High-Reflectance Roof Strategy

High reflectance of roofs, identified as albedo strategy, is able to maximize the di ffuse reflection of solar radiation, reducing the overheating of buildings and the surrounding urban context and maintaining lower surface temperatures [24,48]. From past studies on UHI mitigation, it has become accepted that a high level of albedo (as white roofs) has the potential to cool cities by 1–3 ◦C, cooling the lower states/layers of the atmosphere [49–51]. In particular, since in the urban canyon white roof has the greatest e ffect on air temperatures when used on buildings of 1–2 stories' height [24], in this work low buildings were identified as suitable for this strategy. Moreover, the beneficial e ffects are greater in a mixed urban morphology context, such as the case of the Turin district analyzed in this work.

As previously mentioned, the *SRI*, used in the main international certification protocols, is a metric for comparing the coolness of roof surfaces. The higher the *SRI*, the cooler the roof will be in the sun [28,52]. For example, a clean black roof usually has an *SRI* of about 0 (with a solar reflectance of 0.05 and an infrared emittance of 0.90), while a clean white roof could have an *SRI* of about 100 (with a solar reflectance of 0.80 and a thermal emittance of 0.90). In general, dark roofs have an *SRI* less than 20 [53].

In this work, the e ffect of albedo strategy on thermal conditions was investigated calculating the *SRI* and the roof surface temperature ( *Ts*) based on solar reflectance (ρ) and infrared emittance (ε). According to ASTM E1980-11(2019) standard, *SRI* can be defined as:

$$SRI = 100 \cdot \frac{T\_b - T\_s}{T\_b - T\_w} \tag{6}$$

with:

$$T\_s = 310.04 + 82.49 \cdot \alpha - 2.82 \cdot \sigma - 54.33 \cdot \alpha \cdot \sigma + 21.72 \cdot \alpha \cdot \sigma^2 \tag{7}$$

where:

*Tb* is the steady-state temperature of a black surface (K) with solar reflectance of 0.05 and infrared emittance of 0.9, under the standard solar and ambient conditions with a solar flux of 1000 Wm−2, ambient air temperature of 310 K, convective coe fficient of 12 Wm−2·K−<sup>1</sup> surfaces, and apparent sky temperature of 300 K;

*Tw* is the steady-state temperature of a white surface (K) with solar reflectance of 0.80 and infrared emittance of 0.9, under standard solar and ambient conditions;

*Ts* is the temperature of the roof surface (K) under the standard solar and ambient conditions;

α is the solar absorptance of the roof surface (-) equal to 1 − ρ;

ρ is the solar reflectance of the roof surface (-); and

σ is the Stefan–Boltzmann constant, 5.67 × 10−<sup>8</sup> (Wm−2·K−4). Table 4 shows typical roofing materials with solar absorption (α), solar reflectance (ρ), and infrared emittance (ε) values used in this work to quantify *SRI* and *Ts* before and after roof renovation using the albedo strategy.

In the analyzed district, the values of roofing material properties refer to 'generic black shingle' for dark and black roofs, 'gray Ethylene-Propylene Diene Monomer (EPDM)' for medium roofs, and 'white EPDM' for white and renovated roofs. The values of *SRI* and *Ts* were calculated both at building scale and at blocks-of-building scale to evaluate the external conditions.

The main problem of this strategy is that over time the solar reflectance values of high- reflectance roofs decrease due to the accumulation of surface dirt and the degradation of the material by about 0.15 mainly during the first year [54]. The emission, however, does not decrease significantly, and washing the roof surfaces could restore the roof solar reflectance to 70%–100% of the original values [55].

Since most roofs are not washed frequently, it is necessary to evaluate aged values of solar reflectance and infrared emittance values to predict energy savings. If aged values of a roof are unknown, it is possible to estimate the aged solar reflectance (*Aged*ρ) based on the initial solar reflectance (*Initial*ρ) by using the following equation:

$$A \gcd\_{\rho} = 0.7 \cdot \left( \text{Initial}\_{\rho} - 0.2 \right) + 0.2 \tag{8}$$


**Table 4.** Solar performance of roofing materials [48,56].

Referring to LEED (Leadership in Energy and Environmental Design) environmental protocol is also possible to assess mixed nonroof and roof measures, using the following relation as a function of area surfaces ( *A*):

$$\frac{A\_{\text{nourrof measures}}}{0.5} + \frac{A\_{\text{high refracance roof}}}{0.75} + \frac{A\_{\text{regetated roof}}}{0.75} \ge A\_{\text{total site}} + A\_{\text{total roof}}\tag{9}$$
