Anyone planning or renovating a building made of aerated concrete cannot ignore one key parameter: the heat transfer coefficient, or U-value for short. For Ytong, the best-known brand of steam-cured aerated concrete, this value plays a decisive role in determining how much heat escapes through a wall component to the outside, what wall thickness is required to achieve a specific energy efficiency level, and whether a structure meets the minimum regulatory requirements. The Ytong U-value is not an abstract number, but rather the result of precise building physics relationships that every designer should understand.

• What the U-value means in physical terms and how it is calculated
• Which material properties of Ytong aerated concrete determine the U-value
• What bulk density classes and strength classes exist and how they influence the Ytong U-value
• What regulatory requirements apply in Germany for exterior walls made of aerated concrete
• How to evaluate single-shell and multi-shell wall constructions with Ytong from a building physics perspective
• What wall thicknesses are required for typical performance levels
• How thermal bridges, plaster, and mortar influence the effective U-value
• What role the Ytong U-value plays in the context of moisture protection and preventing condensation

What the U-value is: Definition and physical basis

The heat transfer coefficient U describes how much thermal energy flows per second through one square meter of a building component when there is a temperature difference of one Kelvin between the two sides. The unit is W/(m²K), i.e., watts per square meter per Kelvin. The lower the U-value, the better the building component insulates, and the less heat is lost. A U-value of 0.20 W/(m²K) means that, with a temperature difference of ten Kelvin between the interior and exterior, only two watts of heat escape to the outside per square meter of wall surface. By way of comparison: An uninsulated solid brick wall from a Wilhelminian-era building can have U-values of 1.5 W/(m²K) or higher.

The U-value is calculated as the reciprocal of the total thermal transmittance RT. This is composed of the thermal resistances R of all layers in the wall structure, as well as the heat transfer resistances at the interior and exterior surfaces. The thermal resistance of a single layer is the quotient of its thickness d in meters and its thermal conductivity λ in W/(mK). For a homogeneous wall made of a single material, the simplified rule is: The thicker the wall and the lower the thermal conductivity of the material, the higher the thermal resistance and the lower the resulting U-value. This is precisely where Ytong aerated concrete excels: Its thermal conductivity is significantly lower than that of conventional masonry materials such as solid bricks or ordinary concrete.

Ytong Aerated Concrete: Material Properties and Their Effect on the U-Value

Ytong is the trade name for a steam-cured aerated concrete (AAC, Autoclaved Aerated Concrete) made from quartz sand, lime, cement, water, and a foaming agent, typically aluminum powder. During setting, the reaction of the aluminum with the alkaline solution produces hydrogen gas, which causes the material to foam and creates a uniform, fine-pored structure. The material is then cured in an autoclave under steam pressure, resulting in the formation of tobermorite as a load-bearing binding mineral. The result is a building material with a pore content of up to eighty percent of its total volume, which explains its exceptionally low bulk density and thermal conductivity.

The thermal conductivity (lambda) is the key material parameter for the Ytong U-value. In aerated concrete, it depends directly on the bulk density: the lighter the material, the more air pores it contains, and the poorer its thermal conductivity. Ytong products are available in various bulk density classes, which are typically designated by the letter D followed by a numerical value, such as D300, D350, D400, D500, or D600. The number corresponds to the bulk density in kg/m³. A D300 block has a bulk density of about 300 kg/m³ and a thermal conductivity of approximately 0.08 W/(mK), while a D600 block has a thermal conductivity of about 0.17 W/(mK). Strength increases with bulk density: Lighter classes are thermally superior, while heavier classes can bear higher loads.

The design value of thermal conductivity, as defined by the standards DIN EN ISO 10456 and DIN 4108-4, is used to calculate the Ytong U-value. This design value is slightly higher than the nominal value because it includes a moisture allowance and a statistical safety factor. In practice, this means that the manufacturer’s specifications for lambda are generally design values that may be used directly in U-value calculations, provided they comply with recognized testing and certification standards.

Apparent Density Classes and Their Typical Thermal Conductivities

Selecting the correct bulk density class is one of the first design decisions in a Ytong construction project. For single-shell exterior walls optimized purely for thermal performance, the lightweight classes D300 through D400 are preferred because they offer the lowest thermal conductivities. Classes D500 and D600 are more commonly used in interior applications, for load-bearing interior walls, or in areas where higher compressive strengths are required. Compressive strength is specified in strength classes designated by the letter PP followed by a number. Higher strength classes are generally associated with higher bulk densities and, consequently, higher thermal conductivities. Designers must carefully balance structural requirements against thermal performance goals in these cases.

Regulatory Requirements for the Ytong U-Value in Germany

The key regulatory requirements for the U-value of exterior building components in Germany are set forth in the Building Energy Act (GEG), which replaced the former Energy Saving Ordinance (EnEV), as well as in DIN 4108-2, the basic standard for minimum thermal insulation. The GEG distinguishes between verification using the reference building method and the simplified method, but in either case sets maximum U-values for individual building components. According to the GEG, a maximum U-value of 0.24 W/(m²K) applies to exterior walls of residential buildings in new construction when the simplified method is used. This value should be understood as a minimum requirement; energy-efficient buildings, such as those built to the KfW Efficiency House standard or the Passive House standard, aim for significantly lower values.

DIN 4108-2 regulates minimum thermal insulation and has a different objective than the GEG: It is not primarily intended to protect against energy losses, but rather against condensation on the interior wall surface and thus against mold damage. The minimum value required by this standard for the temperature factor fRsi on the interior surface is 0.70 for residential buildings. This value must be met to ensure that the interior wall surface does not drop below the dew point temperature of the indoor air under the standard boundary conditions (indoor temperature 20 degrees Celsius, 50 percent relative humidity; outside minus five degrees Celsius). For a single-shell Ytong exterior wall without thermal bridges, this requirement can generally be met without difficulty provided the wall has sufficient thickness.

According to the GEG, requirements also apply to renovation measures on existing buildings, though these differ from those for new construction. For modifications to existing exterior walls affecting more than 10 percent of the wall area, maximum U-values must be adhered to, which are slightly more lenient than the requirements for new construction. Anyone retrofitting an existing building with a Ytong cladding shell or retrofitted interior insulation made of aerated concrete must be familiar with these limits and provide evidence of compliance. Separate requirements apply to commercial non-residential buildings, which vary depending on the type of use and interior temperature.

Single-shell Ytong exterior walls: Wall thicknesses and achievable U-values

The particular advantage of Ytong aerated concrete lies in the ability to construct single-shell, monolithic exterior walls without an additional thermal insulation layer, while still meeting code requirements. This construction method significantly simplifies the design, avoids the issues associated with composite structures, and reduces susceptibility to thermal bridges. Whether a single-shell Ytong wall is sufficient depends on the selected bulk density class, the wall thickness, and the target energy standard.

As a guide: A Ytong exterior wall of bulk density class D400 with a thermal conductivity of approximately 0.10 W/(mK) and a wall thickness of 36.5 cm achieves a U-value of about 0.25 W/(m²K), which is just below the GEG minimum requirement. With a wall thickness of 42.5 cm in the same class, the U-value drops to about 0.22 W/(m²K). If you choose the lighter Class D300 with a thermal conductivity of around 0.08 W/(mK), a wall thickness of 36.5 cm already achieves a U-value of about 0.20 W/(m²K) with a wall thickness of just 36.5 cm, coming close to KfW-55 requirements. These values are approximations; the exact results depend on the design values of the respective product, the plaster system, and the heat transfer resistances.

For passive houses or KfW-40 buildings, sufficiently low U-values of 0.10 to 0.15 W/(m²K) generally cannot be achieved with single-shell Ytong alone without the wall thicknesses becoming uneconomically large. In these cases, Ytong is often used as the inner shell of a double-shell construction, combined with exterior thermal insulation made of mineral wool or polystyrene, or special Ytong products with particularly low bulk density are used. Some manufacturers also offer Ytong STEINS with integrated insulation or tongue-and-groove systems that minimize thermal bridges at the joints.

Effect of Plaster and Mortar on the Ytong U-Value

In practice, a Ytong exterior wall never consists solely of the STEINS themselves. Exterior and interior plaster, as well as the mortar joints between the STEINS, influence the effective U-value of the entire building component. Ytong-based exterior plasters typically have thermal conductivities ranging from 0.20 to 0.40 W/(mK), with interior plasters having similar values. With typical plaster thicknesses of one to two centimeters, the impact on the U-value is small but not entirely negligible. More important is the choice of mortar for the bed joints: Standard mortar has a thermal conductivity of about 1.0 W/(mK) and thus forms a thermal bridge between the STEINS. Ytong thin-bed mortar, which is designed for bed joints only one to three millimeters thick, significantly reduces this effect. For a precise U-value calculation according to DIN EN ISO 6946, the influence of the mortar joints should be taken into account, which—with thin-bed mortar and small joint cross-sections—usually results in a negligible correction.

Thermal Bridges and Their Effect on the Effective U-Value

The calculated U-value of a homogeneous wall cross-section is an idealized value that accurately describes a building’s actual heat loss only if there are no thermal bridges. Thermal bridges are geometric or material-related weak points in the thermal envelope where the heat flow is locally increased. In Ytong constructions, thermal bridges typically occur at window reveals, lintel and sill areas, ceiling supports, balcony slabs, building corners, and connections to the foundation or roof.

For the energy assessment of a building according to GEG, thermal bridges are either accounted for on a flat-rate basis via a surcharge on the average U-value of the building envelope or calculated in detail using linear heat transfer coefficients (Psi values) for each individual type of thermal bridge. According to DIN 4108 Supplement 2, the flat-rate surcharge is 0.05 W/(m²K) for buildings with thermal bridge-minimized design and 0.10 W/(m²K) for standard design. For Ytong constructions with consistently implemented thermal bridge-minimized detailing, the lower allowance generally applies, which noticeably improves the overall thermal performance.

Compared to other masonry materials, Ytong aerated concrete offers a structural advantage in preventing thermal bridges: The material can be sawed, milled, and sanded using simple tools, which facilitates the precise fabrication of reveals, lintels, and connections. Ytong lintels made of aerated concrete, which have the same thermal conductivity as the wall blocks, prevent the thermal bridge that a precast concrete lintel would inevitably create. This design feature is a key reason why Ytong buildings often achieve better actual energy consumption values in practice than those predicted by calculations.

Ytong U-Value, Moisture Protection, and Condensation Prevention

The U-value of a Ytong wall is not only relevant to the energy balance but also has a direct impact on moisture protection. A well-insulated wall maintains its inner surface close to room temperature, ensuring that the temperature of the wall surface does not fall below the dew point temperature of the indoor air. Conversely, a U-value that is too high—i.e., insufficient thermal insulation—means that the interior wall surface cools down significantly at low outdoor temperatures, promoting condensation. This is the physical relationship that DIN 4108-2 addresses normatively with the temperature factor fRsi.

Ytong aerated concrete is a vapor-permeable building material: it allows water vapor to pass through, but also stores moisture in its pores. The vapor diffusion resistance of aerated concrete is low; the water vapor diffusion resistance factor Mu ranges between approximately five and ten, depending on the bulk density. This means that Ytong walls can absorb moisture from the indoor air and release it again under favorable conditions—a process known as moisture buffering. This property is beneficial for the indoor climate but requires special care when planning interior insulation or vapor-tight coatings to prevent moisture accumulation within the wall cross-section.

To verify condensation protection within the wall cross-section, the Glaser method according to DIN 4108-3 or a hygrothermal simulation is used. In the Glaser method, the distribution of temperature and vapor pressure of saturation through the wall cross-section is calculated to determine whether the dew point is exceeded at any point. For single-shell Ytong exterior walls without interior insulation, the result is generally uncritical because the vapor pressure profile does not create a plane where condensation could occur. The situation becomes more critical with retrofitted interior insulation or in constructions with vapor-tight layers on the interior side, which require careful planning.

Ytong U-Value in the Context of Overall Planning

The Ytong U-value is not an isolated parameter but part of a comprehensive building physics concept. Anyone planning a building made of aerated concrete must consider the U-value of the exterior wall in conjunction with the U-values of the roof, floor slab, and windows, as well as the air exchange rate and the absence of thermal bridges in the structure. An Ytong exterior wall with excellent insulation loses its effectiveness if thermal bridges at ceiling connections or window reveals dominate the total heat loss. Conversely, a Ytong construction that consistently minimizes thermal bridges—even with moderate wall thicknesses—can achieve better actual energy consumption values than a construction that is theoretically optimized but poorly executed.

The strength of Ytong aerated concrete lies in the combination of good thermal insulation, ease of installation, sufficient load-bearing capacity for multi-story residential construction, and diffusion-open properties that are advantageous from a building physics perspective. These properties make it one of the most widely used masonry materials for single-shell exterior walls in Germany. Anyone who understands the basics of the Ytong U-value, knows how bulk density, wall thickness, and thermal conductivity are related, and is familiar with the code requirements can use this building material purposefully and efficiently. Calculating the U-value is not an end in itself, but rather a tool that designers use to ensure that a building remains energy-efficient, moisture-resistant, and comfortable over the long term.