In times of resource scarcity, energy shortages and energy transition, questions about the energy efficiency of buildings and building components are much more urgent than in the past. It is therefore all the more essential to deal with the relevant terminology. Correctly calculating the transmission heat loss (HT) is a decisive factor for the energy efficiency of buildings.
A transmission heat loss calculation provides information on how much heat is lost through walls and ceilings as well as windows, doors and the roof and is therefore an important limit value in the Building Energy Act and in the energy performance certificate. A lower value generally also means lower heating costs.
In the following, we summarize the most important calculation principles for calculating transmission heat loss, break down the meaning of the term and show you how you can sustainably reduce transmission heat loss.
The most important facts in brief
The loss of thermal energy through components of the building envelope is referred to as transmission heat loss
Is determined room by room and takes into account all areas adjacent to outdoor areas or rooms
Specialized software facilitates calculation and integration into ongoing construction processes
What exactly is meant by the term transmission heat loss (HT)?
The loss of thermal energy through the façade or envelope of a building is referred to as transmission heat loss (HT for short). If this were completely sealed, there would be no energy loss and the temperature would always remain constant. However, the reality is usually different.
All building materials used are permeable to heat to varying degrees - some more, some less. This property is also referred to as thermal transmittance.
The loss of thermal energy through transmission is particularly significant if the insulation is inadequate. This means that the more permeable the outer layer of the building is, the more thermal energy escapes into the environment via walls, doors, windows and the roof. The heating system must therefore compensate for this - and therefore use more energy.
In building physics, the specific transmission heat loss is abbreviated as HT. It is measured in watts per Kelvin (W/K). Watt per Kelvin indicates the power in watts that is generated at a temperature difference of one Kelvin between the interior and exterior.
What is the significance of transmission heat loss?
The loss of heat through transmission is not only an indicator of the quality of a house's thermal insulation, but is also regulated by law.
The Building Energy Act (GEG) sets specific maximum values for new buildings and existing residential and non-residential buildings. Developers who receive subsidies from the Federal Subsidy for Efficient Buildings (BEG) must also comply with certain maximum values.
How can the transmission heat loss (HT) be calculated?
In order to calculate the transmission heat loss, the surface area and the U-value of the component must first be determined. Both values are then multiplied by a special temperature correction factor.
This factor takes into account, for example, the smaller temperature difference between the inside of the building and the ground compared to the outside air. DIN EN 12831 for heat load calculation lists various values that must be taken into account in the calculation.
In the next step, the thermal bridge allowance is multiplied by the component area - either using standard values or dimensions calculated individually for the component. The specifications of DIN 4108 are decisive for the correct choice of standard values.
Border Temperature correction factor
to the ground 0,4
adjacent buildings 0,5
unheated room 0,8
to the outside via a roof 0,9
Outside air 1,0
To calculate the transmission heat loss (HT), the overall formula is therefore
Transmission heat loss = (U-value x temperature correction factor x area) + (thermal bridge allowance x area)
Air heat loss as an additional factor
When planning a heating system, in addition to the loss through heat conduction, the loss through ventilation (HV) is also important and is also determined room by room.
This relates to heat losses due to leaks in the building envelope, personal ventilation behavior and any existing ventilation systems.
What are the legal requirements?
Since 2020, the Building Energy Act (GEG) has combined the previous regulations of the Energy Saving Ordinance (EnEV), the Energy Saving Act (EnEG) and the Renewable Energies Heat Act (EEWärmeG) and is therefore decisive for building owners.
It establishes binding specifications and guide values not only for planned new buildings, but also for existing residential and non-residential buildings. The annual primary energy requirement, specific heat transfer coefficients (U-values) and maximum values for specific transmission heat losses are defined by technical reference designs in the GEG.
The Energy Saving Ordinance (EnEV)
Since 2009, the Energy Saving Ordinance (EnEV) has required building owners in Germany to comply with energy efficiency criteria. There are requirements regarding thermal insulation and permissible heat loss, particularly for newly constructed buildings.
For example, a maximum heat loss limit of 0.45 watts per Kelvin and square meter applies to a residential building that is attached to another building. In comparison, a detached house may not lose more than 0.40 watts per Kelvin and square meter of heat.
Decisive for funding opportunities
Precisely calculating the transmission heat loss is not only crucial for the realization of energy-efficient refurbishments or new buildings, but also decisive for possible financial support from the federal government.
When renovating or constructing new buildings, the federal subsidy for efficient buildings (BEG) can be used as long as the maximum permissible values for energy consumption are adhered to. The transmission heat loss and the annual primary energy requirement are decisive for the energy quality of a building and a possible subsidy.
How can transmission heat loss be reduced when planning a building?
Transmission heat loss can be sustainably minimized in advance. For example, powerful planning software that can include and display all the key factors in the building is recommended.
The choice of appropriate building elements is also crucial. This is because the transmission heat loss of a building can also be reduced with the help of improved or additional insulation of the building and the use of energy-efficient windows. The U-values, which characterize the thermal insulation quality of a building element, serve as a guideline.
The following individual measures also influence the energy quality and transmission heat loss of a building:
energy-saving doors
Installation of new roof insulation
Insulation of the cellar ceiling
Renovation of the exterior façade of existing properties
Exchange and replacement of windows with special thermal insulation glazing
Planning for greater energy efficiency
Which of these measures results in the greatest reduction in transmission loss can vary greatly from building to building.
This makes careful planning and documentation in advance of the construction process all the more important. It is therefore advisable to calculate the transmission heat loss and ensure that it meets the requirements of the relevant regulations such as the Energy Saving Ordinance, particularly before construction of new buildings begins. In this way, expensive reworking can be avoided if the building does not meet the requirements.
In principle, many of these energy optimization measures are eligible for subsidies, provided that the respective limit values for primary energy and transmission heat loss are complied with - both for existing properties and new buildings.
Calculating the transmission heat loss is easy
Transmission heat loss is a key indicator for the sustainable and efficient planning of construction projects. Especially in times of scarce resources and the need for effective energy management, it can significantly reduce costs to calculate the transmission heat loss precisely in advance and to implement the relevant processes at the planning stage.
In most cases, a modern software solution can speed up calculations and make them transparent and comprehensible for all project participants.
Trimble Nova is a powerful CAD and calculation software that can realistically model even highly complex building technology and heating plans - from conception to final implementation. The integrated calculations allow the results to be processed and integrated in real time.
The integrated interface to Trimble Connect is essential for a shared data environment and at the same time ensures that the ready-to-execute key figures on transmission heat loss are accessible to all relevant project participants - without any loss of efficiency.
This software-supported workflow lays the foundation for optimized energy and resource management as early as the planning phase - and makes it possible to precisely calculate the transmission heat loss and use the results for the construction process without delay.
