf values of 0.8 W/(m²·K). Combined with triple glazing (Ug 0.5) and warm edge spacers (ψ 0.03), this delivers a whole-window Uw of 0.78 W/(m²·K) for a standard 1200×1400mm casement window — below the 0.8 threshold.

PHI Certification vs CE Marking: What European Importers Need

European window importers face two distinct regulatory frameworks, and confusing them can lead to costly mistakes. The Passive House Institute (PHI) component certification is a voluntary, performance-based certification that confirms a window meets Passive House thermal requirements. CE Marking under the Construction Products Regulation (CPR) is a mandatory legal requirement for placing construction products on the EU market.

PHI Component Certification

PHI certification evaluates the window’s thermal performance (Uw), solar heat gain coefficient (g-value), and thermal comfort criteria. A PHI-certified component receives a data sheet with the exact Uw value, frame dimensions, and installation recommendations. This certification is voluntary but is required if the window is to be used in a PHI-certified building.

CE Marking (CPR)

CE Marking under EN 14351-1 is mandatory for all windows sold in the EU. It declares the product’s performance characteristics (air permeability, watertightness, wind resistance, acoustics, thermal transmittance). CE Marking does not verify Passive House-level performance — it simply declares what the product achieves and that it meets essential safety and performance requirements.

As a certification-ready manufacturer, StarveilBuild engineers window systems to meet PHI Uw≤0.8 thresholds and provides complete thermal calculation documentation (ISO 10077-1/2) and test specimens for PHI component certification application. The PHI certificate is issued under the buyer’s brand. For CE Marking, we provide Declaration of Performance (DoP) documentation and test data per EN 14351-1 that the importer uses to affix the CE mark under their own name.

Frequently Asked Questions