Indoor Climate and Comfort Analysis through Simulation

Thermal comfort can be objectively assessed through dynamic building simulation: the simulation calculates hourly operative temperatures, PMV/PPD values according to ISO 7730, and compliance with comfort categories I—IV according to DIN EN 16798-1 for every room. This allows overheating in summer, draught risks in winter, and the effectiveness of countermeasures (solar shading, night ventilation, temperature reduction) to be quantitatively evaluated during the design phase — rather than discovering comfort problems only after construction.

What is thermal comfort?

Definition and influencing factors

Thermal comfort is defined in ISO 7730 as the state in which a person expresses satisfaction with the thermal environment. This state depends on six factors, four of which concern the environment and two the person:

Environmental factors:

• Air temperature $\theta_a$: the temperature of the room air, measured at the occupant’s position.
• Mean radiant temperature $\bar{\theta}_r$: the weighted mean temperature of all surrounding surfaces. Cold window surfaces in winter or sunlit walls in summer significantly influence $\bar{\theta}_r$.
• Air velocity $v_a$: air movement increases the convective heat transfer at the body and may be perceived as draught.
• Relative humidity $\varphi$: affects the evaporative cooling of the skin. In the range of 30 to 60 %, humidity has only a minor influence on comfort; at very high humidity (> 70 %), heat dissipation is noticeably impaired.

Personal factors:

• Metabolic rate $M$: the body’s heat production, dependent on activity. Sedentary office work corresponds to approximately 1.2 met (70 W/m²), light industrial work to approximately 2.0 met.
• Clothing insulation $I_{cl}$: the thermal resistance of clothing. Summer office attire corresponds to approximately 0.5 clo, winter clothing to approximately 1.0 clo.

Operative temperature

In practice, the operative temperature $\theta_{op}$ is often used as the central comfort quantity. It combines air temperature and radiant temperature into a single value:

At low air velocities (< 0.2 m/s), this simplifies approximately to:

The operative temperature is the quantity used as the evaluation criterion in most comfort standards. It is also used in the verification of summer thermal protection in accordance with DIN 4108-2.

The PMV/PPD model according to Fanger

PMV — Predicted Mean Vote

The PMV model (Predicted Mean Vote) developed by P.O. Fanger calculates the expected mean thermal sensation of a group of people on a scale from -3 (cold) to +3 (hot), where 0 represents the thermally neutral state. The PMV value is based on the energy balance of the human body:

Here $M$ is the metabolic rate and $L$ the thermal load on the body — the difference between internal heat generation and heat loss to the environment. The thermal load $L$ depends on all six comfort factors and is calculated using complex heat transfer equations.

PPD — Predicted Percentage of Dissatisfied

From the PMV value, the PPD value (Predicted Percentage of Dissatisfied) is calculated, which indicates the expected percentage of dissatisfied occupants:

Even at a PMV of 0 (thermally neutral), the PPD is 5 % — there is always a proportion of people who are dissatisfied. For the comfort range (PMV between -0.5 and +0.5), the PPD is below 10 %.

Comfort categories according to DIN EN 15251

DIN EN 15251 (replaced by DIN EN 16798-1, largely identical in content) defines four comfort categories that set different levels of requirement for the indoor climate:

For most new buildings, category II is targeted. The simulation calculates how many hours per year the building is in each category and identifies periods in which comfort is not maintained.

How simulation predicts indoor climate

Calculation of hourly room temperatures

Dynamic building simulation calculates the air and operative temperature for each thermal zone at each time step. The air temperature results from the zone energy balance taking into account transmission, ventilation, solar gains, internal loads and HVAC capacity. The mean radiant temperature is calculated from the surface temperatures of the surrounding building components.

Typical simulation results for a south-facing office show, for example:

• Winter: With the heating running, the operative temperature lies between 20 and 22 °C. With large window surfaces, the radiant temperature near the window may be 2 to 3 K below the air temperature, which is locally perceived as uncomfortable.
• Transition period: On sunny spring days, solar gains may drive the room temperature to 25 to 27 °C even though the heating is switched off. Without solar protection, overheating is imminent.
• Summer: Without active cooling, the operative temperature can rise to 30 °C and more during heat waves. The simulation shows how long and how frequently critical temperatures are exceeded.

Comfort assessment

From the hourly temperature values, the simulation calculates the over-temperature degree hours (for verification in accordance with DIN 4108-2), the number of hours in each comfort category (for evaluation in accordance with DIN EN 16798-1) and, optionally, the PMV/PPD value for each time step.

Practical applications

Office buildings

In office buildings, thermal comfort is directly linked to employee productivity. The simulation evaluates whether passive cooling (night ventilation, solar protection) is sufficient or whether active cooling (chilled ceilings, recirculation chillers) is required. The results also influence the design of heating curves. Typical questions: Is external solar protection sufficient to keep the room temperature below 26 °C? How does thermally activated building systems (TABS) affect temperature peaks?

Schools and educational facilities

Classrooms present special requirements: high occupancy densities (down to 2 m² per person) produce considerable internal loads. At the same time, ventilation is essential for hygienic reasons (CO₂ concentration) and for comfort. The simulation shows how different ventilation concepts (window ventilation, mechanical ventilation) affect temperature and air quality.

Residential buildings

In residential buildings, summer thermal protection is of primary importance, especially in attic apartments and buildings with large window areas. The simulation evaluates the effectiveness of shading measures, night ventilation and thermal mass. Winter comfort too — in particular the radiation asymmetry near windows — can be analysed.

Special uses

Museums (constant temperature and humidity for exhibits), server rooms (cooling load), swimming halls (high humidity) and hospitals (special comfort requirements) require differentiated comfort analyses that go far beyond the standard evaluation.

Relationship to HVAC sizing

Comfort analysis and HVAC sizing are closely linked. The simulation shows not only whether comfort is maintained, but also what heating or cooling power is required to achieve it. The peak power determines the sizing of the system, while the annual energy determines the operating costs.

A frequent finding in practice: the heating load according to DIN EN 12831 (steady-state method) overestimates the power actually needed, as it does not take into account solar gains and internal loads. Dynamic simulation can show that a smaller system is sufficient — provided that the building has adequate thermal mass and that the warm-up reserve is appropriate.

Conversely, the cooling load is often underestimated when only steady-state methods are used. Dynamic simulation shows which cooling load peaks actually occur under a combination of high outdoor temperature, maximum solar radiation and full occupancy.

Local comfort and draught risk

In addition to the general indoor climate, simulation can also identify local comfort problems. Cold window surfaces generate cold air drop that is perceived as draught at floor level. The draught rating (DR) according to ISO 7730 assesses the risk:

with the local air temperature $\theta_a$, the air velocity $v_a$ (in m/s) and the turbulence intensity $Tu$ (in %). A DR below 15 % is considered comfortable.

Conclusion

Indoor climate simulation and comfort analysis combine building-physics calculations with the human perception of well-being. They answer the decisive question of building design: Will occupants feel comfortable in this building? The operative temperature, the PMV/PPD model and the comfort categories according to DIN EN 16798-1 provide objective evaluation criteria. Dynamic simulation calculates these quantities for every hour of the year and identifies critical periods and rooms. The results feed directly into HVAC sizing and enable planning that optimises both energy efficiency and user satisfaction. VICUS Buildings calculates these comfort parameters for every zone and every time step, enabling a differentiated evaluation already during the design phase. In a time of rising summer temperatures and growing comfort expectations, comfort analysis through simulation is becoming an indispensable component of building design.

Further reading: Dynamic Building Simulation — the simulation method behind comfort prediction, Summer Thermal Protection in Accordance with DIN 4108-2 — overheating analysis and verification, Steady-state vs. Dynamic Calculation — why dynamic methods are essential for comfort evaluation.

References and Standards

• DIN EN 16798-1 — Energy performance of buildings — Ventilation for buildings — Part 1: Indoor environmental input parameters
• ISO 7730 — Ergonomics of the thermal environment — Analytical determination of thermal comfort using PMV and PPD indices
• ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy