U-Value (Thermal Transmittance)

What does U-value measure?

U-value, or thermal transmittance, is a measure of how much heat flows through a construction element when there is a one degree Kelvin (or one degree Celsius) temperature difference between its two surfaces. It is expressed in watts per square metre per Kelvin, written as W/(m²·K). The lower the U-value, the better the thermal insulation, meaning less heat escapes in winter and less enters in summer. A wall with a U-value of 0.20 W/(m²·K) is twice as insulating as one with 0.40 W/(m²·K).

How does U-value relate to R-value?

R-value (thermal resistance) is the inverse of U-value: R = 1/U. If a window has a U-value of 1.0 W/(m²·K), its R-value is 1.0 m²·K/W. In older European practice, particularly in Slovakia, R-value was sometimes used, but U-value has become the standard metric in EU building regulations and is far more common in modern specifications. The relationship is mathematically simple but practically important: when comparing insulation performance, remember that the inverse operation is required to convert between the two scales.

Why do windows have different U-values for glass, frame, and the whole assembly?

A window is not a single material but three distinct thermal systems. The glazing (the glass panes themselves) has a U-value determined by the number of panes, the gaps between them, and any special coatings. The frame (the aluminium or timber structure holding the glass) has its own U-value, which is typically much worse than the glass because frames conduct heat readily. The whole-window U-value, written as Uw, is a weighted average of the glass area, frame area, and edge effects. This is a common source of confusion in Slovakia: a specification that quotes "U = 0.70" for the glazing alone may become Uw = 1.0 or worse when the frame is included. Always ask whether a U-value figure refers to glazing only (Ug), frame only (Uf), or the complete window (Uw). Only the whole-window value matters for compliance and real-world performance.

What are the STN 73 0540 requirements for different building elements?

Slovakia's national standard STN 73 0540 sets minimum thermal performance requirements for buildings. These values apply to new construction and deep renovations. For walls, the current minimum required value is approximately U = 0.28 W/(m²·K), though better practices recommend U ≤ 0.20 W/(m²·K). For roofs and top-floor ceilings, the requirement is stricter: approximately U = 0.20 W/(m²·K), with recommendations at U ≤ 0.15 W/(m²·K). Ground floors and basement walls have less stringent requirements, typically around U = 0.40 W/(m²·K). Windows are always the weakest point thermally: the minimum is approximately Uw = 1.2 W/(m²·K), but modern standards recommend Uw ≤ 0.90 W/(m²·K), and passive houses require Uw ≤ 0.80 W/(m²·K).

What U-values define passive-house and low-energy standards?

Passive houses are defined by extremely low heating and cooling demands, achieved partly through exceptional thermal performance. A passive house must achieve: walls U < 0.15 W/(m²·K), roofs and top ceilings U < 0.10 W/(m²·K), and windows Uw < 0.80 W/(m²·K). Ground floors below heated spaces should be U < 0.15 W/(m²·K). These figures are not negotiable targets but absolute requirements. A low-energy house, the next tier down, typically aims for walls U ≤ 0.20 W/(m²·K) and windows Uw < 1.0 W/(m²·K). The difference sounds small numerically but requires fundamentally different design: triple-glazing becomes essential, thermal bridges must be nearly eliminated, and airtightness strategies become critical. Without heat recovery ventilation, even a perfectly insulated passive house cannot meet its heating targets.

Element	STN 73 0540 (Current)	Good Practice	Passive House
Walls (above ground)	< 0.28 W/(m²·K)	< 0.20 W/(m²·K)	< 0.15 W/(m²·K)
Roofs and top ceilings	< 0.20 W/(m²·K)	< 0.15 W/(m²·K)	< 0.10 W/(m²·K)
Ground floors (unheated below)	< 0.40 W/(m²·K)	< 0.25 W/(m²·K)	< 0.15 W/(m²·K)
Windows (whole assembly)	< 1.20 W/(m²·K)	< 0.90 W/(m²·K)	< 0.80 W/(m²·K)
Doors with glazing	< 1.50 W/(m²·K)	< 1.10 W/(m²·K)	< 1.00 W/(m²·K)

How do you verify U-values in practice?

U-values cannot be measured on a finished building; they are calculated during design based on the thermal properties of each material layer and published using standardised methods (EN ISO 6946 for opaque elements, EN ISO 10077 for windows). Every window supplier and insulation product comes with certified U-values from laboratory testing. When specifying materials, always request the U-value from the manufacturer or installer. For windows, demand the whole-window value (Uw), not just the glass. In Slovakia, when planning a renovation or new build, the energy auditor and your architect must calculate and verify all U-values as part of the technical design. This calculation directly determines whether your building meets STN 73 0540 and qualifies for Obnov Dom grants or Plan Obnovy funding.

What role do thermal bridges play in U-value performance?

The U-values in a specification assume perfect construction, but in reality thermal bridges concentrate heat flow at weak points: junctions between walls and roofs, around window frames, at concrete lintels, and at balcony connections. A thermal bridge can locally have an effective U-value far higher than the surrounding element. In a passive house, these must be minimised to less than 0.01 W/(m·K) additional heat loss per lineal metre, a level achievable only with careful detailing and continuous insulation. A single poor detail can derail passive-house certification, which is why thermal bridge analysis and blower-door testing are non-negotiable in high-performance projects. Standard construction practice in Slovakia often tolerates thermal bridges; passive house demands their elimination.

Why does glazing orientation and coatings affect practical performance even if U-value is constant?

U-value describes conductive and convective heat transfer only; it does not account for solar gain or radiant properties. A south-facing window with solar-gain coatings (low-emissivity coatings that allow sun in but trap infrared heat) might have the same U-value as an identical north-facing window but deliver vastly different thermal performance over a year. This is why a thermal-energy model is essential in passive-house design: U-values alone do not tell the full story. Two buildings with identical U-values can have very different heating needs depending on orientation, shading, thermal mass, and occupancy patterns. Nevertheless, U-value remains the single most important specification because it sets the floor for what is physically possible.

Glazing Type	Number of Panes	Typical U-Value (glass only, Ug)	Typical Window U-Value (Uw)
Single glazing	1	5.0–5.5 W/(m²·K)	5.5–6.0 W/(m²·K)
Double glazing (standard)	2	2.7–3.0 W/(m²·K)	2.8–3.2 W/(m²·K)
Double glazing (low-E coating)	2	1.1–1.5 W/(m²·K)	1.3–1.8 W/(m²·K)
Triple glazing (standard low-E)	3	0.5–0.7 W/(m²·K)	0.7–0.95 W/(m²·K)
Triple glazing (premium low-E, argon fill)	3	0.4–0.5 W/(m²·K)	0.6–0.8 W/(m²·K)