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Transmission heat loss is the heat transferred through the building envelope to the outside or to adjacent areas due to a temperature difference. The main determining factors are the surface areas and U-values of external walls, the roof, windows, doors and floors, as well as additional losses caused by thermal bridges.
Transmission heat loss is an important parameter for assessing a building's energy performance, calculating its heating load and planning energy renovation measures.
Why is transmission heat loss important?
- Assessing a building's energy performance: It indicates how much the building envelope contributes to the building's heat loss.
- Identifying renovation potential: Building components with high heat losses can be identified and improved in a targeted manner.
- Calculating the heating load: Transmission heat loss is a key component of a building's heating load.
- Comparing building envelopes: Relevant parameters make it possible to assess different building components and building conditions.
- Preparing thermal insulation compliance documentation: Specific transmission heat loss plays an important role under German building energy law.
How does transmission heat loss occur?
When there is a temperature difference, heat moves from the warmer area to the colder area.
In winter, this typically means that heat from heated rooms is transferred through the building envelope to the colder outdoor air, the ground or adjacent unheated areas.
Typical heat-transferring surfaces include:
- external walls
- windows and glazed doors
- external doors
- roof surfaces
- top-floor ceilings
- basement ceilings
- floor slabs
- building components adjacent to unheated rooms
In addition, thermal bridges can increase heat flow at junctions and construction details.
Which factors influence transmission heat loss?
Building component area
In general, the larger the heat-transferring area of a building component, the greater its potential contribution to heat loss.
Accurate building dimensions and component areas are therefore an important basis for the calculation.
U-value
The U-value, also known as the thermal transmittance, describes the rate of heat transfer through a building component and is expressed in W/(m²·K).
A low U-value indicates better thermal insulation. A high U-value means that more heat is transferred for the same area and temperature difference.
Temperature difference
The temperature difference between the heated interior and the adjacent area also influences heat flow.
The greater the difference between the indoor and outdoor temperatures, the higher the instantaneous heat loss under otherwise identical conditions.
Thermal bridges
Thermal bridges occur, for example:
- at building corners
- around window junctions
- at balcony slabs
- at roof junctions
- at plinths
- where materials change
They can cause additional transmission heat loss and must be taken into account depending on the calculation method.
Building component construction
The materials and layer composition determine heat transfer through a building component.
Insulation, wall construction, glazing and frame design therefore have a direct influence on the energy performance of the building envelope.
How is transmission heat loss calculated?
1. Determine the thermal building envelope
To calculate transmission heat loss, the first step is to determine which building components enclose the heated or conditioned area.
This defines the heat-transferring envelope area.
2. Record the building component areas
The actual dimensions of external walls, the roof, windows, doors and other relevant surfaces are required.
Up-to-date geometry data is particularly important for existing buildings, as available plans are often outdated or incomplete.
3. Determine the U-values
Suitable thermal transmittance values are then assigned to the building components.
Depending on the available data, these can be derived from:
- known building component constructions
- planning documents
- building physics calculations
- information about the existing building
- suitable measurement or assessment methods
4. Account for thermal bridges
Depending on the calculation method, additional heat losses at building component junctions are included using appropriate thermal bridge values.
5. Combine the contributions
In simplified terms, the transmission heat transfer coefficient can be calculated from the contributions of the individual building components and thermal bridges using the formula:
Hₜ = Σ(Uᵢ × Aᵢ × bᵢ) + Σ(ψⱼ × lⱼ × bⱼ)
Where:
- U and A represent the U-value and building component area,
- ψ and l represent the linear thermal bridge coefficient and the corresponding length, and
- b represents the relevant temperature correction factor.
An individual external wall with a high U-value can therefore be just as relevant as a very large surface with a comparatively good U-value.
What is the transmission heat transfer coefficient Hₜ?
The transmission heat transfer coefficient Hₜ describes heat transfer through the building envelope based on a temperature difference of one kelvin.
Its unit is:
W/K
It combines the transmission contributions of the relevant building components and thermal bridges at building level.
When this value is combined with a specific temperature difference between the inside and outside, the resulting heat flow under these conditions can be determined.
What is the specific transmission heat loss H'ₜ?
The specific transmission heat loss H'ₜ relates transmission heat transfer to the heat-transferring envelope area of the building.
Its unit is:
W/(m²·K)
This makes it easier to assess the energy performance of buildings and building envelopes independently of their absolute size.
In principle, a low H'ₜ value indicates a building envelope with better thermal performance than a high value.
Under German building energy law, the specific transmission heat loss in relation to the heat-transferring envelope area is a key requirement for the structural thermal insulation of residential buildings.
Transmission heat loss vs. U-value
The two parameters are directly related but apply to different levels.
U-value
- describes an individual building component
- unit: W/(m²·K)
- accounts for heat transfer through a wall, roof or window, for example
Transmission heat loss
- considers heat transfer through several or all relevant building components
- accounts for building component areas
- can also include the effects of thermal bridges
- enables assessment of the entire thermal building envelope
A good U-value for an individual building component therefore does not automatically mean that the building as a whole has low transmission heat loss.
Transmission heat loss vs. heating load
Transmission heat loss and heating load are not the same.
Transmission heat loss describes heat lost through the building envelope.
A heating load calculation also considers other heat losses and boundary conditions, particularly:
- ventilation heat loss
- air changes
- design temperatures
- any other project-specific boundary conditions
Transmission heat loss is therefore a key component of the heating load calculation, but it is not the entire heating load.
Transmission heat loss in existing buildings
For existing buildings, the quality of a calculation depends heavily on the available input data.
Typical challenges include:
- missing or outdated floor plans
- unknown wall and roof constructions
- subsequent structural alterations
- different renovation standards
- replaced windows
- undocumented insulation measures
- unclear thermal bridges
- inaccurate building component areas
Geometry data in particular can be recorded systematically through a digital building survey.
One possible workflow is:
Digital building survey → Building geometry → Building component areas → Building envelope model → U-values and thermal bridges → Transmission heat loss → Heating load or energy performance assessment
Precise geometry data does not replace the building physics assessment of the components. However, it improves one of the key foundations of the calculation: correctly determining the relevant areas.
Transmission heat loss and energy renovation
Improving the building envelope generally reduces transmission heat loss.
Typical measures include:
- external wall insulation
- roof insulation
- insulation of the top-floor ceiling
- insulation of basement ceilings or floor slabs
- replacement of windows and doors with poor thermal performance
- optimisation of building component junctions
- reduction of thermal bridges
The effectiveness of an individual measure depends, among other factors, on the affected area, the previous and future U-values, and the overall construction.
Renovation measures should therefore be assessed in the context of the entire building envelope rather than in isolation.
Quality criteria for a reliable calculation
- Complete recording of the thermal building envelope
- Correct assignment of building component areas
- Transparent and verifiable U-values
- Consideration of relevant thermal bridges
- A consistent reference and area model
- Documented assumptions for unknown components in existing buildings
- Application of the calculation method relevant to the respective compliance assessment
- Consistent building data throughout planning and energy consulting
For existing buildings in particular, even a highly detailed calculation is only as reliable as its input data.
Common errors and misconceptions
- Equating transmission heat loss with the U-value: The U-value describes an individual building component, not the entire building.
- Considering only external walls: The roof, windows, doors, floors and other boundary surfaces also contribute to heat loss.
- Ignoring thermal bridges: Building component junctions can cause additional heat loss.
- Using incorrect building component areas: Inaccurate geometry has a direct impact on the calculation.
- Using old plans without verification: Alterations and renovations change surface areas and the thermal quality of building components.
- Confusing H'ₜ with Hₜ: Hₜ describes a building's heat transfer coefficient, while H'ₜ relates this coefficient to the heat-transferring envelope area.
- Equating heating load with transmission heat loss: Heating load also includes ventilation heat loss, among other factors.
- Using thermography as a direct calculation method: Thermal images can reveal thermal anomalies but do not replace a complete calculation of transmission heat loss.
Standards and regulatory context
DIN EN ISO 13789:2018-04 describes methods for calculating the transmission and ventilation heat transfer coefficients of buildings and parts of buildings.
Specific transmission heat loss in relation to the heat-transferring envelope area is also an important parameter under German building energy law. Among other purposes, it is used to assess the structural thermal insulation of residential buildings.
The calculation method to be applied depends on the relevant compliance requirements and the applicable legal or normative context.
FAQ
Which building components influence transmission heat loss?
The most relevant components include external walls, the roof, windows, doors, ceilings and floor surfaces, as well as thermal bridges at building component junctions.
Is transmission heat loss the same as heating load?
No. Transmission heat loss is an important component of the heating load. A heating load calculation also takes ventilation heat loss and defined design conditions into account.
How can transmission heat loss be reduced?
Primarily by improving the building envelope, for example through insulation measures, more energy-efficient windows and doors, and the reduction of thermal bridges.