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Glossary · Energy industry & energy transition

Building envelope

building envelope heat demand Energy Efficiency Heating load heat pump renovation building

Definition and scope

The building envelope comprises all building components that separate heated or cooled interior spaces from outside air, the ground or unheated parts of the building. These include external walls, the roof or top-floor ceiling, windows and French doors, external doors, the basement ceiling or floor slab, and the junctions between these components. In German building energy law, this surface is called the "heat-transferring enclosure surface" (wärmeübertragende Umfassungsfläche). The top-floor ceiling is defined there as the accessible ceiling of heated rooms facing the unheated roof space. In winter, heat leaves the building through the envelope; in summer, heat enters through it.

For energy assessment, it is not only the individual component that counts but the interaction of the entire envelope. A well-insulated external wall helps little if window junctions are leaky, thermal bridges cause high losses, or a lot of heat escapes unnoticed through the basement ceiling. The building envelope is therefore not a decorative surface but has a building-physics function: it limits heat losses, influences air exchange, protects against moisture and helps determine how stable the indoor temperature remains in cold, heat and wind. For new buildings, the law explicitly addresses these aspects with separate provisions on minimum thermal insulation, thermal bridges, airtightness and summer heat protection.

U-value and other characteristics

The most important characteristic of a building component is the thermal transmittance coefficient, or U-value for short. It indicates how much heat output flows through the component per square metre of component area at a temperature difference of one kelvin between inside and outside. The unit is watts per square metre and kelvin, W/(m²K). An example: a wall of 100 m² with U = 0.24 W/(m²K) and a temperature difference of 30 K (20 °C inside, −10 °C outside) loses roughly 720 W by calculation. For an uninsulated older wall of about 1.4 W/(m²K), it would be around 4,200 W. The lower the U-value, the lower the loss.

However, U-values of individual components are not sufficient for the building as a whole. Area and compactness, thermal bridges at junctions, airtightness, ventilation behaviour and solar gains through windows change the actual heat demand. For the airtightness of new buildings, there is a dedicated measured value: in the differential pressure method (blower door test), the amount of air flowing through leaks is measured at 50 pascals. If the measured value is to be used in the energy balance of a new building, the volume flow may be at most three times the heated air volume per hour without a mechanical ventilation system and at most 1.5 times with such a system. For large buildings, a limit value related to the envelope area applies.

Envelope, heating system and heating load

The heating system must be distinguished from the building envelope. The envelope determines how much heat a building loses under certain conditions; the heating system supplies that heat. An inefficient heat generator can increase energy use even though the envelope is good. Conversely, even a modern heating system in a poorly insulated building needs a lot of energy and high output. In practice, heating technology is often discussed as if heat demand could be solved by a new boiler or a heat pump alone. However, the envelope determines a large part of the load that the system technology has to handle.

Equally important is the distinction from the heating load. The heating load is the heat output required on a cold design day, usually stated in kilowatts. Annual heat demand, by contrast, is a quantity of energy in kilowatt-hours per year. A poor envelope usually increases both, but not necessarily in the same proportion. For example, ventilation losses and thermal bridges weigh disproportionately in severe cold, while solar gains reduce annual demand mainly in the transitional season.

Significance for heat pumps and the power system

The quality of the envelope shapes the conditions for the electrification of heat. A heat pump works more efficiently the lower the flow temperature can be. Low flow temperatures are favoured by large heating surfaces, hydraulic balancing and an envelope that only has to compensate small losses. Refurbishing the envelope can therefore improve the seasonal performance factor, reduce electricity costs and lower the required unit capacity. It does not replace good planning of the heating system, but it changes its operating conditions.

In the power system, simultaneity matters above all. When many buildings are heated with heat pumps or directly with electricity, high heating loads often occur in the same cold hours. A poorly insulated building stock then increases not only annual electricity consumption but above all the peak load and the strain on distribution grids. Good envelopes reduce the required amount of heat and dampen these power peaks.

In addition, there is the time-buffering effect. Strictly speaking, insulation does not increase a building's storage mass, since that lies in walls, ceilings and screeds. However, insulation reduces heat loss. As a result, it takes longer for the stored heat to dissipate: a well-insulated, massive building cools down much more slowly. This thermal inertia creates room for flexibility. A heat pump can, for example, heat somewhat more when electricity prices are favourable or when there is a lot of renewable feed-in, and then pause for a while without comfort noticeably suffering. The envelope is not itself an electricity store, but it helps determine how well buildings can serve as flexible heat sinks.

Common misconceptions

  • "Building envelope means facade insulation": The facade can be a major lever but is only one part of the envelope. Often the technically and economically obvious measures lie first with the top-floor ceiling, the basement ceiling, leaky windows or poorly insulated roller shutter boxes. Junctions, penetrations and thermal bridges can also cause high losses and moisture damage. A sensible assessment therefore considers the condition of the building, the component areas, existing damage, use and the renewal cycle, not just the insulation thickness.

  • "An airtight house cannot breathe": Airtightness is meant to prevent uncontrolled leaks through which warm, moist indoor air escapes and can condense in components. Ventilation is still necessary, but in a controlled way: through window ventilation, a ventilation concept or a ventilation system with heat recovery. In terms of building physics, the issues are diffusion, convection, moisture protection and hygienic air exchange. The image of the "breathing wall" is misleading because air exchange should not take place through leaky components.

  • "Modern heating technology makes the envelope superfluous" or conversely "insulation solves everything": Both simplifications separate what works together in the building. Heat pumps also work in many existing buildings, but their efficiency, their electricity costs and their grid compatibility depend strongly on heat losses, heating surfaces and flow temperatures. A good envelope saves energy but does not resolve questions of heat distribution, control or financing.

Legal framework in Germany

The former Building Energy Act (Gebäudeenergiegesetz, GEG) was comprehensively amended in 2026 and renamed the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG). The Bundestag adopted the reform on 10 July 2026, and it was promulgated on 28 July 2026. The main part has applied since 29 July 2026. Further parts, including the implementation of the EU Energy Performance of Buildings Directive and provisions on proof of compliance, are to apply from 1 January 2027. Many sections were renumbered, so older references such as "§ 48 GEG" are therefore no longer necessarily correct.

For new buildings, the law sets requirements for structural thermal insulation, thermal bridges and the permanent airtightness of the envelope. For existing buildings, there is no general obligation to insulate. Requirements apply when external components are renewed, replaced or installed for the first time. The maximum values are set out in Annex 7, which in the current consolidated version is assigned to § 36. For residential buildings and zones of non-residential buildings with a target room temperature of at least 19 °C, the following apply, for example:

  • External walls when replaced, installed for the first time, or when external insulation, cladding or renewal of plaster is carried out: U = 0.24 W/(m²K). For zones with low heating (12 to below 19 °C), 0.35 W/(m²K) applies.

  • Windows and French doors when the entire component is replaced: Uw = 1.3 W/(m²K); roof windows: Uw = 1.4 W/(m²K). New external doors: U = 1.8 W/(m²K).

These values apply only to the component that is changed in each case. There are exceptions and special rules, for example for listed buildings, for small areas of change or for certain insulation methods. Whether a specific measure falls under them must be checked against the legal text in each individual case. Funding programmes may impose stricter requirements than the law.

Cost-effectiveness and stakeholders

Whether a measure on the envelope pays off depends strongly on timing. Many measures make particular sense when components are being renewed anyway: when the roof is re-covered, the facade is refurbished or the windows are replaced. Scaffolding, planning and surface work are then needed in any case, so only the additional costs of the energy improvement need to be assessed. The question "Is insulation worthwhile?" can therefore hardly be answered seriously without knowing the state of maintenance. The assessment also takes into account energy prices, investment costs, funding, service life, comfort, preservation of value and the question of whether today's decisions prepare or obstruct later steps.

Institutionally, the envelope touches several levels. Owners decide on investments, while tenants often bear a large part of the running energy costs. Municipalities consider buildings in municipal heat planning in connection with heat networks, grid expansion and neighbourhood solutions. A technically sensible measure can nevertheless fail because of financing, ownership structure, heritage protection, a lack of skilled tradespeople or an unresolved distribution of costs and benefits.

Overall assessment

The building envelope shows that heat supply does not begin in the boiler room. It is where the demand arises that heat generators, networks, power plants and storage must then meet. Anyone discussing the heat transition, power grids or security of supply must consider this physical level too: every kilowatt-hour of heat that is not lost thanks to a better envelope need not be generated, transported or paid for, nor supplied as capacity in a cold hour.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

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