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The impact of exterior climate in the design of the driving rain protection of the facade in the context of interior insulation retrofit solutions
Abstract
It is well known that the application of interior insulation systems increases consistently the energy performance of historic buildings, but severely affects the drying potential of their envelopes, by reducing the evaporation potential towards the internal side. Several strategies have been implemented to face this side effect: among the others, in the cases where the wind-driven rain represents the most impacting environmental factor that could cause interstitial condensation, it is recommended to reduce the penetration of the wind-driven rain through the replacement of the original plaster with a "water-repellent" one. This, in order to decrease the permeability of the external surface to the liquid water coming from the wind-driven rain. Since such interventions represent big economic and environmental costs, there is a big need to identify whether they are necessary or not. In the German standard DIN 4108-3, the need for this type of intervention is decided only on the basis of the annual precipitation amount; this approach can hardly be extended to lower European latitudes, as it totally excludes the impact of the radiation, which may play a significant role on the water content of the envelopes as a consistent driver for evaporation phenomena. Moreover, the wind effect may be taken into account by considering the wind driven rain on the studied façade. These hypotheses are investigated in the framework of the "HyLAB" project, for the conservation and refurbishment of a protected industrial building located in Bolzano (Italy); the results from hygrothermal simulations are analyzed and compared to other climates with similar annual precipitation and different annual radiation amount, at lower and higher latitudes. Results shows that, regarding highly rainy climate with consistent amounts of solar radiation, the use of a water repellent exterior coating could be avoided. Moreover, regarding lower latitudes of Europe, there is currently the need of a more elaborated method for identifying the necessity of replacing the external coating; this method should include external climate factors, e.g. annual radiation and wind-driven rain.
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Historic buildings are defining elements of city centres but also responsible for a large share of greenhouse gas emissions. In order to reduce their energy demand, it is important to improve envelopes’
thermal transmittance without damaging the historic integrity of their façades by applying interior insulation. Thus, a detailed planning is needed to guarantee that case-specific variables are well considered. In this study, a holistic performance-based evaluation method is developed. Firstly, the hygrothermal risks are investigated through dynamic hygrothermal simulation. Secondly, an energy assessment is performed, followed by a life-cycle assessment to investigate the environmental impact of the intervention. The developed method is applied in the framework of a conservation and rehabilitation case of a residential building, to evaluate six natural-based insulation systems: perlite filled bricks, wood fibre, cellulose, cork, mineral foam, and calcium silicate. Results show that vegetal-based materials have the lowest initial environmental impact. In any case, retrofit interventions proved to be crucial to reduce the overall environmental impact associated with the use of historic buildings. The developed method, proving to be suitable for application to internally insulated historic buildings, supports a clear identification of the design solution with the lowest environmental impact by using a unique indicator.
In the 1960s and 70s extensive experimental investigations were carried out at the open air test site of the Fraunhofer-Institute of Building Physics concerning the driving rain protection of Autoclaved Aerated Concrete (AAC) walls coated with innovative synthetic resin renders. Since some of the exposed test walls did not perform as well as others, a correlation between the water absorption and the vapour diffusion properties of the façade coatings was established. This correlation was subsequently introduced into the German Standard for exterior rendering systems with a slight modification to account for the special characteristics of mineral renders.
Is it reasonable to invest -thoughts and money -in the energy refurbishment of historic buildings? This paper quantifies the potential impact in terms of climate protection and enhanced living conditions – looking not only at exemplary listed buildings, but more generally historic "cityscapes". Statistics reveal that 14% of EU-27 building-stock dates before 1919, other 12% between 1919 and 1945 (with considerable national differences), corresponding to 30 resp. 55 million dwellings and 120 million Europeans living there. With information on climatic regions and building performance a heating-demand of 855 TWh corresponding to more than 240 Mt CO 2 can be estimated. Refurbishment can save 180 Mt CO 2 within 2050 (3.6 % of 1990's EU-27-emissions), while bringing indoor comfort increases (higher surrounding temperatures, less draughts, ...) and energy-costs decrease. Finding conservation-compatible solutions enhances therefore long-term-conservation and sustainable management of our towns.
Wind-driven rain (WDR) is one of the most important moisture sources with potential negative effects on the hygrothermal performance and durability of building facades. The impact of WDR on building envelopes can be understood in a better way when the WDR intensity distribution can be accurately predicted. Most field experiments of WDR reported in the literature focused on either stand-alone buildings or on buildings in geometrically complex environments. There is a need for high-resolution measurements in more generic and idealized multi-building configurations. The present study reports WDR measurements that were conducted with high spatial and temporal resolution in a test setup consisting of an array of 9 low-rise cubic building models, located in Dübendorf, Switzerland. Detailed descriptions are provided of the building models, the surroundings, the measuring instruments, the measurements of WDR, wind speed, wind direction, horizontal rainfall intensity and air temperature during three selected rain events, as well as error estimates for the WDR measurements. The datasets of rain events and WDR measurement results are made available online to download and are intended for WDR model development and validation.
The moisture design of exterior walls in a building envelope is an important task that needs to be carried out systematically to generate a sustainable and healthy built environment. Many conventional methods or practice guidelines are available for this purpose, based primarily on local traditions and with limited performance assessment records. In recent years, with the rapid development of global free trade and economy, new wall systems and unconventional materials have been introduced in every part of the world for reasons such as aesthetic appeal, cost effectiveness and so on. However, neither the long-term moisture management performance of these new wall systems nor the uses of unconventional materials have been assessed in a systematic way. The primary reason for this lack of assessment is the absence of a design-oriented methodology to perform the task. This paper presents selected results from a recently completed research project that demonstrate that it is indeed possible to assess the moisture management performance of exterior walls in a systematic way, using a hygrothermal modeling tool together with key inputs from a limited number of laboratory and field investigations. In this project the hygrothermal responses of exterior walls and their components were assessed with a novel moisture response indicator, called the RHT index, which is derived from relative humidity and temperature data over a time period. The results and discussion presented in this paper clearly show the need and usefulness of the application of hygrothermal simulation tool for the optimum moisture design of exterior wall systems in various geographic locations, when sufficient information is available from laboratory and field experiments.
An inevitable measure when energy retrofitting historic buildings in Europe, is the reduction of building envelope heat loss. On preservation-worthy facades where external insulation is not an option, installation of internal insulation is gaining pace. The historic buildings in Denmark are often constructed with solid masonry facades and wooden decks. The internal insulation may, however, entail potential hygrothermal risks in walls and embedded wood. Measures such as vapour barriers and capillary active insulation materials are continuously evolving and the subject of much current research. The hygrothermal conditions are of great importance for the durability of the building constructions, and for the health and wellbeing of occupants. Wind-driven rain (WDR) is a central factor contributing to water penetration and moisture loads of the exterior walls. Numerous studies have shown that WDR loads influence the moisture conditions in masonry walls and embedded wooden beams, and can even affect interior relative humidity. In the present paper WDR loads on existing façades in a cold temperate climate were determined by measurements and compared to a semi-empirical model. Simultaneously, the hygrothermal conditions within internally insulated walls with exposed brick and embedded wooden beams were monitored. Furthermore, numerical simulations were implemented for clarification of WDR impact. Hygrothermal simulations and previous studies, inevitably show that high WDR loads result in higher moisture content behind the interior insulation. Results from the field measurements of WDR however, cannot directly be referred to the moisture content measured in walls behind interior insulation or beam ends. However, fluctuations in external air humidity proved to be influential on condiditons in the construction. Implementation of a semi-empirical model for calculations of WDR agreed with previous studies in predictions being too conservative when compared to measured WDR.
Energy efficiency in historical buildings with a worth-preserving appearance can be improved with interior thermal insulation. However, interior thermal insulation may change significantly the hygrothermal performance of the building envelope and increases the risk of moisture problems in the assembly components. This paper studies the influence of different wall structures, types of exterior render and brick and vapor control strategies on the moisture performance of internally insulated walls with vapor-open insulation for different climatic conditions in Switzerland. The hygrothermal performance is simulated with a coupled heat and mass transport model and evaluated using a hygrothermal indicator, called the RHT Index. The numerical analysis indicates that the hygrothermal performance of interior insulated walls depends mainly on the moisture performance of the exterior finishing render. A capillary active render with larger liquid permeability leads to higher moisture contents in the building envelope. The influence of parameters such as masonry structure and brick type on envelope hygrothermal performance is rather small. For wall envelopes with less capillary active renders, the need to use a vapor barrier depends on the risk for water leakage and the vapor resistance of the render.
Exterior walls in historic multi-storey buildings compared to walls in modern buildings have low thermal resistance, resulting in high energy loss and cold surfaces/floors in cold climates. When restrictions regarding alteration of the exterior appearance exist, interior insulation might be the only possibility to increase occupant comfort.
This paper describes an investigation of the hygrothermal influence when applying 100 mm of diffusion open interior insulation to a historic multi-storey solid masonry spandrel. The dormitory room with the insulated spandrel had a normal indoor climate with a maximum observed monthly average humidity by volume excess of 3.2 g/m³ during the experiment.
Relative humidity and temperature were monitored manually using wooden dowels over 2 years and 8 months in two solid masonry spandrels: one insulated wall and one untreated wall. The investigation showed that installing insulation on a solid masonry spandrel induced hygrothermal changes: Uniformly distributed higher relative humidity and lower temperature throughout the masonry, compared to an un-insulated wall. The relative humidity of the un-insulated masonry wall was in the range 50 % on the inside to 60 % on the outside, while the insulated wall showed uniformly distributed values around 80 %.
The risk of moisture-induced damage was evaluated based on mathematical models for mould and decay of wood, visual inspection for frost and mould, and on-site measurements for presence of mould spores. The damage evaluation showed no risk of damage from the changed hygrothermal conditions when applying interior insulation to a solid masonry spandrel.
Wind-driven rain (WDR) is one of the main sources of moisture damages in buildings. Roof overhangs are a common feature that can be used to reduce WDR on building facades. However, there is very limited information on the quantitative evaluation of the effectiveness of overhangs in reducing WDR on building façades, especially through field measurements. A six-story building with a low-sloped roof located in Vancouver has been equipped with a retractable overhang along with a rooftop weather station measuring wind speed, wind direction and horizontal rainfall and a total of 31 WDR gauges measuring WDR on building facades. The spatial distribution of WDR on the building façade has been studied without and with overhangs. The effectiveness of roof overhang is studied with respect to wind speed, wind direction and rainfall intensity. Field measurements show that for the particular climate characterized by long rainy winters with mild wind and rain, the overhang is effective and significantly reduces WDR for this six-story building, especially for areas directly underneath the overhang. The protection increases from the side edge to the center and from the bottom to the top of the façade. As expected, the larger overhang provides greater protection. The relationship between overhang effectiveness and distance from the roofline is quasi-linear with smaller gradient for the larger overhang. The effectiveness of the overhang is highly dependent on wind speed and wind direction - it increases for oblique winds but decreases with the increase of wind speed.
Installation of thermal insulation is one of the most effective approaches to improve energy efficiency of the building envelope. Interior insulation retrofit of the masonry construction will change its original thermal and moisture balance, thus a careful pre-phase design is needed to avoid the structural and moisture impairments. This study evaluates the performance of four interior insulation systems with application of the capillary-active mineral materials: autoclaved aerated concrete, calcium silicate, mineral foam, and perlite. Hygrothermal properties of these four materials are experimentally measured and analyzed. They present quite different moisture storage and transport characteristics, which influence the hygrothermal behavior of the refurbished construction. A non-capillary-active polyurethane rigid foam insulation system is also presented for comparison. Moreover, the influence of the indoor humidity on the performance of the insulation systems is investigated.
Hygrothermal models allow designers to evaluate the hygrothermal performance of building envelopes. A key question in hygrothermal modeling however remains in the proper selection of representative exterior moisture conditions, i.e. a moisture reference year. A Climatic Index combining wind-driven rain load and potential evaporation is developed allowing considering the effect of climatic variability on the performance of the building envelope. The proposed Climatic Index combines thus both the wetting load and evaporation potential. This index is suitable for the evaluation of the level of moisture damage risk of wall assemblies where typical moisture problems are mainly caused by rainwater uptake or ingress.
The Climatic Index is determined for four cities in Switzerland. The hygrothermal performance of the assemblies is simulated and evaluated using a hygrothermal indicator, called the RHT Index. A clear correlation between Climatic Index and RHT Index is found. However, the moisture reference year selected based solely on a 10% level criterion of the Climatic Index may not correspond to the year with the most significant moisture damage risk. Therefore, a new procedure is proposed which combines a first selection of three years around the 10% level criterion, followed by a careful comparison of these years based on hygrothermal simulations and selection of the year with the largest RHT Index as moisture reference year. The combination of Climatic Index and RHT Index from hygrothermal simulations allows for the selection of moisture reference years that are around the 10% level, but excluding the ambiguity.
Until recently, the most common use of reference year data has been related to energy calculation. Representative weather data for use in moisture design calculations is as critical as other parameters required for heat, air, moisture (HAM) simulations e.g., material data or indoor climate data. The objective of ASHRAE's research project, RP-1325 (2010), was to develop a method for generating more representative design weather years for use in heat and moisture performance predictions of building enclosure systems. Existing methods for selecting moisture design reference years have been reviewed. A new method was developed that provides an improved approach for selecting the most critical years in terms of hygrothermal performance. During the development of the method, eight U.S. locations were investigated, with 30 years of hourly weather data for each location. Another four locations were used to validate the accuracy of the method. The new method was applied in selecting the hygrothermal weather year for European location. The performance of six different building enclosure components, 4 walls and 2 roofs, was investigated using existing hygrothermal simulation models. The effect of weather data on the performance of building enclosure components was evaluated. The new method allows for selection of weather years that are among the most severe for the simulated structures. The results show that weather data is an essential component of design criteria.
This paper aims to study the effects of wind-driven rain load and vapor diffusion on the hygrothermal performance of wall systems in a wet and mild climate through a field experimental study. In the study, four test panels with a combination of vapor barrier and capillary break are manufactured, instrumented and installed in a field experimental facility. The wetting and drying potentials of the test panels in response to a predominately vapor diffusion and a wind-driven rain load are discussed based on the analysis of 15 months of measurement data. The experimental result shows that, in a yearly basis, the wetting and drying rates of a wall without a capillary break are about two times higher than that of the wall with a capillary break. While the wetting and drying rates are comparable in a wall system with a vapor barrier, the drying rate is 38% higher than the wetting rate in a wall with no vapor barrier. In general, a wall with no vapor barrier wets and also dries faster than a wall with a vapor barrier. For the wall types and climate considered in this paper, the wetting rates of walls with a predominately wetting mechanism of vapor diffusion and wind-drive rain load are comparable. In general, the experimental data suggest that even in a mild climate, vapor diffusion is a critical moisture load with comparable effect that wind-driven load induces.
The concept of time of wetness (TOW) is extensively used in the atmospheric corrosion disciplines for such purposes as corrosion prognostics and environmental corrosivity classification. Time of wetness can generally be defined as the amount of time a metal surface remains wet during atmospheric exposure. Although the basic concept and importance of TOW in governing atmospheric corrosion is generally agreed upon, what is meant by " wet " has generally remained ambiguous and its practical determination has been widely varied throughout its history. The objective of this review is to demystify TOW in terms of definition and measurement and identify issues that inhibit this parameter from providing deeper insight into the effect wetness duration has on atmospheric corrosion. Specifically, this paper summarizes the current state of TOW determination methods along with the parallel concept leaf wetness used in environmental science. Furthermore, wetting and drying phenomena associated with atmospheric corrosion and thought to control TOW are overviewed. From this framework, current issues and limitations of TOW are identified and avenues for further improvement suggested.
Rainfall that is pushed along horizontally by the wind is referred to as wind-driven rain (WDR) or driving rain. Water penetration of exposed building surfaces is enhanced with WDR, and structural damage is accelerated with repeated WDR exposure. Additionally, WDR is an agent for enhanced soil erosion and for dispersal of certain plant diseases. This research provides a descriptive climatology of WDR for the contiguous United States. WDR intensity is estimated from a linear regression model that develops a relationship between rainfall on a horizontal surface and rainfall on a vertical surface. The WDR estimates are used to create maps of WDR intensity, WDR event duration, WDR annual frequency, WDR total receipt, and WDR direction. A 25–year period of record (POR) of hourly data is used for 182 stations across the United States. Areas of highest WDR intensity, duration, and frequency are found in the southern United States, especially along the Gulf Coast. Directional indices indicate that WDR occurs most often from easterly directions in locations in the southeast and central United States, while WDR is predominantly from the south along the West Coast. [Key words: wind-driven rain, rainfall, climatology, United States.]
Representative weather information is essential for a reliable building energy performance evaluation. Even if detailed energy analyses can be carried out considering the multi-year weather data, generally a single reference year is adopted. Thus, this artificial year has to correctly approximate the typical multi-year conditions. In this work, we investigate the representativeness of the method described in the technical standard EN ISO 15927-4:2005 for the development of reference years. Energy performance of a set of different simplified buildings is simulated for 5 North Italy locations using TRNSYS. The energy needs computed using the reference year are compared to those of a multi-year simulation. The annual variability of energy results for the studied thermal zones is investigated, paying attention to its effects on the building envelope energy ratings according to a proposed classification. Also, those configurations more influenced by the annual weather changes are identified by means of statistical indexes. The analyses demonstrate that the representativeness of the reference year results can vary significantly in the considered locations–and, consequently, the accuracy in building energy assessment and classification can be reduced, especially for some building envelope configurations.
Building surface erosion is a common phenomenon observed on historic building façades due to wind-driven rain (WDR) impact. Recently, studies on climate change and the effect this might have on increased extreme rainfall events has renewed the scientific interest on determining the risk of accelerated erosive effects. Given the fact that WDR loads on building façades is proportional to rainfall and represents the main moisture source and erosive physical impact for building façades, an assessment method that quantifies the severity of erosion is the first step towards recommending remedial measures. The paper discusses the major factors escalating the gradual loss of surface material, considering value, hazard, vulnerability and exposure in order to examine the WDR drop impact on the aesthetic significance and the structural integrity of heritage buildings, within a parametric framework. The study investigates the effects of different size water drops, with different impact speeds on a range of masonry materials with different surface asperities and varying moisture absorption features, at various impact angles. For the relative quantification of the long-term surface erosion, straightforward and globally adaptable experiments are proposed based on site-specific climatic data and materials. Finally, strength decline of exposed sample units proves the strength-degrading effect of erosive WDR.
A formula was developed to yield an index of the relative potential of a climate to promote decay of off- the- ground wood structures. The distribution of climate indexes for the United States is shown by regions on a contour map. The quantitative measure of decay potential is needed to estimate needs for protective measures, especially preservative treatments. Possible adaptation of the formula to meet conditions of wood in contact with the ground is considered.
This paper presents a new method for selecting the energy reference year, which may be used for many applications, such as the energy performance of buildings, the simulation of active or passive solar energy systems, HVAC system performance or indoor climate analyses. The EN ISO 15927-4 method was modified by using monthly dependent weighting factors for the main climatic parameters in the calculation of heating and cooling energy demand in buildings. We used data from the cold Finnish boreal climate to demonstrate the method. In a cold boreal climate, during the summer temperature and solar radiation have a similar influence on heating and cooling energy demand, whereas air humidity and wind speed have a minor effect. The principle for determining the weighting factors and modified method for the selection of the energy reference year can also be used for other climatic zones. The weighting factors determined in this study can be used for other countries in a cold climatic area.
Wind-driven rain (WDR) or driving rain is rain that is given a horizontal velocity component by the wind. WDR research is of importance in a number of research areas including earth sciences, meteorology and building science. Research methods and results are exchangeable between these domains but no exchanges could yet be noted. This paper presents the state-of-the-art of WDR research in building science. WDR is the most important moisture source affecting the performance of building facades. Hygrothermal and durability analysis of facades requires the quantification of the WDR loads. Research efforts can be classified according to the quantification methods used. Three categories are distinguished: (1) experimental methods, (2) semi-empirical methods and (3) numerical methods. The principles of each method are described and the state-of-the-art is outlined. It has been the intent of the present paper to bring together the reports, papers and books—published and unpublished—dealing with WDR research in building science to provide a database of information for researchers interested in and/or working in WDR research, independent of their field of expertise.
This paper gives an onset to whole building hygrothermal modelling in which the interaction between interior and exterior climates via building enclosures is simulated under a moderately cold and humid climate. The focus is particularly on the impact of wind-driven rain (WDR) on the hygrothermal response, mould growth at interior wall surfaces, indoor climate and energy consumption. First the WDR load on the facades of a 4 m × 4 m × 10 m tower is determined. Then the hygrothermal behaviour of the brick walls is analysed on a horizontal slice through the tower. The simulations demonstrate that the impact of WDR loads on the moisture contents in the walls is much larger near the edges of the walls than at the centre. The obtained relative humidity and temperature at the interior wall surfaces are combined with isopleths of generalised spore germination time of fungus mould. The results show that WDR loads can have a significant impact on mould growth especially at the edges of the walls. Finally, for the case analysed, the WDR load causes a significant increase of indoor relative humidity and energy consumption for heating.
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