Equivalence between the methods established by ISO 15927-3 to determine wind-driven rain exposure: Reanalysis and improvement proposal

https://doi.org/10.1016/j.buildenv.2020.106777Get rights and content

Highlights

  • Equivalence analysis between the ISO 15927-3 methods to calculate wind-driven rain.

  • Detailed climate data analysis of 12 Spanish sites with varied exposure conditions.

  • Comparison of several methodological interpretations for the non-hourly data method.

  • Improvement proposal to reinforce the reliability of this alternative method.

  • Calculation of a best-fit relationship to extrapolate results between both methods.

Abstract

ISO standard 15927-3 establishes two calculation methods to characterise wind-driven rain exposure on building façades, with different approximations according to the available climatic data: (i) a reference method, using hourly wind and rain data, and (ii) an alternative method using average wind records and the present weather code for rain in half-day intervals when hourly data are not available. However, the equivalence between the two methods was only validated in three British cities with similar environmental conditions (London, Manchester, and Edinburgh), and for only four façade orientations. In addition, the description of the second method also admits different interpretations that can influence its results. This study re-examines the real equivalence between the two methods, comparing their results in multiple façade orientations of 12 Spanish locations subjected to varied amounts of wind-driven rain exposure. The analysis shows that the reliability of the method based on non-hourly data varies significantly according to the methodological interpretation applied, making it advisable to review its current description. Finally, an improvement is proposed that reinforces the equivalence and reliability of the alternative method with respect to the reference method, regardless of the climate of the analysed location.

Introduction

The penetration of rainwater into building façades causes multiple types of deterioration in porous construction materials (such as frost attack, salt migration, crystallisation stress, cracking, and loss of adherence), reductions in the efficiency of the thermal insulation of the building envelope, impacts on the health conditions of residents, and biodeterioration phenomena [[1], [2], [3], [4], [5], [6], [7], [8]]. The wind-deflected rain that impacts the vertical surface of the façades (wind-driven rain (WDR)) is the main source of moisture for this penetration process [9,10]. Accurately determining the exposure of each façade to the rainwater supply is therefore a key aspect for defining watertight designs.

ISO standard 15927-3 constitutes an international reference for the calculation of this exposure, and enables the WDR that would impact on a given façade orientation to be quantified. This is totalled over one year, and over the worst WDR period likely to occur in any given 3-year period [[11], [12], [13], [14], [15]]. Accordingly, the standard uses hourly rainfall and wind velocity data (i.e., speed and direction) collected at the sites for a minimum of 10 years. This calculation procedure is based on the methodology applied by BS standard 8104, but it does not include the WDR maps and directional roses included in the British standard, and is thus applicable to any geographic area (hereafter termed the reference method) [16].

In turn, this standard incorporates an alternative method for locations without hourly climate data, which uses average wind records and the present weather code defined by the World Meteorological Organization [17]. This method (hereafter termed the alternative method) characterises periods when the façade is likely to moisten. This means it does not quantify the amount of water that impacts the façade, but the severity of these ‘wetting periods’. For this purpose, each 12 h interval (‘half-day’) is assigned a wetting (+1), drying (−1) or neutral nature (0), according to the presence and intensity of the rainfall, mean wind speed, wind direction and relative humidity during each half-day. The number of wetting half-days accumulated during a single wetting period that might be exceeded once every 10 years should provide a priori a WDR exposure characterisation on a given façade orientation equivalent to the reference method [11].

Sanders analysed the equivalence between the results of both methods at several British locations (cooperative project IEA-Annex 41 of the International Energy Agency), and this comparison was graphically collected in the Informative Annex D of the standard ISO 15927-3 [11,18]. However, neither the number of sites studied (London, Manchester, and Edinburgh airports, three cities with a similar rainfall amount of between 600 and 830 mm/year) nor the orientations analysed (North, South, East, and West) are sufficiently representative to validate the equivalence between the two calculation methods [19]. Furthermore, the description of the alternative method does not define how to identify the end of each period of accumulated wetting half-days, thus allowing different interpretations that could lead to different results.

To address this issue, this research assesses the real equivalence between the two calculation methods by analysing a representative number of locations and façade orientations, subjected to varied climates and WDR exposures. Accordingly, 12 Spanish locations distributed throughout the regions of Galicia (northwest of the Iberian Peninsula), La Rioja (Ebro river valley), and the Canary Islands were studied. These regions have average rainfall amounts ranging from 72 mm/year to 1734 mm/year, mean wind speeds ranging from 1.2 m/s to 6.4 m/s, and climates ranging from hot desert to oceanic [20,21].

This reanalysis will allow the reliability of the alternative method to be quantified under different climatic conditions, discuss the validity of different methodological interpretations and identify the one that is capable of achieving better agreement with respect to the reference method. Finally, an improvement for this alternative method is proposed. This advancement of the international ISO standard 15927-3 will provide a more reliable characterisation of the WDR exposure in locations without simultaneous records of hourly climatic data, thus enabling more suitable designs for building façades in these locations.

Section snippets

Background. Description of standard ISO 15927-3

The reference method of ISO standard 15927-3 uses a semi-empirical approach to calculate the WDR exposure, based on the overall formulation established by the ‘WDR relationship’ (Eq. (1)). This relationship links simultaneous records of wind speed U (m/s) and rainfall intensity Rh (mm) to obtain the water amount impinging on the vertical façade surface [22,23].WDRθ=kURhcos(Dθ)

To determine the WDRθ (l/m2) on each specific façade orientation θ (°), the WDR relationship incorporates a cosine

Methodology

Although the alternative method aims to determine the number of wetting half-days accumulated in a single wetting period (i.e., wetting period severity), the ISO standard does not establish any rule to define the end of these wetting periods. Further, the definition applied for the spell of the reference method is not applicable because it is based on hourly records. Logically, the duration of these periods affects the number of wetting half-days accumulated in them.

After a detailed analysis of

Reanalysis of the alternative method, considering different methodological interpretations

From the same 10-min and 30-min records used with the reference method, the necessary climate series (precipitation values, average wind speed, average wind direction, atmospheric relative humidity, and occurrence of precipitation in intervals of 3–12 h) have been developed to apply the alternative method. In this way, any discrepancy between the baseline data used by both methods is avoided. All results obtained by means of the alternative method for each façade orientation, location, and

Discussion and improvement proposal

The disparity of the previous results demonstrates the need to review standard ISO 15927-3 to clarify the procedure that should be followed to quantify the length of the wetting periods. In turn, although Interpretation B increases the reliability and precision of the alternative method, it still does not guarantee adequate equivalence with the reference method in any situation (some poor correlations persist, such as Calahorra, Pazuengos and Vecindario). Therefore, it is necessary to propose a

Conclusions

The continuous improvement of international standards is a task of great interest in the respective fields in which these standards are applied and a challenge for researchers. The alternative method supported by the international standard ISO 15927-3 provides a necessary tool to characterise the WDR exposure in locations without exhaustive hourly climatic data. This research has demonstrated the need to reformulate this alternative method—not only to clarify its calculation methodology but

Acknowledgements

This research was partially funded by the Foundation for the Promotion of Applied Scientific Research and Technology in Asturias (FICYT) through the GRUPIN project Ref. IDI/2018/000221, and The Spanish Ministry of Science, Innovation and Universities through the State Plan for Scientific and Technical Research and Innovation with the project Ref. PGC2018-098459-B-I00, both co-financed with EU FEDER funds. The authors acknowledge engineer Francisco J. Simón Polo for his help with data collection

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