Summer Thermal Protection in Accordance with DIN 4108-2

Verification of summer thermal protection in accordance with DIN 4108-2 is legally required for all new buildings and major extensions under the German Building Energy Act (GEG, section 14) and can be carried out using two methods: the simplified solar heat gain coefficient procedure (section 8.3) or thermal building simulation (section 8.4) with over-temperature degree hours as the evaluation metric (residential limit: 1200 Kh at 25 degrees C). Particularly for glazing proportions above 50 %, corner rooms or attic apartments, the simulation-based verification is often the only way to pass, because it physically models thermal mass, night ventilation and solar shading strategies.

Why summer thermal protection?

Overheated rooms impair the performance of office workers, disturb sleep in residential buildings and can have health consequences for vulnerable population groups. Studies show that productivity drops measurably above an operative room temperature of 26 °C — a key finding of comfort analysis. At the same time, the heat load demand for active cooling rises when the structural thermal protection is not sufficient. Good summer thermal protection avoids or reduces the need for mechanical cooling and thereby lowers investment and operating costs.

The German Building Energy Act (GEG) requires in § 14 compliance with summer thermal protection in accordance with DIN 4108-2 for all new buildings and major extensions. The verification is part of the building approval procedure.

The two verification methods

Method 1: Solar heat gain coefficient (§ 8.3)

The simplified method according to section 8.3 of DIN 4108-2 compares an existing solar heat gain coefficient $S_{vorh}$ with an admissible value $S_{zul}$. The verification is provided when:

The existing solar heat gain coefficient is calculated as:

Here $A_{W,j}$ is the window area of the j-th glazing, $g_{total,j}$ the total solar energy transmittance including solar protection, and $A_G$ the net floor area of the room. The total solar energy transmittance $g_{total}$ results from the g-value of the glazing and the reduction factor $F_C$ of the solar shading:

Typical values for the reduction factor $F_C$ are 0.2 to 0.3 for external venetian blinds, 0.5 to 0.7 for internal roller blinds and 0.7 to 0.9 for glazing without solar protection.

The admissible solar heat gain coefficient $S_{zul}$ depends on the climate region (DIN 4108-2 distinguishes three summer climate regions in Germany), the building type (lightweight/heavyweight), the availability of night ventilation and the window area proportion. For heavyweight buildings in moderate climate regions with night ventilation, higher values are permitted than for lightweight buildings in hot regions without night ventilation.

Advantages of the simplified method: Quick calculation, suitable for hand calculation, little input data.

Limitations: The method does not take into account room-specific geometries, detailed thermal mass effects, or complex shading situations. For buildings with a high glazing proportion or unusual geometry, it often produces overly conservative results — the room fails the verification even though it would be uncritical in reality.

Method 2: Thermal simulation (§ 8.4)

The simulation method based on dynamic building simulation according to section 8.4 of DIN 4108-2 calculates the room temperatures over a period of at least one summer period (typically 1 June to 31 August or the entire year) in hourly or finer time resolution. The over-temperature degree hours serve as the evaluation quantity.

What are over-temperature degree hours?

Over-temperature degree hours $\Theta_{ÜGS}$ quantify how much and for how long the operative room temperature exceeds a threshold temperature $\theta_{grenz}$:

Here $\theta_{op,i}$ is the operative room temperature at time step $i$, $\theta_{grenz}$ is the threshold temperature (depending on usage type, e.g. 25 °C or 27 °C) and $\Delta t$ is the time step size in hours. Only hours in which the threshold temperature is exceeded enter the sum.

DIN 4108-2 defines maximum admissible values of $\Theta_{ÜGS}$ for different usage types. For residential buildings, the limit value is 1200 Kh (Kelvin-hours) relative to a threshold temperature of 25 °C during the hours of use.

When is the simplified method not sufficient?

In practice, the solar heat gain coefficient procedure often fails for:

• Rooms with glazing proportions above 50 % of the façade area
• Corner rooms with glazing facing two or more cardinal directions
• Attic apartments with roof windows
• Buildings in summer climate region C (Upper Rhine Rift, parts of Bavaria)
• Rooms with high internal loads (server rooms, densely occupied offices)

In such cases, the thermal simulation under § 8.4 can show that summer thermal protection is nevertheless maintained — provided that the building has sufficient thermal mass, effective solar shading and ventilation options. The software used should comply with recognised validation standards.

Factors influencing summer thermal protection

Glazing and solar shading

Glazing is the most important path by which solar heat enters. The g-value of modern triple glazing is 0.50 to 0.55, for solar protection glazing 0.25 to 0.35. External solar shading (external venetian blinds, louvres) reduces the solar input most effectively because the absorbed radiation is dissipated to the outside before it reaches the room. Internal solar shading is significantly less effective because the radiation has already passed through the glazing.

Thermal mass

Heavy components (concrete, masonry, screed) absorb heat during the day and release it again at night. This buffering effect smooths out temperature peaks. A 20 cm thick concrete ceiling with direct thermal connection to the room (without a suspended ceiling) can lower the maximum room temperature by 2 to 4 K compared to a lightweight construction. Suspended ceilings and carpeting reduce the effective thermal mass and worsen summer thermal protection.

Ventilation and night-time cooling

Increased ventilation, especially night ventilation, is one of the most effective passive measures. On cool nights, the heat stored in the building components is dissipated, so that the building starts the next morning with lower temperatures. An air exchange rate of 2 to 4 h⁻¹ during night hours (10 p.m. to 6 a.m.) can reduce the over-temperature degree hours by 40 to 60 %. The prerequisite is that the night air temperature is sufficiently below the room temperature and that ventilation is feasible from a safety perspective (burglary protection, weather protection).

Orientation and shading

South-facing windows receive less direct radiation in summer than east- or west-facing windows, because the sun is high in summer and overhangs provide effective shading. West and east façades are particularly critical because the low-angle afternoon or morning sun penetrates deep into the room. Horizontal overhangs are effective on south façades but hardly effective on east and west façades.

Practical tips for the verification

1. Prioritise solar shading: An external, automatically controlled solar shading is the most effective single measure. $F_C$ values of 0.20 to 0.25 are realistically achievable.

2. Activate thermal mass: Avoid suspended ceilings and raised floors where possible, or at least partially leave them open so that the concrete ceiling remains thermally effective.

3. Provide for night ventilation: Plan openable windows or mechanical night ventilation. In the simulation-based verification, the actually achievable ventilation rate must be substantiated. The results can serve as a basis for a digital twin of the building.

4. Limit glazing proportion: A window area proportion of 40 to 50 % of the façade area is still manageable in most climate regions. Beyond that, the effort required for solar shading and cooling rises considerably.

5. Use simulation: If the simplified method is not passed, the simulation-based verification is almost always worthwhile. The physically more accurate calculation in many cases leads to successful verification without the need for additional structural measures.

Conclusion

Summer thermal protection in accordance with DIN 4108-2 is a central requirement of building design that is gaining further importance due to climate change. The simplified solar heat gain coefficient procedure offers a quick overview but reaches its limits for demanding buildings. Thermal simulation in accordance with § 8.4 represents the actual building behaviour in a realistic way and shows whether a building remains comfortable even under extreme summer conditions. The combination of effective solar shading, thermal mass and night ventilation is the key to good summer thermal protection. In VICUS Buildings, the simulation-based verification in accordance with DIN 4108-2 can be carried out directly — the software quantifies the contribution of each measure and makes well-founded design decisions possible.

Further reading: Dynamic Building Simulation — the simulation fundamentals behind the verification method, Indoor Climate and Comfort Analysis — comfort criteria and thermal well-being evaluation, Steady-state vs. Dynamic Calculation — when each method is appropriate for thermal protection verification.

References and Standards

• DIN 4108-2 — Thermal protection and energy economy in buildings — Minimum requirements for thermal insulation
• DIN EN 16798-1 — Energy performance of buildings — Ventilation for buildings — Part 1: Indoor environmental input parameters
• VDI 6007 Part 1 — Calculation of transient thermal response of rooms and buildings — Room model