September 15, 2009

Abstract: 

The balance between wetting, drying, and safe storage is critical to the long term performance of building enclosures. Where wetting cannot be controlled to acceptable levels, safe storage and drying become critical. The use of one-dimensional hygrothermal simulation software has been well established for a wide range of wall and roof assemblies. However the use of such software has previously had a limited ability to accurately model the physics of enclosures with ventilated claddings. The most recent version of WUFI 4.1 has added the ability to model enclosure systems that incorporate embedded sources and sinks of moisture and heat. This capability can be used to model source effects such as air and rain leakage within a wall assembly or sinks such as drainage and ventilation.

Introduction

The balance between wetting, drying, and safe storage is critical to the long term performance of building enclosures. Where wetting cannot be controlled to acceptable levels, safe storage and drying become critical. Many common building materials have little safe storage capacity, that is, they cannot be exposed to high levels of moisture for long periods of time.

The sheathing is one building component often made of moisture-sensitive materials placed directly behind the cladding, separated by only a thin sheathing membrane and air gap. For some periods of time, the sheathing can be expected to be exposed to rainwater wetting from the exterior or condensation wetting (air leakage or vapour diffusion) from the interior. Protecting the sheathing from moisture is seen as important and has been the goal of many product manufacturers, builders, and practitioners over several decades. However experience has shown that accidental leaks can still occur, and hence the role of drying is very important to the moisture balance.

Moisture can be transported by airflow (convection), diffusion, or gravity into and through an enclosure wall assembly. Drainage will remove much of the bulk moisture by gravity, when a drainage path is provided, however moisture can still remain adhered or absorbed to materials within the wall assembly. The amount of moisture that can be safely absorbed or stored depends on the material properties. Drying can occur by vapour diffusion, evaporation, desorption, or by air convection (ie. ventilation). Vapour diffusion is shown to be a relatively slow process particularly when low permeance materials are used within the wall assembly. Evaporation or desorption can only occur when moisture is able to get to the surface of the material (often only at the cladding or interior surface), and be removed by the flow of air. Allowing evaporation or desorption to occur at layers within the wall assembly, particularly at the sheathing and removing the excess moisture by ventilation to the exterior provides an effective means to remove additional moisture directly from sensitive materials and improve the drying potential of some wall assemblies.

It is becoming more common in North America to construct walls with claddings separated from the framed wall by an air cavity. This is used as a rain control strategy to eliminate capillary flow between the cladding and sheathing, provide drainage of incidental moisture, and provide some venting or ventilation to remove evaporated/desorbed moisture. Practitioners and builders have sometimes found this gap to be beneficial, particularly in rainy climates such as coastal British Columbia where so-called "rainscreen" wall assemblies are now required by code for most new buildings. The separation of the cladding from the wall assembly has sparked much debate among the building science community. The functions and benefits of providing this cavity are not seen as necessary by all those parties involved, and the actual characteristics of the cavity and vent/drains has not been scientifically determined as a function of performance required. The minimum size of the air gap is also debated; however recent work has shown that walls with even very small continuous gaps (<1 mm) can drain well (Smegal 2006). Although the drainage and a capillary break are obvious improvements, the need for and role of ventilation in improving drying is still debated. Recent ASHRAE-sponsored research however has been able to predict ventilation rates and show the benefits of ventilation on ventilation drying and reduction of inward solar-driven vapor (Burnett et al 2004).

The ability to model the impacts of ventilation within wall assemblies using hygrothermal models has so far been limited to a few research-grade two-dimensional research models. Recently IBP/ORNL enhanced their one-dimensional hygrothermal software, WUFI 4.1, which is used by many practitioners worldwide. The new enhancement can account for the two-dimensional effects of ventilation within wall assemblies, by modeling heat and moisture sources or sinks at any layer within the wall. In addition, the 1% driving rain load mentioned in the proposed ASHRAE 160P Standard can be easily simulated.

This paper discusses how source and sinks can be used in a hygrothermal model to simulate rain leaks and ventilation drying. The model results are compared to measured field data for common wall assemblies with ventilated claddings, and guidance is provided as to calculating cladding ventilation rates and performing accurate simulations.

The role of ventilation in wall performance, the fluid flow mechanics, and previous research are reviewed first to provide the foundation for the research presented here.

Background

It is well accepted that moisture is one of the primary causes of premature building enclosure deterioration. Excess moisture content combined with above-freezing temperatures for long enough will cause rot, mold growth, corrosion, and discoloration of many building materials. The four major moisture sources and transport mechanisms that can damage a building enclosure are (Figure 1 left side):

  1. precipitation, largely driving rain, or splash-back at grade);
  2. water vapor in the air transported by diffusion and/or air movement through the wall (both to interior and exterior);
  3. built-in and stored moisture, particularly for concrete or wood products;
  4. liquid and bound groundwater, driven by capillarity and gravity.

At some time during the life of a building, wetting should be expected at least in some locations. In the case of a bulk water leak, drainage, if provided, will remove the majority of the moisture from the wall cavity. However a significant amount of water will remain absorbed by materials and adhered to surfaces. This remaining moisture can be removed (dried) from the wall by the following mechanism (Figure 1 right side):

  1. evaporation (liquid water transported by capillarity to the inside or outside surfaces;
  2. evaporation and vapor transport by diffusion, air leakage, or both either outward or inward;
  3. drainage of unabsorbed liquid water, driven by gravity;
  4. ventilation by convection through intentional (or unintentional) vented air cavities behind the cladding.

Figure 1: Wetting (Left) and Drying (Right) Mechanisms for Walls

A balance between wetting, drying, and storage is required to ensure the long term durability of the building enclosure. Some commonly used building materials are more sensitive to moisture (eg. paper faced gypsum and untreated wood based sheathings) and hence require a higher drying potential than the more durable materials they have replaced (eg. concrete, masonry, or solid sawn timber). Several wide-spread building enclosure failures in the past decade including those in Vancouver BC, Wilmington NC, Minneapolis, MN and other locations in North America have further raised the awareness and impact of moisture and its impact on building materials (Crandell & Kenney 1996, Morrison Hershfield 1996, Brown et al. 1997, Barrett 1998, RDH 2001, Brown et al. 2003).

Recent building enclosure failures have shown that the drying potential of some wall assemblies in certain climates may be insufficient when exposed to accidental wetting or leaks. As a response to these failures, drained walls have been widely recommended to deal with rainwater penetration. However, cladding ventilation may be needed or useful to increase drying for some wall assemblies in some climates. Ventilated claddings can also control wetting due to inward driven vapor from rain wetted absorbent claddings. The use of large ventilated and drained cavities has already been mandated by some building codes (NBCC 2005).

Some definitions are useful. A ventilated wall is one which has vent openings at the top and bottom of an air cavity, to promote air circulation. A vented wall has only vent openings at the bottom of the wall, usually provided for drainage (Straube & Burnett, 1999). Some exchange of air between the exterior and cavity will occur in a vented wall, however the volume will be small and the area over which it acts is limited in comparison to a ventilated wall.

In both ventilated and vented walls, the cladding is separated from the rest of the wall assembly by a gap or cavity. A WRB (water resistive barrier), which acts as a drainage plane and secondary capillary break, is usually provided to the interior of the cladding and ventilated cavity. The cladding and gap, while significantly limiting the amount of rain penetration, are not relied upon to stop all water. The WRB is also not expected to be completely water tight and may allow some small amount of liquid water penetration. The gap must be drained to the exterior using flashings at penetrations and at the base of wall.

A rainscreen wall as discussed in this paper is comprised of a cladding (stucco, vinyl, cement board, wood) over a ventilated and drained cavity, with flashed details at windows, penetrations, and other transitions.

Not all drained walls are ventilated, and simply providing a drainage cavity does not ensure ventilation will occur. Vent locations and details are important and should be understood by designers.

The principle of using drained claddings with a vented or ventilated cavity behind is not new, and has been used for several centuries. For example, brick veneer has typically been installed away from the sheathing since the late 19th century (although the cavity was often blocked with mortar droppings or filled with insulation). The benefits of providing this vented or ventilated cavity has been debated and the topic of much research in the past few decades.

Previous Research

The previous field research, ventilation mechanics and driving forces are discussed.

Field Research

As early as the late 1970’s and early 1980’s the role of ventilation behind wood claddings was being investigated in Atlantic Canada as problems with warping and paint deterioration of wood sidings became apparent (Marshall 1983) in some climates. Wood siding manufacturers performed in-house tests and found that placing wood siding over a strapped air cavity reduced the occurrence of such moisture problems (Morrison Hershfield 1992).

Throughout the 1980’s a growing number of moisture-related failures were discovered in the Canadian housing stock. Field exposure test huts were constructed in different Canadian climates to study the drying of wood-frame walls, particularly when constructed with initially saturated lumber as was common practice for parts of the country (McCuaig 1988, Forest & Walker 1990, Burnett & Reynolds 1991). These studies showed that drying built-in moisture was practical and possible, and also provided some evidence that cladding ventilation could improve drying. However, the studies were not conclusive, as test variables were insufficiently controlled to isolate the role of ventilation and its specific impact on drying.

In Europe, the Franhofer-Institut für Bauphysik (IBP) conducted field monitoring of ventilation flow and drying effectiveness for different panel claddings in several different projects. Popp et al. (1980) found that the drying rate of an initially wetted aerated concrete block work wall was significantly faster when the cladding was ventilated or even vented compared to an impermeable cladding which was adhered directly to the concrete.

Similar results of ventilation drying effectiveness were also shown by Mayer and Künzel (1983) who measured ventilation behind large cladding panels on a three-storey building in service. The two forces affecting ventilation were found to be wind induced pressure differences and solar-induced thermal buoyancy. Hourly air velocities were measured between 0.05 and 0.15 m/s when the windspeed was between 1 to 3 m/s. Wind direction influenced the ventilation air velocity more than wind-speed. From the testing they concluded that a clear cavity depth of 20 mm was generally sufficient for panel-type claddings, and although a large vent area is not absolutely necessary for acceptable wall performance, it is a practical means of removing trapped moisture. Finally it was recommended that if moisture sensitive materials are used in the backup wall, the upper and lower vent openings should be as large as possible for increased ventilation rates.

In the United States, the impacts of cladding ventilation on wood frame walls was also investigated by TenWolde and Carll (1992) and TenWolde et al (1995). These studies found that in walls with little or no air leakage (from the interior), cavity ventilation promoted drying. When air leakage was allowed it dominated the results.

In full-scale Canadian field studies, Straube and Burnett (1995) and Straube (1998) investigated the role of airspaces in ventilation drying and pressure moderation behind brick veneer and vinyl siding. The study outlined methods to calculate ventilation flow and found that cladding ventilation could be useful as a means to control inward vapor drives behind brick veneers.

Two Canadian laboratory studies investigated the role of ventilation drying of walls in Vancouver, BC in the late 1990’s. The studies were directly as a result of the “leaky-condo crisis”, where a large number of moisture failures were observed in the recently constructed residential housing stock in coastal British Columbia (Morrison Hershfield 1996, Barrett 1998). Both Morrison Hershfield (1999) and Forintek (2001) undertook laboratory studies to determine the impact venting or ventilation had on the performance of wood-frame wall assemblies.

In the Morrison Hershfield study (1999), full-scale insulated wall assemblies with stucco cladding were constructed and initially wetted on the interior side of the sheathing . The walls were exposed to approximately 10°C exterior conditions with no air movement or solar radiation. The major conclusions of the study were that drying was slow for all wall types and that the ventilated rainscreen wall design did not enhance drying of water that penetrates into the stud cavity. Even though the parameters were untested, the authors concluded that solar radiation and wind would have no significant effect on drying, nor would other types of cladding. Applying the physics of thermal and moisture buoyancy described in the next section, calculated natural ventilation rates and driving temperature differences are very low for these walls and in hindsight it is clear why ventilation drying would not have been effective in these test conditions.

The Forintek Envelope Drying Rate Analysis (EDRA) study (2001) was larger and studied more parameters in simulated environments. Two phases were completed, one without simulated exterior wind and solar effects and one with. Solar radiation was simulated up to a 120 W/m2 peak, equivalent to diffuse radiation on a north facing wall in Vancouver. Wind pressure differences of 1 to 5 Pa between top and bottom vents were also simulated. The walls were initially soaked to pre-wet the sheathing and studs, and hence had a relatively uniform distribution of moisture. The sample walls included both stucco and vinyl siding, vented and ventilated designs, SBPO and building paper sheathing membranes, and OSB and plywood sheathing. Some of the conclusions from the study included:

  • Walls with cavities (vented and ventilated) dried faster than comparable panels without cavities (face-sealed). There was a substantial range in the drying rates: as much as three times higher drying rate for comparable walls with a ventilated cavity than for those without.
  • Ventilation (top and bottom vents) resulted in marginally faster drying than vented (bottom vents) walls. The width of cavity was also important, and those walls with cavities of 19 mm dried faster than 10 mm.
  • Walls with plywood dried faster than comparable walls with OSB sheathing. OSB has a lower vapour permeance than plywood and may have restricted the drying through the sheathing to the exterior.
  • Solar radiation increased drying rates of the ventilated walls but had little effect on the face-sealed walls (all walls were restricted from drying to the interior by a low permeance interior vapor barrier)

Recently ASHRAE sponsored a large research and development project (ASHRAE TRP-1091) to study the mechanics of ventilation in wall systems and assess the potential for ventilation drying of common, above-grade residential wall assemblies. Three institutions were involved in this project, namely, the Pennsylvania Housing Research/Resource Center at Penn State (PHRC/PSU), the Building Engineering Group at the University of Waterloo (BEG/UW) and the Building Technology Center at Oak Ridge National Laboratory (BTC/ORNL). The project produced a total of 12 reports and numerous conference and journal papers and is summarized by Burnett et al. (2004).

A review of the literature and theory was performed, hygrothermal properties of several materials were determined, a study of ventilation flows was performed for brick veneer and vinyl siding, the impact of ventilation drying was determined, CFD simulations were performed, and the Moisture-Expert hygrothermal model was validated using the field data which allowed further parametric simulations to be performed. The following conclusions were made from the study:

  • Ventilation rates are dependent on the cladding and venting configuration (size and type of openings) and strongly influenced by weather events (wind and solar radiation). Brick veneer walls had lower ventilation rates than vinyl siding walls.
  • Solar-driven vapor diffusion can act to redistribute vapor from within the wall to the interior, where it can condense and in some cases, cause damage. Cladding ventilation reduces the magnitude of this flow as this vapor is directly removed to the exterior. . .

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