What U-Value Measures
U-value, formally called the thermal transmittance coefficient, expresses the rate of heat transfer through a building element — in this case a window — under steady-state conditions. It is measured in watts per square metre per kelvin (W/m²·K). A lower number indicates better thermal performance.
For windows, the total U-value (Uw) covers three distinct components: the glazing unit (Ug), the frame (Uf), and the linear thermal bridge at the perimeter junction between frame and glass (Ψg, the psi value). Each component contributes to the overall result in proportion to its area.
Uw = (Ag × Ug + Af × Uf + lg × Ψg) / (Ag + Af)
Where Ag = glazing area, Af = frame area, lg = perimeter length of glazing edge, Ψg = edge thermal bridge coefficient.
Glazing Configurations and Their U-Values
The number of glass panes and the type of gas fill in the cavities between them directly determine the Ug value. Single glazing is rarely used in new European residential construction due to its very high heat loss. Double and triple glazing with low-emissivity coatings and inert gas fills are the standard approaches.
| Glazing type | Ug W/(m²·K) | Common application |
|---|---|---|
| Single glazing (4 mm) | ~5.8 | Older stock, not permitted in new Polish construction |
| Double glazing, air-filled | ~2.8 | Renovations pre-2000 |
| Double glazing, Low-E + argon | 1.0–1.4 | Standard renovation glazing |
| Triple glazing, Low-E + argon | 0.5–0.7 | New builds, energy-efficient renovation |
| Triple glazing, Low-E + krypton | 0.4–0.5 | Passivhaus standard |
Low-emissivity (Low-E) coatings are thin metallic layers applied to one or more glass surfaces. They reflect long-wave infrared radiation — the heat radiated by interior surfaces — back into the room, significantly reducing radiative heat transfer through the glazing cavity.
The Role of the Frame
Frame materials vary considerably in their thermal conductivity. Traditional aluminium frames without a thermal break have high conductivity and produce Uf values often exceeding 3.0 W/(m²·K). Modern frames designed for energy-efficient construction fall into three main categories.
| Frame material | Typical Uf W/(m²·K) | Notes |
|---|---|---|
| PVC, standard (3 chambers) | 1.6–2.2 | Common in Poland, affordable |
| PVC, advanced (6+ chambers) | 0.9–1.3 | Required for near-passive performance |
| Wood, 78 mm | 1.0–1.4 | Natural material, requires maintenance |
| Aluminium + thermal break | 1.0–1.6 | Depends on break depth and material |
| Timber-aluminium composite | 0.7–1.0 | Combines durability with low Uf |
Edge Spacer and Perimeter Thermal Bridges
The spacer bar that holds the glass panes apart at the perimeter of a glazing unit is a significant source of heat loss. Traditional aluminium spacers conduct heat readily across the cavity, creating a cold zone at the glass edge visible as condensation in cold weather.
Warm-edge spacers — made from stainless steel, polymer composite or thermoplastic materials — reduce this junction loss. The psi value (Ψg) for a warm-edge spacer typically falls in the range of 0.03–0.06 W/(m·K), compared to 0.06–0.10 W/(m·K) for aluminium spacers. For a window with 3 m of perimeter glazing edge, the difference amounts to approximately 0.09–0.21 W/K.
Polish Regulatory Requirements
The Warunki Techniczne 2021 (WT 2021) regulation sets mandatory maximum U-values for windows and roof windows installed in new and substantially renovated buildings in Poland. These limits apply to the whole window unit (Uw), not just the glazing.
| Element | Max Uw W/(m²·K) since 2021 |
|---|---|
| Windows and balcony doors (heated rooms) | 0.9 |
| Roof windows | 1.1 |
| Windows in non-heated stairwells | 1.4 |
Prior to 2021, the limit was 1.1 W/(m²·K) for standard windows. The 2021 update pushed Poland toward greater alignment with Passivhaus and near-zero energy building (NZEB) requirements. For Passivhaus certification, the window U-value requirement under PHPP (Passive House Planning Package) methodology is typically below 0.8 W/(m²·K) for central European climates.
Solar Heat Gain and g-Value
U-value alone does not fully describe window energy performance. The solar heat gain coefficient (g-value or SHGC) measures the fraction of solar radiation that passes through the glazing. In Polish climate conditions — characterised by cold winters and moderately warm summers — a higher g-value (0.5–0.6) can reduce heating demand during the heating season by contributing solar gains, but may require shading in summer to avoid overheating.
Passive house design typically targets a balanced approach: high-performance south-facing glazing with good solar gain, combined with north-facing windows using the lowest possible U-value. This thermal zoning is part of the standard PHPP optimisation process used for buildings seeking Passivhaus certification under PHI (Passive House Institute, Darmstadt) criteria.
Measurement and Verification
Window U-values in Europe are determined according to EN ISO 10077-1 (simplified method) or EN ISO 10077-2 (numerical simulation). Product declarations must comply with EN 14351-1 for external windows. In Poland, CE marking under this standard is mandatory for windows placed on the market. The declared U-value must reflect actual tested or calculated performance, not a nominal specification.
References: Passive House Institute — PHPP documentation · Rozporządzenie WT 2019/2021 — ISAP · EN ISO 10077-1:2017