# Impact of roof watering on urban cooling during heat waves

Maxime Chaumont, Frédéric Filaine, Sophie Parison, Martin Hendel, Laurent Royon

► To cite this version:

Maxime Chaumont, Frédéric Filaine, Sophie Parison, Martin Hendel, Laurent Royon. Impact of roof watering on urban cooling during heat waves. Energy and Buildings, 2025, 337, pp.115693. 10.1016/j.enbuild.2025.115693. hal-05106129

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Submitted on 10 Jun 2025

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# Impact of roof watering on urban cooling during heat waves

Maxime Chaumont<sup>a,*</sup>, Frédéric Filaine<sup>a</sup>, Sophie Parison<sup>a</sup>, Martin Hendel<sup>a,b,●</sup>, Laurent Royon<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Urban cooling

Heatwave

Roof spraying

Thermal insulation

Cool roof

## ABSTRACT

Due to their morphology, lack of vegetation and the materials used, cities overheat compared to their natural surroundings. This phenomenon, coupled with heatwaves, can cause public health problems and increased energy consumption. In response, public authorities are deploying a range of cooling techniques, such as vegetation, reflective materials and urban watering. In response, various cooling techniques are deployed by public authorities. This study focuses on the cooling effects of roof watering conducted in a lab-scale experiment under heatwave conditions. The experimental set-up includes a temperature and humidity-controlled climate chamber with a solar simulator and a watering system to analyse the thermal and microclimatic behavior of different roof structures. Temperature, heat flow and the surface heat balance are measured or calculated to this aim. When the watering rate is sufficient, regardless of the roof sample studied, convective exchanges become negative during the day. As the surface temperature of the roof is lower than that of the outside air, the latter is cooled on contact with the roof. On the other hand, watering has a different impact depending on the position of the insulation in the roof structure. For external thermal insulation (ETI), watering cools the air both day and night. For internal thermal insulation (ITI), watering significantly reduces the heat stored during the day and therefore the amount of heat released at night. For these structures, watering cools the air during the day and limits their contribution to nighttime urban heat island (UHI). This study also shows that there is a different optimum watering rate for each structure, which maximizes the cooling effects of watering while minimizing water consumption. Finally, watering applied to cool roofs reduces the water consumption associated with this cooling technique, while being highly effective in moderating urban overheating during heatwaves.

## 1. Introduction

Climate change is increasing the frequency, intensity and duration of heat waves [1]. These heatwaves, combined with the urban heat island phenomenon, increase the vulnerability of urban population and lead to a significant increase in space cooling energy requirements [2–4]. In response, several solutions have been put in place by local authorities to reduce urban overheating during the summer season and mitigate its consequences.

Buildings are particularly well-suited to the implementation of cooling techniques since they contribute to urban heating via convective exchanges between their various facets (roofs and façade) and the outside air, and are one of the most energy-intensive sectors for cooling needs. Indeed, air-conditioning already accounts for around 6 % of total energy demand in the building sector (which alone represents around 36 % of final energy consumption), and is growing extremely fast, such as in China (+13 % per year for the residential sector) [5]. According to

various economic, climatic, technological and sociological factors, energy consumption for cooling purposes in the residential sector could increase by 750 % by 2050 [6]. The most popular techniques used to reduce building energy consumption and its contribution to urban heating are thermal insulation [7–9], cool roofs [10–12] and green roofs [13–16]. However, for architectural, economic and technical reasons, it is not always possible to apply some of these solutions. Indeed, for many European cities with a significant architectural heritage, it is not always possible to add insulation or modify its position, to apply cool coatings or a green roof in existing buildings [17]. Roof watering however can be an interesting alternative in this case.

Evaporative cooling is a passive cooling strategy that aims to make use of the high latent heat of vaporization of liquid water to cool the air or a given surface without additional energy use. The evaporation temperature of the water depends on local hygroscopic conditions. The use of water to cool the various facets of a building is a relatively old method. Indeed, according to several authors [18,19], one of the first

<footnote><sup>a</sup> Université Paris Cité, LIED, UMR 8236, CNRS, F-75013 Paris, France</footnote>

<footnote><sup>b</sup> Université Gustave Eiffel, ESIEE Paris, Département SEED, F-93162 Noisy-le-Grand, France</footnote>

<footnote>* Corresponding author.
E-mail address: maxime.chaumont@urbancoolingsolutions.eu (M. Chaumont).</footnote>
<table><caption>**Table 1**
Impact of watering on external surface temperature of roofs and pavements.</caption><thead><tr><th>Authors</th><th>Surface watering</th><th>Maximum surface temperature reduction</th></tr></thead><tbody><tr><td>Ketan Nayak et al. [28]</td><td>Roof</td><td>5.5 °C</td></tr><tr><td>Carrasco et al. [29]</td><td>Roof</td><td>14 °C</td></tr><tr><td>Mao et al. [30]</td><td>Roof</td><td>19.5 °C</td></tr><tr><td>Hendel et al. [31]</td><td>Pavement/road</td><td>13 °C</td></tr><tr><td>Parison et al. [32]</td><td>Pavement/road</td><td>23.1 °C</td></tr><tr><td>Takebayashi et al. [33]</td><td>Pavement/road</td><td>10 °C</td></tr></tbody></table>

scientific studies on the subject was carried out by Houghten et al. [20] in 1939. They showed that by spraying water on a roof, or leaving a permanent layer of water a few centimeters thick, the heat flow entering the building could be reduced, whatever the type of roof. In fact, when a roof is watered, part of its solar gains are used for water evaporation. As a result, its temperature is lower and the heat flow entering the roof is reduced, as are convective exchanges between the surface and the ambient air.

A pretty common and quite well studied technique involves leaving a permanent layer of water on the roof (roof pond). The thickness of this layer can vary from a few centimeters to several tens of centimeters (water basin) [21]. During the day, solar radiation is captured by the water basin. The water temperature rises and a certain amount of water evaporates. At night, some of the heat stored in the water is released to the surrounding environment via convective exchanges with the outside air and radiative exchanges with the sky. There are many variants of this technique with varying degrees of sophistication. For example, it is possible to add ventilation, shading or insulation systems to improve the performance of these roofs. Sharifi et al. [21] summarize these design variations in a literature review. It should be noted that, depending on the technique used, several factors can have a significant impact on the performance of these roofs, such as atmospheric humidity [18], the depth of the water basin [22] or the material composition of the roof [23,24]. Ultimately, whatever the configuration, the aim is to passively cool or heat (depending on the season) the underlying building. These roofs therefore aim to improve indoor comfort and save energy. For example, Erell et al. [25] show that, for a building air-conditioned to 25 °C, a roof with a water basin (equipped, among other things, with a removable insulation system) provides energy savings of 50 % to 100 % compared with a conventional roof in various European cities. Givoni [26] shows that a building equipped with a “Skytherm System” (an innovative system combining a roof pond with mobile roof pond insulation) can reduce indoor temperatures by 6 °C to 8 °C compared with a conventional building. Another study by Spanaki et al. [27] in Greece showed that a roof with a water basin equipped with a ventilation system reduces daily indoor temperature fluctuations and maximum value by 30 %.

Although this type of watered roof and its variants offer interesting thermal performance, they cannot be easily deployed on existing buildings. Indeed, even in the simplest case where the roof is covered with a water basin, few existing roof structures are able to support such a significant load increase. This would require major renovation work, generating significant energy and financial costs. These costs would be much higher in the case of other variants that include ventilation, shading or insulation, for example. Furthermore, roofs with water basins can only be deployed on flat roofs.

Another technique, which involves spraying water droplets onto the roof surface at regular intervals, seems more readily generalizable and suitable for existing buildings. This technique can be applied regardless of the mechanical strength of the structure or the slope of the roof. With each spray, the roof is covered either with water droplets or with a film a few millimeters thick. Between sprays, the water evaporates, significantly cooling the surface. Watering can take place all day long or only

at certain times, for example during the hottest hours of the day. This technique can be carried out by an operator using a hose, or automated using a network of hoses and sprinkler systems and can be installed quickly because there is no need for a preliminary study to ensure the strength of the structure, as may be the case when installing a roof pond on an existing building. Moreover, this technique does not require large quantities of expensive materials, as can be the case for roofs with water basins (polystyrene panels, louver plates, movable insulation, floating insulation, pump ...) such as described by Sharifi et al. [21].

Several authors have taken an interest in this watering technique and its effectiveness, as summarized in Table 1. Nayak et al. [28] studied the impact of sprinkling droplets on a roof composed of a waterproofing membrane, an insulating layer and lightweight concrete. The experiment was carried out under real-life conditions during Japan's hot season, when air temperatures reach a maximum of 35 °C. Watering took place between 7 am and 7.30 am at a watering rate of 0.167 kg. min⁻¹, with a maximum surface temperature cooling of 5.5 °C. Another study carried out by Carrasco et al. [29] on the roof of a non-air-conditioned building at Texas A&M University showed that spraying droplets onto the roof surface reduced the temperature by around 14 °C and the heat flux transmitted to the building by 75 %. Finally, Mao et al. [30] experimented sprinkling an ethylene tetrafluoroethylene (ETFE) roof of a building complex in the city of Baiyun in the Chinese province of Guangzhou. The experiment was carried out under real-life conditions in October 2020, with air temperatures reaching up to 29 °C. The optical properties of the roof, determined using a numerical model, were 0.7 for albedo and 0.85 for emissivity. The roof was watered every hour for nine hours with a maximum surface temperature reduction of 19.5 °C.

This technique has also been applied to other urban surfaces. For example, Hendel et al. [31] experimented watering asphalt concrete pavement and sidewalk on rue du Louvre in Paris. The experiment took place in real-life conditions during the summer of 2013. Among other things, they compared surface temperatures with and without watering. Depending on the area studied (sidewalk or roadway), watering reduced surface temperatures by 2 °C to 4 °C in the morning (in the shade) and 6 °C to 13 °C in the afternoon (in the sun). Parison et al. [32] investigated the impact of watering on the thermal behavior of an asphalt pavement. This study was carried out in the laboratory on an asphalt sample with an albedo of 0.08 and emissivity of 0.99. Air temperature was set to a constant 35 °C throughout an 8-hour day phase with a halogen lamp solar simulator. At a watering rate of 1.25 mm·h⁻¹, the authors observed a surface temperature reduction of up to 23.1 °C. Other authors such as Takebayashi et al. [33] have also studied the impact of watering on pavements and roads. They found a surface temperature reduction of up to 10 °C. Finally, other authors, such as Wang et al. [34] and Liu et al. [35], have focused on the cooling effect of this technique when applied to porous materials.

The number of studies of this watering technique applied to roofs is relatively limited, and the number of those, such as Kabore et al. [36], studying its impact on urban climate even more so. Furthermore, we have found no studies proposing a method for optimizing this watering technique when applied to roofs, i.e. to find the optimum flow rate that minimizes water consumption while maximizing the cooling effect. Finally, no study has analyzed the impact of roof radiative and thermal properties on the efficiency of this technique, i.e., on the optimum flow rate. The present study aims to provide some answers to these gaps.

Previous work by the authors has shown that the composition of the roof (particularly the position of the insulation in the structure) and its solar reflectivity play an important role in atmospheric heating [37]. This study aims to quantitatively investigate how the roof's contribution to urban heating is affected and reduced by watering. It also proposes a methodology for determining the optimum watering rate to maximize the cooling effects of this technique while minimizing water consumption. Finally, the present work investigates how the effectiveness of this technique, and therefore the optimum watering rate, are affected by the position of the insulation in the structure and by the roof's solar
<figure><img src="image_3.png" /><figcaption>Fig. 1. Roof samples and their instrumentation.</figcaption></figure>

**Table 2**

Albedo (α) and hemispherical emissivity (ε) of the two waterproofing membranes (dry and wet).

<table><thead><tr><th rowspan="2"></th><th colspan="2">Albedo (α)</th><th colspan="2">Emissivity (ε)</th></tr><tr><th>Dry</th><th>Wet</th><th>Dry</th><th>Wet</th></tr></thead><tbody><tr><th>Standard</th><td>0.07 ± 0.01</td><td>0.03 ± 0.01</td><td>0.887 ± 0.008</td><td>0.96</td></tr><tr><th>Cool</th><td>0.7 ± 0.01</td><td>0.69 ± 0.01</td><td>0.902 ± 0.020</td><td>0.96</td></tr></tbody></table>

reflectivity.

This study is carried out in the laboratory, using an experimental device to subject roof samples to heatwave conditions. First, the composition of the roof samples and the experimental set-up are presented. The heat budget analysis method used to determine the impact of watering on the various roof samples is then detailed. The results are then presented, analyzed and discussed. The last section provides a conclusion and suggestions for future work.

## 2. Materials and methods

This section presents the general methodology and details the modifications made to that of Chaumont et al. [37] for the analysis of watering experiments.

### 2.1. Roof samples

Four structures are studied according to their waterproofing membrane (standard or cool) and insulation configuration (ITI or ETI), as shown in Fig. 1.

The roof samples are cylindrical in shape, measuring 28 cm in height and 16 cm in diameter. They are instrumented at the center of the upper surface with a radiative and thermal sensor to measure surface temperature and quantify convective exchanges between the surface and ambient air. They are also equipped with thermoflowmeters positioned at different depths in the center of the samples. All the sensors are connected to a central data acquisition unit, which records a measurement every 10 s. The optical properties of the two EPDM waterproofing

<figure><img src="image_4.png" /><figcaption>Fig. 2. Experimental set-up.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Left: total (blue), short- (SW, red) and longwave (LW, green) irradiance in the laboratory (solid line) and in situ (dashed line); right: air temperature (red) and relative humidity (blue) imposed during the experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Heat balance of the upper surface of roof samples and its instrumentation.</figcaption></figure>

membranes were characterized for both wet and dry surfaces using a UV-VIS-NIR Cary 5000 spectrophotometer and 15-cm integrating sphere. These properties are presented in Table 2.

Finally, the arrangement of the layers within the structures and the optical properties of the surface are the only two parameters that vary from sample to sample.

## 2.2. Experimental set-up

Once constructed, each sample is placed in the experimental set-up shown in Fig. 2: Experimental set-up, which comprises a climate chamber and a solar simulator. The solar simulator consists of a 150 W xenon arc lamp, placed at the focal point of an aluminum parabolic mirror 20 cm in diameter. To simulate an air-conditioned building, the

underside of the roof samples is maintained at a set point temperature of 25 °C using a thermostatic bath. Their lateral surface is thermally insulated with a 14 cm-thick polyurethane box, ensuring that heat ex- changes mainly take place via the upper surface of the samples, and is unidirectional.

To study the impact of surface watering on the thermoclimatic behavior of the roof structures, a sprinkler mechanism is used to uni- formly spray water droplets onto the top surface of the samples. For each of the four structures, three watering rates are studied in addition to the case without watering. Note that the watering technique used in this study involves spraying water droplets onto the upper surface of the roof. As a result, it can either be covered with droplets (low watering rate) or a water film (high watering rate). It should also be noted that watering only takes place during the daytime, and that the same amount of water is sprayed each time. Only the frequency of spraying varies between experiments. Finally, in addition to the case without watering, each of the structures will be studied for 3 different watering rates, i.e. $Q_1 = 0,42 \text{ mm.h}^{-1}$, $Q_2 = 0,84 \text{ mm.h}^{-1}$ and $Q_3 = 1,26 \text{ mm.h}^{-1}$. Given the dimensions of the supply pipe, the temperature of the water sprayed is that of the air temperature imposed inside the climate chamber.

## 2.3. Climate cycle

Each experiment begins with a 24-hour initialization phase, during which the temperature inside the experimental set-up is set to 25 °C. During the second phase of the experiment, which also lasts 24 h, the sample is subjected to heatwave conditions as illustrated in Fig. 3 (right) below. The temperature and humidity cycle is representative of Parisian heatwave conditions, oscillating between 25 °C and 40 °C and 23 % and 50 % humidity, with a fixed absolute humidity of $11.5 \text{ g.m}^{-3}$. The complete experimental cycle lasts 26 h, from 7 am to 9 am the following day.

The imposed irradiance seeks to reproduce real-world conditions on a clear summer day in France. This is compared (Fig. 3, left) with global horizontal irradiance (dotted lines) measured in the field on July 23rd, 2019 in the town of Hyères in the South of France.

While total (SW and LW) cumulative irradiance is comparable to field observations, they differ in spectral distribution. Cumulative SW irradiance over 24 h in real conditions is just over 1.5 times greater than that measured inside the enclosure. For LW irradiance, the opposite is true: that measured inside the enclosure is about 1.5 times greater than that measured in real conditions.
<figure><img src="image_7.png" /></figure>

● Characterized before experiments

● Measured during experiments

● Residual surface heat balance

<figure><img src="image_8.png" /><figcaption>Fig. 6. Surface temperature measurements for the four roof structures at different watering rates.</figcaption></figure>
<table><caption>**Table 3**
Maximum reduction ($T_{sdry}^{max} - T_{swet}^{max}$) and fluctuation ($F_{Ts}^{max}$) in surface temperature as a function of watering rate for the four roof samples.</caption><thead><tr><th></th><th>Watering rate</th><th><i>T</i><sub>sdry</sub><sup>max</sup> - <i>T</i><sub>swet</sub><sup>max</sup></th><th><i>F</i><sub>Ts</sub><sup>max</sup></th></tr></thead><tbody><tr><td rowspan="3">Standard ITI</td><td>Q<sub>1</sub></td><td>5 °C</td><td>7 °C</td></tr><tr><td>Q<sub>2</sub></td><td>14.5 °C</td><td>5.5 °C</td></tr><tr><td>Q<sub>3</sub></td><td>20 °C</td><td>1.5 °C</td></tr><tr><td rowspan="3">Standard ETI</td><td>Q<sub>1</sub></td><td>7 °C</td><td>13 °C</td></tr><tr><td>Q<sub>2</sub></td><td>18 °C</td><td>7 °C</td></tr><tr><td>Q<sub>3</sub></td><td>30 °C</td><td>1 °C</td></tr><tr><td rowspan="3">Cool ITI</td><td>Q<sub>1</sub></td><td>6 °C</td><td>6 °C</td></tr><tr><td>Q<sub>2</sub></td><td>9.5 °C</td><td>4 °C</td></tr><tr><td>Q<sub>3</sub></td><td>15 °C</td><td>&lt; 0.5 °C</td></tr><tr><td rowspan="3">Cool ETI</td><td>Q<sub>1</sub></td><td>6 °C</td><td>9 °C</td></tr><tr><td>Q<sub>2</sub></td><td>16.5 °C</td><td>3.5 °C</td></tr><tr><td>Q<sub>3</sub></td><td>21 °C</td><td>&lt; 0.5 °C</td></tr></tbody></table>

## 2.4. Surface heat balance

The heat balance for the roof's external surface with watering is shown in Fig. 4.

It is written as:

$$S + L = S_{\text{ref}} + L_{\text{up}} + H + V_{0} + \Phi \quad (1)$$

where S and L are the SW and LW irradiance, respectively, $S_{\text{ref}}$ and $L_{\text{up}}$ are the SW and LW radiosity of the surface, respectively, H is the convective flux between the surface and the atmosphere, $V_0$ the conductive flux absorbed by the surface and $\Phi$ the cooling flux due to surface watering.

With:

$$S_{\text{ref}} = \alpha S \quad (2)$$

$$L_{\text{up}} = (1 - \varepsilon)L + \sigma T_{s}^{4} \quad (3)$$

where $\alpha$ and $\varepsilon$ are the surface albedo and emissivity, respectively, $T_s$ the surface temperature and $\sigma$ the Stefan-Boltzmann constant ($5.67 \times 10^{-8} \text{ W.m}^{-2}.\text{K}^{-4}$). We can rewrite the heat balance (1) as follows:

$$(S + L) - (\alpha S + (1 - \varepsilon)L + \sigma T_{s}^{4}) = H + V_{0} + \Phi \quad (4)$$

$$R_n = H + V_0 + \Phi \quad (5)$$

where $R_n$ is the net radiation received by the surface.

For the four experiments without watering (one per sample), the surface heat balance is written:

$$R_n = H + V_0 \quad (6)$$

To quantify the impact of each roof on urban heating during a heatwave, it is necessary to know the evolution of these four quantities ($R_n$, $V_0$, H, $\Phi$) throughout the experiment. The rest of this section describes the method used to determine each of these variables.

### 2.4.1. Net radiation

To determine net radiation, average irradiance is measured using a radiant sensor that measures radiation over a spectral band ranging from 0.3 µm to 100 µm with a surface area identical to that of the roof sample (16 cm disk). This measurement is carried out under identical climatic and radiative conditions to those imposed during the roof sample experiments. The methodology is detailed in Chaumont et al. [37].

### 2.4.2. Convection

The convective flux is calculated by multiplying the convective heat exchange coefficient $h$ by the temperature difference between the air ($T_a$) and the surface ($T_s$) as follows:

$$H = h(T_s - T_a) \quad (7)$$

The coefficient $h$ is determined using experiments without watering, as described in detail in Chaumont et al. [37], while air and surface temperature are measured during each experiment.

### 2.4.3. Conduction

When the surface is dry, the conductive flux $V_0$ is deduced from the surface heat balance (6):

$$V_0 = R_n - H \quad (8)$$

When the surface is wet, the conductive flux $V_0$ is calculated as follows:

$$V_0 = U_{\text{dry}}(T_s - T_{2\text{cm}}) \quad (9)$$

with $T_{2\text{cm}}$ that measured at a depth of 2 cm and $U_{\text{dry}}$ the effective conductance of the 2 cm-thick surface layer. This effective conductance is estimated for each of the four structures studied on the basis of measurements made during experiments without watering, as explained in the Supplementary Material.

### 2.4.4. Cooling flux

The cooling flux $\Phi$ is deduced from the surface heat balance (5):

$$\Phi = R_n - (H + V_0) \quad (10)$$

It is then possible to differentiate the cooling flow attributable to the evaporation of water (latent heat) and that attributable to the temperature difference between the water and the surface (sensible heat). The cooling flow can be rewritten as follows:

$$\Phi = \text{IE} + c_w \rho_w \frac{q_w}{T_a} (T_s^a - T_w) \quad (11)$$

with 1 the latent heat of vaporization of water (2260 kJ.kg$^{-1}$), E the evaporation rate, $c_w$ the specific heat of water (4.18 kJ.kg$^{-1}.K^{-1}$), $\rho_w$ the density of water (1000 kg.m$^{-3}$), $q_w$ the quantity of water delivered per spray, $t_0$ the time between two sprays, $T_s^a$ the surface temperature when sprayed and $T_w$ the temperature of the water sprayed on the surface. In our experiment, $T_w = T_a$, with $T_a$ the air temperature.

The cooling flux determined with this methodology is validated against the water mass balance for experiments with low watering rates, as detailed in the Supplementary Material. Indeed, with low watering rates, all the sprayed water evaporates between sprays and there is no water runoff. Therefore, the average evaporation rate is known. This method validation is presented in the Supplementary Material.

Finally, the characterizations and experimental measurements carried out enable us to determine all the terms of the surface heat balance for each experiment. The methodology used to estimate each heat balance term, whether the surface is watered or not, is summarized in Fig. 5.

## 2.5. Energy partitioning

The cumulative heat budget will be analyzed to determine how roof sample irradiance is partitioned into radiosity, atmospheric convection and downwards conduction, with this latter term divided into heat storage by the concrete structure and heat transmission through the structure, i.e. into the building. This analysis aims to help quantify the contribution of the studied roof structures to outdoor and indoor overheating, i.e. to the urban climate and to the indoor microclimate.

For outdoor overheating and the contribution of each roof sample to the urban heat island effect, the samples' day and night convective exchanges with the atmosphere are relevant, i.e. the term H defined earlier.

In addition, the heat stored in the concrete layer of the roof structure can also contribute to outdoor overheating in the case of the ITI structures for which the stored heat will more readily be released outdoors than indoors. However, the relative impact of this contribution will
<figure><img src="image_9.png" /><figcaption>Fig. 7. Temperatures (left) and heat flux densities (right) measured 2 cm-deep for the different watering rates and roof configurations.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 8. Surface heat balance for the standard ETI roof at different watering rates.</figcaption></figure>

depend on when the stored heat is released, i.e. during or after a heat-wave event.

From the building's point of view, the heat transmitted through the entire roof structure to the underside (e.g. the underlying apartment) is relevant. Similarly, the heat stored in the concrete layer can also contribute to indoor overheating in the case of the ETI structures, since it will more readily be released indoors than outdoors. The timing of this release is also important (during or after a heatwave).

Energy partitioning, as opposed to the heat flow, will be analyzed, obtained by integrating the different heat balance terms measured during the experiment over the duration of the experiment and represented in the form of a Sankey diagram to analyze how the roof surface irradiance is partitioned into day and night radiosity, convection, heat storage and transmission as well as water cooling. A compacted version of Sankey diagrams will be used to be able to show all four experiments on the same figure for each sample.

# 3. Results

## 3.1. *Temperature and heat flux density*

### 3.1.1. *Surface temperature*

Fig. 6 shows the evolution of surface temperature of the four roof samples presented in Section 2.1 over time for different watering rates. During the day, independently of the considered structure, the higher

the watering rate, the more gradual the increase in surface temperature, and the lower the maximum temperature reached.

At low watering rates ($Q_1$), the maximum measured surface temperature is reduced by more than 7 °C for the standard ETI roof and by 5 °C for the standard ITI roof (Table 3). For the two cool roofs, the maximum surface temperature is reduced by 5.9 °C. It can also be seen that surface temperature fluctuations are greater for the ETI roof samples, with an amplitude of up to 13 °C and 9 °C for the standard and cool ETI samples, respectively, and up to 7 °C and 6 °C for the ITI samples. We also note that these fluctuations are lower for the two cool structures than for their standard counterparts.

When the watering rate doubles ($Q_2$), the daytime surface temperature is reduced for all four roof samples compared to watering rate $Q_1$. However, ETI structures are more affected by the increased watering rate. Indeed, compared to the experiments without watering, the maximum surface temperature is reduced by 18 °C and 16.5 °C for the standard and cool ETI samples, respectively, whereas this drop is 14.5 °C and 9.5 °C for the ITI samples.

Increasing the watering rate also reduces the amplitude of surface temperature fluctuations. Once again, ETI samples are most affected. Indeed, the fluctuations are reduced by up to 54 % for the standard ETI roof and by 66 % for the cool ETI roof. For the two ITI samples, the fluctuations are reduced by only 38 % for the standard roof and 29 % for the cool roof. Finally, with the exception of the standard ETI sample, surface temperature fluctuations in the morning (between 9 am and 1
<figure><img src="image_11.png" /><figcaption>Fig. 9. Surface heat balance for the standard ITI roof sample at different watering rates.</figcaption></figure>

pm) are very low (<1 °C), indicating that the surface no longer has time to dry out between two sprays at watering rate $Q_2$. At watering rate $Q_3$, the surface temperature hardly fluctuates, the surface is permanently wet with a continuous water film. The surface temperature reaches a maximum of 37 °C for the two standard samples and around 34 °C for the cool samples.

When the solar simulator is switched off and the surface has received little or no water during the day, the surface temperature decreases during the entire night phase for all four roof samples. For watering experiments with $Q_2$ and $Q_3$, the surface temperature varies slightly between 7 pm and 10 pm when the solar simulator is switched off. It should be noted that the drying time is longer for cool samples than for standard samples once the lamp is switched off.

Once the surface is dry, no residual nighttime watering effect can be seen on surface temperature for the two ETI samples. This does not apply to the ITI structures for which nighttime temperatures decrease with increasing daytime watering rates.

Surface temperature reductions observed with watered experiments are of the same order of magnitude as those observed in the literature, depending on the watering rate imposed. For experiments with $Q_3$, the results are similar to those obtained by Parison et al. [38] for equivalent watering rates. However, there are a few differences, notably in the case of the standard ETI sample for which the maximum surface temperature reduction is 30 °C, compared with 23.1 °C in one of Parison et al.'s experiments using an asphalt with similar radiative properties to our

black EPDM. This can be explained by the differences in thermal properties between the insulating layer just below the surface of the ETI sample and the asphalt used by Parison et al. [38]. Indeed, the ETI roof's lower thermal conductivity causes its surface to heat up more than the asphalt's, resulting in stronger water evaporation and surface cooling. Additionally, differences in climatic conditions and watering protocol also play a role.

### 3.1.2. Temperature and heat flux 2 cm deep

Fig. 7 illustrates temperature (left) and heat flux (right) measured 2 cm-deep for different watering rates for the four roof samples. Overall, the observations made at the surface remain relevant with a few differences.

Independently of the sample and the watering rate, unlike surface temperatures, 2 cm-deep temperatures fluctuate very little during the day when watering is activated. Furthermore, the surface temperature fluctuations observed between 7 and 10 pm for $Q_2$ and $Q_3$ flows are almost non-existent 2 cm-deep for the ITI samples. For the ETI samples, the amplitude of these variations is even smaller. Finally, for experiments with watering rates $Q_2$ and $Q_3$, the maximum temperature at a depth of 2 cm for cool structures is no longer reached during the diurnal phase, but during the early hours of the night.

During the diurnal phase, when the watering rate is low, the heat flux measured at a depth of 2 cm within the four structures fluctuates slightly, which is consistent with the observations made on surface
<figure><img src="image_12.png" /><figcaption>Fig. 10. Surface heat balance for the cool ETI roof sample at different watering rates.</figcaption></figure>

temperatures.

For the ETI structures, the heat flux measured 2 cm-deep during the day is only slightly affected by watering in absolute terms. Indeed, the maximum flux is lowered by 12 W.m⁻² between the dry experiment and the Q₃ watering rate experiment for the standard ETI sample, and by 8 W.m⁻² for the cool ETI sample. However, judging by the amount of heat transferred through the sample at a depth of 2 cm over the whole day, it appears that watering has a greater effect on the cool structure. Indeed, when watering rate Q₃ is applied, the heat transferred 2 cm-deep is reduced by 80 % for the standard structure and by 96 % for the cool structure, compared with the dry experiments. At night, watering has little or no residual impact.

For the ITI structures, the 2-cm-deep heat flux is strongly impacted by watering. It can be seen that the maximum flux is lowered by 70 W. m⁻² between the dry and highly watered (Q₃) experiments for the standard ITI sample, and by 33 W.m⁻² for the cool ITI sample. The heat transferred through the sample 2 cm deep during the day is reduced by 73 % for both ITI structures. At night, the greater the watering rate applied during the day, the lower the heat flux (in absolute terms) and the later it changes sign at night. For both samples, the difference in nighttime flux amplitude for watering rate Q₂ and Q₃ is almost negligible.

### 3.1.3. *In-depth temperature and heat flux*

For the ETI samples, watering significantly reduces the heat flow

through the insulation. Comparing the heat flux measured at the insulation/concrete interface in the dry experiment to that with watering rate Q₃, we can see that it is divided by 3 for the standard sample and by 4 for the cool sample. Below, in the concrete layer, the impact of watering is less noticeable, although the heat flux entering the building is reduced.

In the case of the ITI samples, watering has a significant effect on the entire concrete layer, but is less marked from the concrete/insulation interface onwards. Within the concrete layer, the greater the watering rate, the weaker and flatter the measured signals. As with the ETI samples, the heat flow to the building is reduced.

## 3.2. Surface heat balance

The surface heat balance for each watering rate is illustrated in Fig. 8 and Fig. 9 for the standard samples, and in Fig. 10 and Fig. 11 for the cool samples.

Independently of the roof sample, watering significantly increases daytime net radiation. Nevertheless, we note that ETI samples are more affected than their ITI counterparts. Indeed, between the dry experiment and that with the Q₃ watering rate, the maximum net radiation increases by 250 W.m⁻² and 150 W.m⁻² respectively for standard and cool ETI samples, whereas this increase is only 175 W.m⁻² and 105 W.m⁻² for the ITI samples. This difference is partly explained by the thermal behavior of the two roof configurations when dry. Indeed, the surface
<figure><img src="image_13.png" /><figcaption>Fig. 11. Surface heat balance for the cool ITI roof sample at different watering rates.</figcaption></figure>

temperature of the dry ETI structures is higher than that of their ITI counterparts, which therefore emit more LW radiation, consequently reducing dry net radiation.

It can also be seen that the increase in net radiation due to watering is greater for standard structures than for cool ones. For all roof samples, daytime watering has little impact on net radiation at night.

For all four samples, as the watering rate increases, daytime atmospheric convection decreases. However, with the exception of the standard ETI sample, the additional reduction becomes relatively small or even negligible as the watering rate increases from Q₂ to Q₃. Apart from the standard ETI sample, convective exchanges become negative throughout the day at a certain watering rate between Q₁ and Q₂ (as Tₛ < Tₐ) beyond which the roof surface no longer heats the ambient air during the day.

Overall, the effect of watering on convection is stronger for ETI structures in the daytime. At night it has no significant impact on the ETI samples, but reduces nighttime convection for the ITI samples.

As far as conduction is concerned, two quite distinct behaviors can be observed. For ETI roofs, the flux absorbed by the surface is only slightly affected by watering. When the watering rate is low (Q₁), the amount of heat absorbed during the day slightly increases. This probably does not correspond to a physical reality and is likely due to the uncertainty associated with estimating effective conductance. At night and for both ETI samples, the conductive flux approaches zero in the first few hours after the solar simulator is switched off, once the surface is completely

dry, and remains so throughout the night phase.

For the two ITI samples, watering significantly reduces conduction during the day, since the maximum values reached are halved between the dry and Q₃ experiments. While increasing the watering rate from Q₁ to Q₂ results in a significant drop in conductive flux, increasing from Q₂ to Q₃ has little additional effect. At night, the greater the watering rate in the day, the later conduction changes sign and the more stored heat is released at night.

Finally, at low watering rates (Q₁), the cooling flux is almost constant and equivalent for all four samples. In theory, at this watering rate, all the sprayed water evaporates and there is no runoff. The cooling flux should therefore be strictly identical from one sample to the next. The small deviations observed are mainly due to experimental uncertainties. When Q₂ is applied, the cooling flux increases more sharply for the two standard structures than for the cool ones.

With Q₃, the cooling flux closely follows net radiation between 9 am and maximum irradiance at 2 pm for all four samples. Thereafter, the cooling flux is greater than net radiation and remains so until the simulator is switched off. Cooling flux peaks of approximately 800 W·m⁻² and 525 W·m⁻², respectively, are observed for the standard and cool ITI samples, and of around 875 W·m⁻² and 600 W·m⁻² for the ETI samples. Regardless of the insulation strategy, the cool waterproofing membrane reduces the maximum cooling flux by around 275 W·m⁻² compared to the standard membrane.

The higher the watering rate during the day, the later the cooling flux
<figure><img src="image_14.png" /><figcaption>Fig. 12. Energy partitioning of standard roof samples for four watering rates.</figcaption></figure>

reaches zero at night, i.e. the later the surface dries. In fact, in the Q₃ experiment, the cooling flux reaches zero at around 10 pm for the cool ETI sample, compared to around 8:30 pm for the other three samples.

### 3.3. Energy partitioning

The partitioning of the 24-hour cumulative irradiance for the two standard and cool roof structures at different watering rates is presented in Fig. 12 and Fig. 13, respectively. These figures describe how incident SW and LW irradiance are partitioned into SW (dotted) and LW radiosity and net radiation (shades of red), convection (blue), surface conduction during the day, residual heat storage at night and transmission into the underlying building (shades of green) and the watering cooling flux (black). Nighttime components are hatched.

Whatever the structure, daytime radiosity decreases with watering. The higher the dry surface temperature, the greater the decrease in daytime radiosity following watering. In the case of the standard ETI sample, for example, this reduction reaches 30 % between the dry experiment and Q₃, whereas it is only 11 % for the cool ITI sample. This decrease leads to an increase in net radiation. The radiation balances of standard structures are more affected by watering than those of cool structures, as are those of ETI structures compared with ITI structures with the same solar reflectivity.

For the ITI structures, conduction decreases by 0.5 kWh.m⁻² (17 %) for the standard sample and by 0.27 kWh.m⁻² (15 %) for the cool sample between the dry experiment and Q₁, while convective exchanges decrease by 1.59 kWh.m⁻² (58 %) and 1.53 kWh.m⁻² (100 %), respectively. With Q₂, convective exchanges are reduced by 1.52 kWh.m⁻² and conduction is lowered by 1.16 kWh.m⁻² for the standard ITI sample. Finally, for the two ITI samples, the increase in watering rate

from Q₂ (0.84 mm.h⁻¹) to Q₃ (1.26 mm.h⁻¹) results in only a slight additional reduction in convective exchanges and conduction.

For the two ETI samples, the previous observations on convective exchanges are similar. On the other hand, surface conduction is almost unaffected by watering, unlike the ITI structures. In fact, between the dry experiment and watering rate Q₁, conduction during the day increases slightly. This last observation is probably due to the change in method used to estimate the watered heat budget (effective conductance), since the difference between the surface temperature and that measured 2 cm down is greater in the dry experiment.

At night, the energy partitioning of ETI structures is virtually unaffected by watering. We can only observe that water evaporation, which continues after the solar simulator is switched off, cools the air near the surface, particularly when the watering rate is high. For ITI structures, the higher the watering rate in the daytime, the lower the amount of heat released at night, namely via atmospheric convection. We also note that the greater the watering rate, the less heat is stored in the concrete layer after 24 h. In particular, for the standard sample, the amount of heat still stored in the concrete layer decreases by 60 % between the dry experiment and that with Q₃ (28 % for the cool ITI sample).

Finally, although already very low thanks to the insulation, watering also reduces the amount of heat transmitted to the building. When Q₃ is applied, heat transmission is reduced by 30 % for the two standard structures. For cool structures, it is reduced by 60 % for the ETI roof and 43 % for the ITI roof.

Regardless of the roof sample, it would appear that the effects of watering on the various heat balance variables are significant up to a certain watering rate, and level off above a certain threshold between watering rates Q₂ and Q₃. It would therefore appear that there is an optimum watering rate that maximizes the effects of watering while
<figure><img src="image_15.png" /><figcaption>Fig. 13. Energy partitioning of cool roof samples for four watering rates.</figcaption></figure>

minimizing water consumption. The following section seeks to estimate this more precisely.

### 3.4. Optimum watering rate

To estimate the optimal watering rate, we propose to adapt the method proposed by Parison et al. [38] pertaining to pavement watering of road materials. This work was carried out in the laboratory and the watering technique used is identical to our own. The method, adapted to our experiment, is illustrated in Fig. 14. Similar to Parison et al. [38], we assume the existence of two distinct regimes for daytime watering, shown by the black dotted lines in Fig. 14 which illustrates cumulative cooling flux $\Phi$ during the day phase.

At low watering rates in the first regime, all the sprayed water evaporates. The surface is thus cooled both by evaporation (latent contribution) and by the sensible heat absorbed by the sprayed water, due to the temperature difference between the delivered water and surface (sensible contribution). The latent contribution increases proportionally to the watering rate. If we neglect the sensible contribution, the slope of the first regime ($s_1$) is known and equal to the integral of the latent heat of water vaporization ($I$) from the beginning ($t_{d,0}$) to the end ($t_{n,0}$) of the day phase:

$$s_1 = \int_{t_{d,0}}^{t_{n,0}} ldt \qquad (12)$$

In the second regime, not all the sprinkled water evaporates between sprays: the surface is permanently covered by a water film, i.e. evaporative cooling has reached its maximum value under the experimental conditions. Additional cooling is therefore only due to the sensible heat absorbed by the sprinkled water. Given the small additional contribu-

tion made by the sensible heating of the water film, which can even be negative if the sprinkled water is warmer than the surface, it is neglected. The slope ($s_2$) in the second regime is therefore approximated to zero.

To plot the second regime, an experiment with a watering rate high enough to continuously maintain a water film on the sample surface is required. The intersection between the first and second regimes corresponds to the optimum watering rate ($Q_{opt}$), where the watering rate corresponds to the maximum evaporation rate under experimental conditions.

By plotting the experimental data points against the proposed model we obtain Fig. 15. The $Q_3$ experiment presents a continuous water film for all samples and is therefore used to identify the second regime and determine the optimum watering rate for each sample, as summarized in Table 4.

From the dataset, we observe that the optimum watering rate is higher for standard samples than for cool samples, and, for the same albedo, it is higher for ETI samples than for ITI samples. It can be seen that the higher the surface temperature in the dry experiment, the higher the optimum watering rate. This observation is consistent with the existing literature pertaining to the optimization of pavement-watering [38].

While for a low watering rate ($Q_1$) the proposed model fits experimental data well, a significant deviation is visible for all roof samples for the intermediate watering rate ($Q_2$) experiment. This deviation does not appear to correspond to either regime, but to a transient state between the alternating-wet-dry state (first regime) without runoff and the continuous-water-film state with runoff (second regime). This anomaly is more pronounced for reflective structures.

For reflective samples in the transient state, a continuous water film
<figure><img src="image_16.png" /><figcaption>Fig. 14. Model of cumulative daytime cooling flux Φ as a function of the watering rate Q over the whole experiment.</figcaption></figure>

is present throughout the early part of the diurnal phase (first regime), but becomes intermittent (second regime) after 12 pm. This analysis is supported by the surface temperature oscillations visible in Fig. 6; they are nearly absent at the beginning of the experiment, but appear and gradually increase around noon.

In the case of the standard samples, this anomaly is less significant. Except for a few moments at the start of the experiment, the surface is almost never covered by a continuous water film, only a few permanent droplets. In fact, Fig. 14 shows that for standard samples, the experimental datapoints are closer to the first regime (alternating between a dry surface and a surface with water droplets) than to the second regime (surface permanently covered by a film of water).

In so far as it accounts for this particularity, the model is deemed satisfactory. For the needs of determining the optimum watering rate, future experiments could therefore be limited to a dry experiment and one with a high watering rate (e.g. Q₃).

## 4. Conclusion

The thermoclimatic behavior of four roof structure samples was studied under heatwave conditions in the lab with different watering rates. A climate chamber was used along with a xenon lamp solar simulator to impose realistic climate conditions as well as a sprinkling device for surface watering. Concrete roof structure samples with standard and cool surfaces were studied with internal and external thermal insulation. Temperatures and heat flows were measured at several depths. These measurements were used to analyze the surface heat balance and the energy partitioning of the different samples and watering rates. A model was proposed and tested against collected experimental data to determine the optimal watering rate.

Overall, results demonstrate significant impacts of watering on roof thermoclimatic behavior, in line with what is expected from evaporative

cooling and reported in the literature. Whatever the structure studied, roof watering drastically reduces surface convection and conduction, while increasing net radiation due to lower radiosity. The effects on convection are relevant as they limit the contribution of roofs to atmospheric heating during the day and to the urban heat island effect at night, with high (Q₃) or even intermediate (Q₂) watering rates providing atmospheric cooling day and night depending on the considered structure. Furthermore, watering can impact heat transmission through the roof structure (into the building) and can therefore be expected to improve thermal comfort inside the building and/or save energy for air-conditioning.

Finally, the combination of cooling solutions (cool roof and watering system) provides the best results, not only in terms of their contribution to atmospheric heating, the UHI effect and building energy savings or indoor comfort, but also in terms of water consumption. Ultimately, the most favorable structure is the cool ETI structure.

A simple model of cooling provided by watering was proposed and tested against experimental data. A watering protocol optimization method was proposed, requiring only a high-watering experiment to adequately estimate the optimum watering rate, i.e. maximizing the cooling effects of watering while minimizing water consumption. The high-watering-rate experiment requires a watering rate able to maintain a permanent water film on the sample surface. This protocol is not specific to the study of roof samples and is expected to be fully applicable to watering applied to any urban surface where runoff is negligible or low at low watering rates. This may not be the case for vertical surfaces and those with strong slopes where runoff occurs even at low watering rates.

Based on these findings, it is possible to consider how the technique's water consumption would scale up with widespread application, for example to all flat Parisian roofs (approx. 460 ha [39]). If we assume that 100% of flat roofs are watered at the highest optimal watering rate
<figure><img src="image_17.png" /><figcaption>Fig. 15. Experimental cooling flux as a function of the watering rate for the four roof samples and optimum watering rate.</figcaption></figure>

<table><caption>**Table 4**
Optimum watering rate for each sample.</caption><thead><tr><th></th><th>Standard ITI</th><th>Standard ETI</th><th>Cool ITI</th><th>Cool ETI</th></tr></thead><tbody><tr><th>Q<sub>opt</sub></th><td>1.05 mm.h<sup>-1</sup></td><td>1.15 mm.h<sup>-1</sup></td><td>0.75 mm.h<sup>-1</sup></td><td>0.90 mm.h<sup>-1</sup></td></tr></tbody></table>

(1.15 mm.h⁻¹) for ten hours per day, this would represent a daily water consumption of approximately 53,000 m³.day⁻¹, equivalent to 7.7 % of Paris' total daily water consumption (approx. 700,000 m³.day⁻¹).

While 7.7 % is significant, this is obtained using the highest optimal watering rate (1.15 ~ mm.h⁻¹) and watering 100 % of flat Parisian roofs. With the average optimum watering rate (0.96 mm.h⁻¹) and watering only 10 % of flat roofs, for example only the most vulnerable buildings as assessed by building characteristics or resident vulnerability to extreme heat, the total water consumption amounts to 4,400 m³.day⁻¹, i.e. 0.6 % of Paris's total daily water consumption. This amount could be further reduced by storing rainwater or shortening the duration of roof watering, e.g. targeting only the hottest periods or only during building occupation (e.g. opening hours for an office building).

Certain limitations of the experimental set-up could be improved in future work. One of these concerns the solar simulator, i.e. the incident radiation. Compared with experiments carried out under real conditions, long-wave irradiance is higher in the experimental set-up, while short-wave irradiance is lower. One consequence of this is to underestimate the impact of reflective roofs. The second limitation is that the experimental set-up does not reproduce the level of radiative cooling observed in real conditions under clear night skies. Under field conditions ITI roofs would therefore likely release more stored heat at night

than in our experiments. Depending on how this heat is partitioned between convection and radiosity, the ITI roofs' contribution to the UHI effect may be increased or decreased. For ETI roofs, their surface temperature would likely be lower than air temperature, regardless of whether the membrane is standard or reflective, watered or not. To overcome this limitation, we could draw inspiration from the work of Trosseille et al. [40], who developed an experimental laboratory set-up for the study of dew water produced by radiative cooling. These two limitations and their impact on the results are explained in Chaumont et al. [37] who describe the experimental set-up in much greater detail, as well as the results obtained for experiments without watering.

Bearing these limitations in mind, other roof structures of varying composition and thermal properties can be studied and compared to the structures analyzed here. Relevant candidate structures include traditional Parisian zinc roofs, roofs with varying slope, or flat roofs with permeable materials, e.g. gravel, or vegetation. Permeable roofs will require adapting the methodology used here since the thermal properties of the surface layer will likely change with the infiltration of water. Furthermore, the two cooling regimes described may not be applicable directly. Alternatively, water evaporation and the latent flux can be determined directly via a high-precision scale. Results from these additional laboratory studies could be used to construct an indicator based on a reduced number of parameters from which this optimized watering technique could be applied to any type of roof.

The impact of nighttime watering on ITI roofs also seems worthy of further exploration as stored heat remains at the end of daytime watering even at the highest watering rates. Nighttime watering could limit heat accumulation over consecutive heatwave days.

Furthermore, this experiment does not account for the long-term
effects of roof watering on their optical properties. While cool roofs may
see a decrease in their albedo and hence an increase in their optimal
watering rate, the albedo of dark surfaces may be increased by watering,
therefore reducing their optimum watering rate.

CRediT authorship contribution statement

**Maxime Chaumont:** Writing – original draft, Software, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Frédéric Filaine:** Software, Conceptualization.
**Sophie Parison:** Methodology.
**Martin Hendel:** Writing – review & editing, Supervision, Methodology, Investigation.
**Laurent Royon:** Writing – review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgments

The conducted research was funded by the Urban Materials for Cool
Cities (UMat4CC) project (grant number ANR-18-CE22-015-01). The
authors would like to acknowledge the technical support of Wladimir
Toutain, Jean-Yves Radenac, Alexandre Di Palma and Yann Le Goas. We
gratefully thank John Lomas for his careful proofreading of the
manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.enbuild.2025.115693.

Data availability

The authors do not have permission to share data.

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