# Performance of natural ventilation in office buildings: impact of climate, building properties and building use

Simon Bal-Fontaine, Arnaud JAY, Simon Rouchier

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Simon Bal-Fontaine, Arnaud JAY, Simon Rouchier. Performance of natural ventilation in office buildings: impact of climate, building properties and building use. Journal of Physics: Conference Series, 2025, Operation - Heating & cooling systems, 3140, pp.022013. 10.1088/1742-6596/3140/2/022013. cea-05517931

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# Performance of natural ventilation in office buildings: impact of climate, building properties and building use

S Bal-Fontaine¹,², A Jay², S Rouchier¹

¹ Université Savoie Mont-Blanc, CNRS, LOCIE, 73000 Chambéry, France

² Université Grenoble Alpes, CEA LITEN, INES, Le Bourget du Lac, France
simon.bal-fontaine@univ-smb.fr

**Abstract.** The aim of this paper is to define the performance of natural ventilation from different levels, taking into account the climatic potential, the energy performance of the building and the use of the building in terms of thermal comfort of the occupants. To do this, Building Energy Simulation has been used to simulate simplified office buildings with different parameters to quantify their impact on the natural ventilation efficiency. These parameters include three levels of energy performance of the envelope, three different cooling modes with air conditioning only, natural ventilation only and a mixed mode between both, two weather files, and two scenarios of natural ventilation (possible all the day long or only during occupied period). Finally, various KPIs were calculated to show the diversity of ways of looking at natural ventilation performance, and the impacts of changes in the models' input parameters on the three defined performance levels. For example, we have shown that a particular building can have good thermal comfort without having the best building potential, demonstrating the need for multiple levels of natural ventilation performance.

## 1. Introduction

Nowadays, assessing the energy efficiency of buildings in Europe is mainly concerned with winter issues. However, climate change is leading to longer warm periods with more frequent and intense heat waves [1]. It is therefore necessary to anticipate summer energy problems, such as increased need for air conditioning and thermal discomfort of building occupants [2].

To reduce energy consumption while ensuring thermal comfort in buildings, the use of natural ventilation (NV) seems to be one of the main alternatives to air conditioning (AC) systems [2]. However, this cooling method is still not widely used in Europe, especially in office buildings. This is due to the design of these buildings, which takes little account of the summer period, and to the habits of the occupants, which are not yet adapted to natural ventilation. In addition, its performance depends on a large number of parameters with large uncertainties, such as climatic conditions [3], airflow rates [4] and occupant behaviour [5]. Finally, naturally ventilated buildings have variable indoor conditions, which makes the characterisation of thermal comfort in these spaces more complex [6].

The aim of this paper is to explore some ways to characterize the performance of natural ventilation from different levels, taking into account the climatic potential, the energy performance of the building and the use of the building on the thermal comfort of the occupants. For that, we will use 3 simplified office buildings simulated with EnergyPlus [7] with 3 different thermal performance. We will then simulate 3 variants with changes in climate and building constraints to illustrate the variability of input parameters on the performance of natural ventilation. Finally, different cooling modes will be modelled to calculate several Key Performance Indicators (KPI) and to analyse the impact of buildings and their variants on the natural ventilation performance.

<figure><img src="image_2.png" /></figure>
## 2. Methodology

### 2.1. *Definition of natural ventilation performance*

The natural ventilation performance depends on many parameters and it is therefore difficult to quantify an intrinsic performance without taking into account all the influencing parameters. For this reason, we propose a three-level description of this performance (see Figure 1):

* The climatic potential, which corresponds to the maximum performance of natural ventilation in a given location and climate

* The building potential, which corresponds to the performance of a specific building in a given location with optimal use of natural ventilation

* The building efficiency, which corresponds to the final performance of a specific building in a given location, taking into account the constraints of the building (intrusion, noise, etc.) and the behaviour of the occupants (management of internal and solar gains, and openings).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Natural ventilation performance schema</figcaption></figure>

### 2.2. *Global framework*

Figure 2 summarises the methodology that we will explain in this article. First, we simulate the 3 cooling modes for each variant, which are simulated for each building. The right part of the figure shows the cooling mode in which each KPI is calculated. We will explain the name and the calculation of all KPI in the next section. Then, we will detail the buildings, the variants, and the cooling modes. The main purpose of the variants is to illustrate the methodology through significant, but not exhaustive, changes.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Global methodology schema</figcaption></figure>

#### 2.2.1. *KPI Calculation.*
In this study, several KPI are calculated to quantify the climatic potential, building potential and efficiency, and the thermal comfort. For some KPIs we use the adaptive comfort model [8], which is incorporated within the Python package "pythermalcomfort" [9]. Additionally, we opt for the EN 16798-1 2019 model, with an airspeed range of less than 0.6 m/s. This gives an indoor
comfort temperature threshold based on the running mean outdoor temperature, but requires a building with uncontrolled conditions. Table 1 details the calculation of all KPI.

<table><caption>**Table 1.** Key Performance Indicators (KPI) names, types, equations and specifications</caption><thead><tr><th>KPI name [unit]</th><th>KPI type</th><th>Equations</th><th>Specifications</th></tr></thead><tbody><tr><td>Natural Ventilation Potential (NVP) [h] [10]</td><td>Climatic potential</td><td>NVP = &Sigma;<sub>with NV<sub>hours</sub> = 1</sub> NV<sub>hours</sub> if T<sub>ext</sub> &lt; T<sub>up</sub></td><td>T<sub>up</sub> the upper threshold of adaptive comfort model</td></tr><tr><td>Climatic Cooling Potential (CCP) [°C.h] [11]</td><td>Building potential</td><td>CCP = &Sigma;<sub>with m = 1</sub> m * (T<sub>int</sub> - T<sub>ext</sub>) if T<sub>ext</sub> &lt; T<sub>int</sub></td><td>T<sub>int</sub> the indoor temperature of the naturally ventilated building in the variant “ref” is the variant with “AC only” and “study” is the studied variant</td></tr><tr><td>Cooling Requirements Reduction (CRR) [%] [12]</td><td>Building efficiency</td><td>CRR = <div style="display: inline-block; vertical-align: middle; text-align: center;"><div style="border-bottom: 1px solid black; padding: 0 5px;">Q<sub>c,ref</sub> - Q<sub>c,study</sub></div><div style="padding: 0 5px;">Q<sub>c,ref</sub></div></div> if Q<sub>c</sub> cooling needs</td><td></td></tr><tr><td>Climate Potential Utilization Ratio (CPUR) [%] [13]</td><td>Building efficiency</td><td>CPUR = <div style="display: inline-block; vertical-align: middle; text-align: center;"><div style="border-bottom: 1px solid black; padding: 0 5px;">Building efficiency KPI</div><div style="padding: 0 5px;">Climatic potential KPI</div></div></td><td>KPIs in the same unit, timeframe and the same period</td></tr><tr><td>Degree-Hours (DH) [°C.h] [14]</td><td>Thermal comfort</td><td>DH = &Sigma;<sub>with &Delta;t = 1 hour</sub> &Delta;t * (T<sub>int</sub> - T<sub>comfort</sub>)</td><td>T<sub>comfort</sub> the middle temperature of the adaptive comfort model</td></tr></tbody></table>

### 2.2.2. Simulated buildings.
We model three simplified office buildings using DesignBuilder that are based on a standard current floor office building with floors and ceilings as adiabatic surfaces. They have the same external geometry (canopy, window size, etc.) and the same room layout (see Figure 3). The grey areas on the plan are occupied areas, the green area is corridors and the orange area is a storage area.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Simulated current floor office building external view (left), and plan (right)</figcaption></figure>

We assume that there is no occupancy in the corridors and storage areas. In the occupied areas, the lighting power is 4 W/m², the occupancy density is 8 m²/person and the internal loads associated with office equipment are 70 W/person. The mechanical ventilation flow rate is set at 25m3/h.pers during occupied periods, with a 70% reduction in unoccupied periods, and at 0.3 vol/h in the corridors and storage areas. We simulate constant occupancy from 7am to 8pm, every day of the period, to avoid the bias introduced by weekends. The three buildings correspond to different levels of insulation and airtightness, ranging from the most efficient (“RE 2020”) to the least efficient (“Old”). Table 2 shows the main differences between these buildings and it should be noted that the solar shading systems in the buildings are activated throughout the day to minimise solar gain.
<table><caption>**Table 2.** Properties differences of the three buildings</caption><thead><tr><th></th><th>« RE 2020 »</th><th>« RT 2000 »</th><th>« Old »</th></tr></thead><tbody><tr><td><i>Solar protection type</i></td><td>External solar shadings</td><td>Internal blinds</td><td>Internal blinds</td></tr><tr><td><i>Wall composition</i></td><td>20cm of concrete + 20cm of external insulation (U = 0.2 W/m².K)</td><td>20cm of concrete + 10cm of internal insulation (U = 0.3 W/m².K)</td><td>20cm of concrete + 2cm of internal insulation (U = 1 W/m².K)</td></tr><tr><td><i>Windows composition</i></td><td>Triple glazing with argon + wood/metal frame (U<sub>w</sub> = 1 W/m².K)</td><td>Double glazing with air + metal frame (U<sub>w</sub> = 3.5 W/m².K)</td><td>Simple glazing + metal frame (U<sub>w</sub> = 6 W/m².K)</td></tr><tr><td><i>DesignBuilder' infiltrations coefficients</i></td><td>Very low</td><td>Medium</td><td>Very high</td></tr></tbody></table>

### 2.2.3. Simulated cooling modes.
The use of natural ventilation in buildings may be sufficient in some cases, but sometimes it is necessary to add air conditioning to ensure the thermal comfort of the occupants. In this study, three different cooling modes are simulated using the EnergyPlus tool and its AirFlowNetwork, which allows us to have airspeed generated by the natural ventilation. The windows and internal doors are considered fully opened, in order to simulate cross ventilation. Table 3 shows the control conditions and schedule for each mode. The mixed-mode ventilation is a change-over system between natural ventilation and air conditioning.

<table><caption>**Table 3.** Cooling modes details</caption><thead><tr><th></th><th>"AC only" mode</th><th>"Hybrid" mode</th><th>"NV only" mode</th></tr></thead><tbody><tr><td><i>Cooling method</i></td><td>Air conditioning (AC)</td><td>Mixed-mode ventilation</td><td>Natural ventilation (NV)</td></tr><tr><td><i>NV control</i></td><td>X</td><td>T<sub>ext</sub> &lt; T<sub>int</sub></td><td>T<sub>ext</sub> &lt; T<sub>int</sub></td></tr><tr><td><i>NV schedule</i></td><td>X</td><td>24h/24h</td><td>24h/24h</td></tr><tr><td><i>AC control</i></td><td>T<sub>int</sub> &gt; 26°C</td><td>T<sub>int</sub> &gt; 26°C</td><td>X</td></tr><tr><td><i>AC schedule</i></td><td>7h-20h</td><td>7h-20h</td><td>X</td></tr></tbody></table>

### 2.2.4. Simulated variants of the models.
We simulate the three buildings according to three different variants to illustrate the variability of the input parameters on the climatic conditions and the use of the building. Table 4 shows the differences in natural ventilation control and weather file between the variants. The weather files are those used by the RE2020 French thermal regulations [14] for the building simulations for the summer period, which includes a period of 10 days of heat wave in the month of August. The natural ventilation control for the "NV during day" variant is intended to represent building use constraints that would limit the use of natural ventilation to occupancy periods.

<table><caption>**Table 4.** Variants details</caption><thead><tr><th></th><th>Base</th><th>NV during the day</th><th>Hot climate</th></tr></thead><tbody><tr><td><i>NV monitoring</i></td><td>24h/24h</td><td>7h-20h</td><td>24h/24h</td></tr><tr><td><i>Köppen climate category</i> [15]</td><td>« Dfb » (Continental)</td><td>« Dfb » (Continental)</td><td>« Csa » (Mediterranean)</td></tr></tbody></table>

## 3. Results

Results are calculated for the period from 15 April to 15 October, which corresponds to the cooling period in France. Bars in followings graphs are overlap bars, i.e. their height corresponds to their value.
### 3.1. Utilization of the climatic potential

Figure 4 shows the CPUR for the buildings and the variants, and the percentage of time when the outdoor conditions are favourable for natural ventilation. The CPUR corresponds to the fill rate of the solid bar by the hatched bar, i.e. the number of hours of natural ventilation in the simulations relative to the climatic potential (NVP). The CPUR value is displayed on each hatched bar and indicates the extent to which the simulated building is using the climatic potential for natural ventilation. In order to have an understanding of the value of the NVP, it should be noted that the maximum number of natural ventilation hours during the period is 4369 hours.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Climatic Potential Utilization Ratio (CPUR) of the buildings and of the variants based on NVP and simulated natural ventilation, according to the natural ventilation climatic potential ratio</figcaption></figure>

We can see that the “NV during the day” variant reduces the CPUR by half and the “Hot climate” variant by less than 10%. On the other hand, the CPUR hardly changes between the buildings. In the same way, the climatic potential ratio hardly decreases according to the variants, because the adaptive comfort model depends on the outdoor temperatures. We can therefore see that the use of the building has a greater influence on the use of the climatic potential of a building than the outdoor conditions and the thermal performance of the building.

### 3.2. Building efficiency

Figure 5 shows the CRR for each building and each variant, with the number of days with AC in “Hybrid” mode for each simulation. The solid part of the bars represents the cooling needs of the “AC only” cooling mode, and the hatched part those of the “Hybrid” mode. The CRR value is shown on each hatched bar, and it corresponds to the percentage reduction between the solid and hatched bars.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Cooling Requirements Reduction (CRR) of the buildings and of the variants, according to the number of AC days in the “Hybrid” cooling mode</figcaption></figure>

It can be seen that the thermal performance of the building has the greatest impact on the cooling demand in “AC only” mode (solid bars). On the other hand, the cooling requirements in “Hybrid” mode are
similar for the “Old” and “RT 2000” buildings and lower for the “RE 2020” building, which explains the higher CRR values (i.e. higher performance) for this building. We can also see that the “NV during the day” variant increases the cooling demand in the “Hybrid” mode by half. Similarly, the “Hot Climate” variant triples the cooling demand compared to the “Base” variant and increases the number of cooling days during the period.

### 3.3. *Thermal comfort*

Figure 6 shows the DH and CCP indicators and the number of uncomfortable days for the buildings and according to the variants. That compare the thermal comfort with the natural ventilation potential of the buildings. The solid part of each bar shows the DH and the hatched part shows the CCP associated with this DH. The DH represents the average daily DH of the most uncomfortable zone during the period. The CCP value corresponds to the average of the daily CCPs calculated between 20:00 of the previous day and 20:00 of the day, and only on days when there are DH for the same variant.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Mean daily DH of the most uncomfortable zone and associated daily CCP of the buildings and of the variants, according to the number of uncomfortable days</figcaption></figure>

We can see that for all buildings, the “NV during the day” variant increases the DH by almost 10°C.h as well as the number of discomfort days over the period compared to the “Base” variant. The “Hot Climate” variant reduces the natural ventilation potential of all buildings, but significantly increases the number of uncomfortable days. However, the average mean daily DH are lower than in the “NV during the day” variant, which means that there are more uncomfortable days, but they are less intense in the “Hot Climate” variant. Finally, we can see that the “RE 2020” building remains much more comfortable than the other two buildings, with lower DH and number of uncomfortable days in all variants despite his lower natural ventilation potential.

## 4. Conclusion

We have seen that a given building, in a given climate and with its own constraints, can be more or less efficient depending on the level of natural ventilation performance. This confirms the idea of analysing natural ventilation at different levels. For example, changes in climatic conditions, building use and building thermal performance have all an impact on the performance of natural ventilation. Changes in building use have a greater impact on the efficiency of the building in using its climatic potential, while changes in climatic conditions have an impact on cooling requirements, and thermal performance of buildings on thermal discomfort.

The different indicators show the diversity of criteria that need to be taken into account when quantifying the performance of natural ventilation. In fact, a building can have high cooling requirements and still be quite comfortable. This supports the idea that thermal comfort in buildings is the final performance of natural ventilation and that it is important to better characterise it in modelling. In addition, the buildings and the variants show the variability of the KPIs used, which demonstrates the importance of simulating many more case studies or studying the uncertainties of the influential input parameters.
## Acknowledgements

This work was done within the framework and the financial support of COOLDOWN project grant ANR-22-CE22-0014. This work received support from of Solar Academy Graduate School, grant ANR-18-EURE-0016. This research did not receive any other specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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