What a U-value expresses
The thermal transmittance of a building component — its U-value — is a measure of heat flow rate per unit area per unit temperature difference. For windows, it is expressed in W/(m²·K): watts of heat transferred per square metre of window area for each degree Kelvin difference between inside and outside air temperature. A lower U-value indicates better thermal insulation.
A single-pane glass window with no frame insulation has a U-value of approximately 5.8 W/(m²·K). A modern triple-glazed window with a well-specified uPVC or thermally broken aluminium frame can reach 0.7–0.9 W/(m²·K). The difference represents an eight-fold reduction in heat loss for the same area and temperature conditions.
How the overall window U-value is calculated
EN ISO 10077-1 defines the calculation method for the overall window U-value (Uw). The formula accounts for three separate contributions:
- Glazing transmittance (Ug) — determined under EN 673 for the insulating glass unit, weighted by the glazed area (Ag)
- Frame transmittance (Uf) — determined by numerical calculation or testing under EN ISO 10077-2, weighted by the frame area (Af)
- Linear thermal transmittance of the glazing edge (ψg) — determined under EN ISO 10077-2, multiplied by the total perimeter length of the glazing (lg)
The combined formula is: Uw = (Ag × Ug + Af × Uf + lg × ψg) / (Ag + Af). In practice, manufacturers publish Uw values for standard reference window sizes — typically 1230 × 1480 mm under EN 14351-1 — which allows comparison between products. Values for non-standard sizes are calculated by inserting the actual dimensions into the formula.
Polish building regulations and the 2021 revision
The technical conditions governing building design and construction in Poland are set out in the Rozporządzenie Ministra Infrastruktury i Rozwoju w sprawie warunków technicznych, jakim powinny odpowiadać budynki i ich usytuowanie. The most recent substantial revision of the window-related requirements took effect on 1 January 2021, following a phased schedule introduced in 2013.
From 2021, the maximum permitted U-value for windows (Uw) in residential buildings is 0.9 W/(m²·K). This applies to new construction and to major refurbishments where window replacement triggers compliance. The threshold is the same for all Polish climate zones — unlike wall insulation requirements, which vary by zone according to heating degree-day data.
| Period | Uw max — residential | Uw max — non-residential |
|---|---|---|
| Until 31 Dec 2016 | 1.8 W/(m²·K) | 1.9 W/(m²·K) |
| 2017–2020 | 1.1 W/(m²·K) | 1.3 W/(m²·K) |
| From 1 Jan 2021 | 0.9 W/(m²·K) | 1.1 W/(m²·K) |
The 0.9 W/(m²·K) threshold for residential buildings from 2021 effectively eliminates most double-glazed windows with standard argon fills and aluminium spacers from new construction, because those products typically achieve Uw values of 1.0–1.3 W/(m²·K) depending on frame type. Double-glazed units with high-performance soft-coat low-E, warm-edge spacers, and an optimised frame can still meet 0.9 W/(m²·K), but the specification needs to be verified rather than assumed.
Climate zones and heating degree-days in Poland
While the maximum Uw threshold is nationally uniform, Poland's geographic spread means that the actual heat loss through a window over a heating season differs considerably by location. The Polish standard PN-EN ISO 13790, used for energy demand calculation, divides Poland into five climate zones based on long-term heating degree-day data referenced to a base temperature of 20 °C.
The coldest Zone V covers north-eastern regions including parts of Warmia-Masuria, Podlaskie, and areas near the Białowieża Forest, where annual heating degree-days reach 4,000–4,200 HDD. Warsaw sits in Zone III at approximately 3,600 HDD. Wrocław and Poznań fall in Zone II at 3,100–3,300 HDD. Zone I, the mildest, covers parts of Lower Silesia and the warmest coastal areas with around 2,800 HDD.
In Zone V, the same window specification loses roughly 40% more heat over the heating season than in Zone I. Projects in colder zones may choose to exceed the regulatory minimum and specify Uw values of 0.7–0.8 W/(m²·K) to improve the energy balance without significantly increasing capital cost, since the marginal price difference between a 0.9 and a 0.7 W/(m²·K) window is small compared with the heating cost difference over a 30-year building lifespan.
Solar heat gain coefficient (g-value)
U-value describes heat loss; the solar heat gain coefficient (SHGC or g-value) describes solar heat gain. The g-value is the fraction of incident solar radiation that enters the building as heat — through direct transmission and secondary emission from the glass. Values range from about 0.62 for a clear double unit to 0.25–0.35 for a high-performance selective coating.
In heating-dominated climates like Poland, higher g-values are generally favourable for south-facing elevations because winter solar gains reduce heating demand. The same window on a west-facing elevation in a highly insulated building may cause overheating in summer. Building energy calculation under PN-EN ISO 13790 accounts for both components; optimising for U-value alone without considering orientation and g-value can produce buildings that pass the static thermal check but underperform on annual energy balance.
Testing and declaration of performance
Under the Construction Products Regulation (CPR, EU 305/2011), windows placed on the EU market must carry a CE mark and a Declaration of Performance (DoP) listing measured performance characteristics. For windows, the relevant harmonised standard is EN 14351-1, which covers resistance to wind load (class 1–5), water tightness (class 0–9), air permeability (class 1–4), and — where declared — thermal transmittance, sound reduction, and resistance to repeated opening.
Not all manufacturers declare Uw in their DoP; declaration remains voluntary for some characteristics. Polish building control practice requires the installer to demonstrate that the specified window meets the regulatory Uw threshold, which typically means providing the manufacturer's technical data sheet with Uw calculated per EN ISO 10077-1 alongside the DoP.
Installation quality and thermal bridging at reveals
A window that meets Uw 0.9 W/(m²·K) in laboratory conditions can perform significantly worse in a building if the installation introduces thermal bridges at the reveal junction. The linear thermal bridge at the perimeter of the window — where the window frame meets the wall — is characterised by a linear thermal transmittance value (ψinstall). In lightweight timber-frame construction, ψinstall values of 0.02–0.04 W/(m·K) are achievable. In masonry construction with shallow reveals and no external insulation at the reveal face, values of 0.10–0.15 W/(m·K) are common.
The practical implication: a window positioned at the outer face of the wall with the reveal thermally bridged can add an equivalent Uw penalty of 0.1–0.3 W/(m²·K) to the overall wall-window junction. Positioning the frame within the insulation layer — common practice in external insulation composite system (ETICS) installations — eliminates most of this bridging. The correct installation position is documented in EN ISO 10211, and ITB publication IT-417 provides Polish-specific guidance on thermal bridge calculation for window reveals.
Standards and references
- EN ISO 10077-1 — Thermal performance of windows, doors and shutters
- EN ISO 10211 — Thermal bridges in building construction
- Rozporządzenie w sprawie warunków technicznych (Polish building technical conditions — Sejm portal)
- Polish Institute of Building Technology (ITB)
- European Commission — Energy Efficient Buildings