Nil Sahal, Ph. D.
Division of Building Technology, Department of Architecture
Istanbul Technical University
Driving rain is rain droplets carried along with wind. It is one of the most significant moisture sources that may cause premature deterioration of external wall systems. Driving rain deposited on the surface of the cladding may drain from the exterior surface of the cladding, be stored in the cladding by capillary absorption or penetrate further into the wall assembly. As well, it may enter through the imperfectly designed, defectively installed and inadequately maintained interface details at through wall penetrations under the driving force of coincident wind pressure or gravity. Water past the cladding through interface details at wall penetrations (for ex. wall-window interface) may drain and accumulate at the spaces between the cladding and the adjacent component and/or be stored in the adjacent component. This process may depend on several factors such as amount of rainwater striking the surface of the wall, quantity of water past the cladding, type of deficiency, moisture content of the components, temperature, time and type of the assembly. Detrimental effects are staining of the surface, blistering, peeling, cracking of paints and renderings, mold, corrosion of metal components, wetting of cellulose thermal insulation material, rotting of wood studs and deterioration of interior finishes. In order to prevent rainwater induced damage, rainwater control strategies must be incorporated in design process of external wall systems.
Basicly, there are two rainwater control strategies; one of them is reducing exposure and the other one is design approaches for rainwater control.
Climate, site, shape and surface features of the building are the factors that determine the amount of driving rain that an external wall system is exposed to. As for climate, most parts of the world experience a significant amount of driving rain while other parts may not. Driving rain map of Turkey, which is illustrated in Fig. 1, demonstrates the severity of driving rain that an external wall system is exposed to in different locations of Turkey. The locations marked as o and • refer to shielded and moderately exposed locations, respectively. It is observed that only 8 locations are prone to driving rain and the remaining locations are in the shielded sites. The moderately exposed locations are Şile, İnebolu, Zonguldak, Antalya, Samandağ, Pazar, Hopa and Amasra.
Figure 1. Driving rain map of Turkey
For low-rise buildings, exposure to driving rain can be defended against by planting and landscaping. The shape of the roof and overhang also has a critical impact on the driving rain exposure, especially for low-rise buildings. Overhangs and steep roof systems reduce driving rain exposure, as illustrated in Fig. 2.
Figure 2. Overhangs and steep roofs reduce driving rain exposure [2]
Once driving rain strikes the wall surface, it will form a film and begin flowing downward under the force of gravity. Wind flowing over the surface of the wall will deflect the flow from this path and, in extreme cases, may force upward. Surface features and surface texture can greatly affect the flow paths of the surface flow, either concentrating or dispersing water on the surface. Drip edges and slopes ensure that surface water is removed from the surface of the wall system.
When driving rain strikes the outer surface of the wall; some of the water is drained from the surface of the wall (run-off water / surface flow), some of the water may be absorbed by the wall by capillary forces and the remaining may penetrate through the wall penetrations under the driving force of gravity, capillary forces and/or pressure differences across the wall. There are three design approaches for rainwater control, based on how rainwater is controlled once it strikes the wall surface. These are the following.
Mass wall
Face seal wall
Screened and drained wall
This approach requires the core to have enough storage mass to absorb all rainwater that is not drained from the surface. In a mass wall, the absorbed water is eventually removed by evaporative drying before it reaches the inner surface of the wall. The maximum quantity of rain that can be controlled is limited by the storage capacity of the core relative to drying conditions. Fig. 3 demonstrates a mass wall.
Figure 3. Mass wall
This approach requires an impervious cladding at the outer surface of the wall. When driving rain hits the outer surface of the cladding, rainwater is drained from the surface of the cladding. The sealed joints of the cladding also prevent penetration of rainwater into the wall. Fig. 4 illustrates a face seal wall.
Figure 4. Face seal wall
1.2.2.1 Concealed Barrier Wall
Concealed barrier wall is a type of face seal wall. It incorporates a porous cladding, a waterproofing material and sometimes flashing if a drainage plane (not cavity) is incorporated between the cladding and the waterproofing material. When driving rain strikes the outer surface of the wall; some of the water is drained from the surface of the cladding (run-off water), some of the water is absorbed by the porous cladding by capillary forces. The absorbed water is then transmitted to the surface of a waterproofing material, which is located behind the cladding. Waterproofing material resists further inward movement of water. The absorbed water is eventually removed by evaporative drying. If a drainage plane is incorporated between the cladding and the waterproofing material, the water moves downward due to the force of gravity until it is drained outside by flashings. Fig. 5 illustrates a concealed barrier wall.
Figure 5. Concealed barrier wall
In screened-drained wall, some rainwater is absorbed by the porous cladding and/or open joints and this water is removed by an assembly that provides drainage within the wall. Based on the type of the assembly, screened-drained wall is further classified as the following.
Ventilated cavity wall
Pressure moderated cavity wall
1.2.3.1 Ventilated Cavity Wall
Vented cavity wall incorporates a porous cladding and/or open joints, an at least 12-mm deep cavity and flashings. Some of the rainwater drains at the outer surface of the cladding and the remaining is absorbed by the porous cladding material. The absorbed water is then transmitted to the back of the cladding where it moves downward due to the force of gravity until it is drained outside by a flashing, (weep holes are required at masonry cladding). The ventilated cavity behind the cladding provides a capillary break, as well as a clear path for gravity drainage and a path for air flow. The cavity allows water vapor diffussion and air mixing between the cavity and the exterior. Ventilation provides a mechanism for the removal of water that does not drain from the cavity, Fig.6.
Figure 6. Ventilated cavity wall
1.2.3.2 Pressure Moderated Cavity Wall
A pressure moderated cavity wall incorporates a porous cladding and/or open joints, an at least 12-mm deep cavity, an air barrier material and flashings; namely it promotes the moderation of air pressure differences across the cladding by incorporating an air barrier material on the interior surface of the core, Fig.7.
Figure 7. Pressure moderated cavity wall
An external wall system, depending on the design approach for rainwater control, incorporates either one component (mass wall) or several components, such as cladding, waterproofing material, etc. Besides, driving rain, the wall is exposed to other external agents such as loads and temperature difference across the wall. Hence, employing other measures/components into the wall system in conjuction with rainwater control approach is required.
When there is a temperature difference across the wall, heat flow occurs through the wall from relatively high temperature to low temperature. The wall must control heat flow in order to prevent heat loss in winter and heat gain in summer, which both increase energy consumption. Some measures to control heat flow are to incorporate a thermal insulation material in the wall and to increase the thickness of the core. Some examples of thermal insulation materials include expanded polystrene (EPS), extruded polystrene (XPS), rock wool and glass fibre.
Core is the component of the wall, which carries and transfers the static (self-weight) and dynamic loads (eartquake, wind load). External wall systems must carry and transfer the static and dynamic loads safely. Buckling, overturning and/or sliding of the walls under the given loads should not occur. When the core supports the loads that are transfered to it from roof and upper floors (in addition to its own weight, wind and eartquake loads) and transfers them safely to the foundation, this type of external wall system is named as "load bearing wall system". In case the core carries only its own weight, wind and earthquake loads and transfers them to the structural system of the building, it is named as "non-load bearing wall system". Hence, external wall systems can be classified according to the means that the core transfers the loads; i.e. load bearing wall system and non-load bearing wall system.
Load bearing walls are further classified as single layer or multi layer wall system.
Single layer load bearing wall incorporates only a core. The core carries the static and dynamic loads and transfers them safely to the foundations. The measure taken against driving rain is evaporative drying; hence single layer load bearing wall is also called a mass wall. Heat flow through the wall is reduced by increasing the thickness of the core to a certain dimension.
Single layer load bearing wall, namely the core is constructed as masonry or panel. Masonry consists of modular building blocks such as brick, concrete block, stone, adobe bonded together to form the core. Precast or on-site cast reinforced concrete forms the panel core. Fig. 8 illustrates some examples of single layer load bearing wall system.
Figure 8. Examples of single layer load bearing wall systems (rubble stone and brick masonry) [1]
In a multiple layer load bearing wall, the core is employed in the wall assembly in order to carry the static and dynamic loads and to transfer them safely to the foundation.
The core of multiple layer load bearing wall is the following.
Masonry: brick, concrete block, aerated concrete block.
Panel: reinforced concrete, aerated concrete reinforced panel.
Stud: wood stud, light steel stud.
In order to control rainwater, the multi layer load bearing wall system is designed either face seal or screened-drained wall. Thermal insulation material is incorporated into the wall assembly by locating it on the outer surface, inner surface of the core or within the core. Fig. 9 illustrates an external thermal insulation composite system; which is a face seal wall and thermal insulation board is located at the outer surface of brick masonry core. Fig. 10 demonstrates a brick veneer steel stud wall; which is a pressure moderated wall and thermal insulation board is located at the outer surface of light steel stud core.
Figure 9. External thermal insulation composite system Figure 10. Brick veneer steel stud wall (load bearing wall)
2.2 Non Load Bearing Wall System
Non-load bearing wall system carries only its own weight, wind and eartquake loads and transfer them to the structural system of the building. It is designed as the same described in multiple layer load bearing wall; i.e in order to control rainwater, the wall system is designed either face seal or screened-drained wall and the thermal insulation material is incorporated into the wall assembly by locating it on the outer surface, inner surface of the core or within the core. Non-load bearing wall is classified as infill wall and curtain wall.
Infill wall can be located in five different positions relative to the columns of the frame building, Fig 11. Fig. 12 illustrates a non-load bearing infill wall system. The cladding is stone panel with a light steel stud core. Due to the impervious cladding and sealed joints, this is a face seal wall. The thermal insulation material is not shown in the figure; however, it can be located at the outer surface of the steel studs and the metal decking floor slab, within the stud cavity or at the inner surface of the stud. Fig. 13 illustrates a non-load bearing infill wall system; i.e. brick veneer light steel stud wall. This is a is a pressure moderated wall and thermal insulation material is located at the light steel stud cavity, which rests on the floor slab.
Figure 11. Location of infill panel walls relative to the structual system Figure 13. Brick veneer light steel stud wall [13]
Curtain wall is further classified as panel wall system and stick system. The difference between infill panel and panel wall system is that the core of the wall rests on the floor in infill panels (core transmits the loads to the floor), where the wall assembly as a whole is hung to the structural system components in the panel wall, usually from floor to floor.
Panel wall system, additionally, involves precast concrete panels, precast concrete sandwich panels, metal sandwich panels and glass fibre reinforced concrete panels. Fig. 14 demonstrates a panel wall system. It is a pressure moderated cavity wall; i.e. the cladding is brick veneer and an air barrier material is located on the outer surface of the core, which is light-steel stud. There also exists weep holes at the brick veneer and the flashing. The thermal insulation material is located in the light steel stud cavity. The wall assembly as a whole is hung to the structural system of the building. Fig. 15 illustrates a precast concrete sandwich panel. It is a face seal wall and the thermal insulation material is located between the precast concrete panels, which is hung to the floor beam.
Figure 14. Brick veneer steel stud wall (panel wall system) [1] Figure 15. Precast concrete sandwich panel (panel wall system) [1]
Stick system is assembled from various components, which are steel or aluminum anchors, mullions, transoms, vision glass, spandrel glass, insulation and metal back pans, Fig. 16. Vertical members of the stick system (mullions) are fixed to the slab edge and transoms are fixed between the mullions. Horizontal (transom) and vertical (mullion) framing members (sticks) are made of aluminum, steel, aluminum clad PVC-U. One frame opening is created for vision to receive an insulating glass unit and one frame opening for spandrel panel cover in order to hide the floor edge, heating equipment etc., Fig 17. Infill units may be fixed and opening glazing, insulated panels (aluminum, steel, glass, stone facings). These units are typically sealed with gaskets. Hence, stick system is face seal system. Heat flow is controlled by thermal insulation material located at the back of the spandrel panel cover and insulating glass unit.
Figure 16. Stick system [14] Figure 17. Vertical section of a stick system [14]