U-value — requirements, calculation and insulation thicknesses 2026

Q: How is the U-value calculated?

The U-value is calculated using the formula U = 1/RT, where RT is the total thermal resistance of the structure. The total thermal resistance is obtained by adding up the thermal resistance of each material layer (d/λ, where d is thickness in meters and λ is thermal conductivity W/mK) plus the surface resistances.

Q: What are the U-value requirements in 2026?

In 2026, the U-value requirements for new construction are: external wall maximum 0.17 W/m²K, roof maximum 0.09 W/m²K, ground floor maximum 0.16 W/m²K, windows maximum 1.0 W/m²K and doors maximum 1.0 W/m²K. The requirements are based on the Ministry of the Environment decree.

Q: How does the U-value affect insulation thickness?

A lower U-value requirement means thicker insulation. For example, an external wall U-value of 0.17 W/m²K requires approximately 220 mm of mineral wool insulation, while PIR insulation only needs about 120 mm because PIR has lower thermal conductivity.

U-value, or thermal transmittance, indicates how well a structure insulates heat. The lower the U-value, the better the insulation. This guide covers U-value requirements, the calculation formula, and insulation thicknesses for different materials.

What is the U-value?

U-value (thermal transmittance) indicates how much heat passes through a structure per unit area and temperature difference. The unit is W/m²K (watts per square meter per kelvin). The lower the U-value, the better the thermal insulation.

In practice, the U-value means how much heat the structure lets through. If an external wall has a U-value of 0.17 W/m²K, each square meter of wall lets 0.17 watts of heat through per degree of temperature difference between inside and outside.

U-value is the basic measure of energy efficiency in construction. The Ministry of the Environment decree sets maximum U-values for different building components, and these have tightened significantly over the decades. In the 1970s, an external wall U-value could be 0.40 W/m²K — today the requirement is 0.17 W/m²K.

U-value requirements 2026

The Ministry of the Environment decree on energy efficiency of new buildings sets reference U-values. These are minimum requirements that must be followed in new construction. The designer can choose more specific U-values for individual building components, as long as the building's total energy consumption (E-value) meets the requirements.

Building component	U-value (W/m²K)	Note
External wall	≤ 0.17	Most common requirement for new construction
Roof	≤ 0.09	Strictest requirement — heat rises
Ground floor (against soil)	≤ 0.16	Different calculation for crawl space
Windows	≤ 1.0	Whole window U-value, not just glass
Doors	≤ 1.0	Exterior doors and vestibule doors

Note that these are reference values. Energy efficiency can be compensated in different ways: for example, the wall U-value can be slightly weaker if the roof insulation exceeds the requirement. Overall, the E-value must still meet the requirements.

U-value calculation

U-value calculation is based on the total thermal resistance of the structure. The basic formula is:

U = 1 / R_T

where R_T = R_si + R₁ + R₂ + ... + R_n + R_se

Thermal resistance R is calculated for each material layer separately using the formula:

R = d / λ

where d = material thickness (m) and λ = thermal conductivity (W/mK)

R_si R_se is the external surface resistance (typically 0.04 m²K/W). These account for the air layer effect on surfaces.

Example: external wall U-value calculation

Let's calculate the U-value of a typical timber-framed external wall. The wall consists of the following layers (from inside to outside):

Layer	d (mm)	λ (W/mK)	R (m²K/W)
Internal surface resistance (Rsi)	—	—	0,13
Plasterboard	13	0,21	0,06
Mineral wool (frame)	200	0,036	5,56
Wind barrier board	25	0,055	0,45
Ventilation gap + cladding	—	—	0,00
External surface resistance (Rse)	—	—	0,04

R_T = 0.13 + 0.06 + 5.56 + 0.45 + 0.04 = 6.24 m²K/W

U = 1 / 6.24 = 0.16 W/m²K — meets the requirement (≤ 0.17).

Insulation thicknesses for different insulation materials

The thermal conductivity of different insulation materials varies significantly, which directly affects the required insulation thickness. The table below shows approximate insulation thicknesses to achieve an external wall U-value of 0.17 W/m²K (insulation material portion only, excluding other layers).

Insulation material	λ (W/mK)	Thickness (mm)
Mineral wool (stone/glass)	0.035–0.040	200–240
EPS (expanded polystyrene)	0.033–0.039	190–230
XPS (extruded polystyrene)	0.030–0.036	170–210
PIR (polyisocyanurate)	0.022–0.027	120–160
Wood fiber insulation	0.038–0.043	220–250
Cellulose insulation	0.039–0.042	220–250

PIR insulation requires the thinnest layer but is more expensive per square meter. Mineral wool is the most affordable but requires a thicker layer. Material choice affects the structure's thickness, costs, and buildability.

Accurate calculation of insulation materials is important in bid estimation. Incorrect insulation thickness means either excess material or failing to meet energy efficiency requirements.

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U-value in renovation construction

In renovation construction, U-value requirements are not as strict as in new construction. The Ministry of the Environment decree requires that the U-value of the renovated building component be improved, but the new construction level does not necessarily need to be achieved.

In practice, renovation construction follows the principle: the U-value of the building component must improve by at least half the difference between the original and new construction requirement. If the old wall U-value is 0.40 W/m²K and the new construction requirement is 0.17 W/m²K, the U-value after renovation must be at most 0.40 - (0.40-0.17)/2 = 0.29 W/m²K.

In renovation construction, the moisture performance of the structure must also be considered. Additional insulation changes the moisture behavior of the structure, and incorrectly done insulation can cause moisture problems. Design should be left to a professional.

U-value, E-value and energy efficiency

U-value and E-value are related but measure different things. U-value describes the insulation of a single building component, E-value the energy consumption of the entire building.

The building's E-value is calculated considering all building component U-values, ventilation, heating system, solar radiation transmission through windows, and many other factors. Better U-values lead to a lower E-value, but they alone do not determine energy efficiency.

Overall optimization of energy efficiency is called energy calculation. Different building component insulation levels can be balanced: for example, roof insulation can be improved more and wall insulation less, as long as the overall E-value requirement is met.

Most common U-value problems

Achieving the U-value on paper does not guarantee that the structure performs as designed. The quality of execution determines the actual energy efficiency. The most common problems are:

Thermal bridges: Timber frame locations, window junctions and foundation connections weaken the actual U-value. The impact of thermal bridges can be 10–30% of the calculated U-value.
Air leaks: Air flow through the insulation significantly reduces insulating capacity. Careful installation and airtightness of the vapor barrier are critical.
Insulation settling: Blown insulation can settle over time, leaving an uninsulated area at the top. Correct density and sufficient over-dimensioning prevent the problem.
Moisture accumulation: If the structure does not ventilate properly, moisture can accumulate in the insulation and significantly reduce its insulating capacity.
Discontinuities: The wall-roof junction, foundation junction and penetrations are typical locations where insulation remains inadequate.

Accurate material calculation also helps with quality assurance. When you know exactly how much insulation is needed, you can ensure the correct amount is also installed. Massoi calculates insulation material areas and quantities directly from drawings.

Frequently asked questions about U-value

What is a good U-value for an external wall?

In new construction, the U-value requirement for external walls is a maximum of 0.17 W/m²K. Passive houses aim for below 0.10 W/m²K. In practice, 0.15–0.17 W/m²K is a typical U-value for a new construction external wall.

How is the U-value calculated?

The U-value is calculated using the formula U = 1/R, where R is the total thermal resistance. The thermal resistance of each material layer is calculated using R = d/λ (thickness divided by thermal conductivity). Layer resistances are added together plus surface resistances.

What are the U-value requirements in 2026?

U-value requirements for new construction in 2026 are: external wall ≤ 0.17 W/m²K, roof ≤ 0.09 W/m²K, ground floor ≤ 0.16 W/m²K, and windows and doors ≤ 1.0 W/m²K. Requirements are based on the Ministry of the Environment decree.

What is the difference between U-value and E-value?

U-value describes the thermal insulation of a single building component (W/m²K). E-value describes the calculated total energy consumption of the entire building (kWh/m²/year). U-values are one factor in the E-value calculation — but ventilation, heating system and other factors also affect the E-value.

How does the U-value affect insulation thickness?

A lower (stricter) U-value requirement means thicker insulation. Insulation thickness also depends on the material's thermal conductivity: PIR insulation (0.022 W/mK) requires only about 120 mm, while mineral wool (0.036 W/mK) requires about 220 mm for the same U-value.

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