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This paper explores the influence and role of both drainage and ventilation drying on the ability of enclosure assemblies to control moisture. Drainage is often the most direct method of removing water from within a wall (i.e., from exfiltration condensation or rain penetration), but it is often not sufficient to provide moisture control. Design ap...
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Rain water was continuously collected during three years and analysed with respect to pH, conductivity, sulphate, nitrate, chloride and ammonium. For 1986/1987 the weighted averages of pH were respectively 5.20 and 5.26. The normal levels of pH, sulphate. nitrate and ammonium are characteristic of non-polluted atmospheres. Pollution episodes, with...
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... Despite many studies, authors disagree about the extent of cavity ventilation and its effects on hygrothermal conditions in the wall (both in the cavity itself, and as a result, in the material of the wall) [33]. Studies of the effects of a cavity range from positive [34] to negligible [35] and even negative [36]. Many simplified models and indices have been developed but there is poor agreement between these models [37], although in specific cases these models can show good agreement with experimental data [38]. ...
Embodied energy is a measure of the energy used in producing, transporting and assembling the materials for a building. Operational energy is the energy used to moderate the indoor environment to make it functional or comfortable—primarily, to heat or cool the building. For many building geometries, the walls make the most significant contribution to the embodied energy of the building, and they are also the path of greatest heat loss or gain through the fabric, as they often have a greater surface area than the roof or floor. Adding insulation reduces the heat flow through the wall, reducing the energy used during operation, but this adds to the embodied energy. The operational energy is not only a function of the wall buildup, but also depends on the climate, occupancy pattern, and heating strategy, making an optimisation for minimum overall energy use non-trivial. This study presents a comparison of typical wall construction types and heating strategies in a temperate maritime climate. The transient energy ratio method is a means to abstract the heat flow through the walls (operational energy for heating), allowing assessment of the influence of walls in isolation (i.e. in a general sense, without being restricted to particular building geometries). Three retrofit scenarios for a solid wall are considered. At very low U-values, overall energy use can increase as the embodied energy can exceed the operational energy; current best practice walls coupled with low building lifetimes mean that this point may be reached in the near future. Substantial uncertainty is present in existing embodied energy data, and given its contribution to total energy use, this is a topic of urgent concern.
... Conversely, several studies have found that low-permeance exterior insulation can decrease the inward flow of water vapor into the wall assembly from absorptive claddings such as brick veneer (Straube and Burnett 1998) and adhered stone veneer (Smegal and Grin 2015). In walls with these types of claddings (without low-permeance exterior insulation or another feature to decrease the inward vapor flow), an interior polyethylene vapor retarder has been shown to impede the inward vapor flow during summer conditions and to act as a condensing surface, resulting in condensed water running down and accumulating in the bottom plate (Wilkinson and others 2007). ...
Continuous exterior insulation is becoming more common in North America in above-grade exterior walls in both retrofit applications and new construction. It is used to improve the overall thermal performance of wall assemblies. The drying capability of wall assemblies with exterior insulation and an interior vapor retarder in cold climates is not well characterized. The moisture performance of wood-framed wall assemblies with and without exterior insulation was monitored during a 2-year period in the cold climate of Madison, Wisconsin, USA, under low and high interior humidity conditions and with intentional wetting of the wood structural panel sheathing. Moisture content and temperature of standard 38- by 140-mm wood framing and 11-mm-thick oriented strandboard (OSB) sheathing were measured in eight different wall assemblies, each with north and south orientation, in a conditioned test structure. Either a kraft paper or a polyethylene vapor retarder was used on the interior in combination with fiberglass cavity insulation. Exterior insulation was mineral wool, expanded polystyrene, or extruded polystyrene. The OSB sheathing was wetted in a controlled manner at three different times of year to investigate drying response. Wintertime moisture accumulation in OSB under the tested conditions was not a concern except in the wall with no exterior insulation and interior kraft vapor retarder, although rapid drying occurred in springtime. Exterior insulation had a predictable effect on wall cavity temperature. All 16 test walls were able to dry out quickly enough to keep moisture content below dangerous levels when challenged by modest water injection onto the interior OSB surface. The observed decrease in OSB moisture content after controlled wetting events was generally more rapid during warm weather than cold weather, more rapid with exterior insulation than without during cold weather, more rapid with vapor-open exterior insulation than vapor-tight exterior insulation during cold weather, and more rapid with interior kraft vapor retarder than with polyethylene.
... Hence to avoid moisture problems, some drying should be provided. (Straube & Burnett 1998). ...
Drainage is widely accepted as one of the most effective measures for reducing moisture damage due to rain penetration. As a result, a significant proportion of residential and commercial cladding systems employ drainage as a rain control mechanism. Although drainage is effective, some water is always retained in the drainage space, either as droplets on the surface or absorbed and adsorbed to materials lining the drainage space. Previous ASHRAE-sponsored research has shown that ventilation behind the cladding can remove significant amounts of this retained moisture.
One-dimensional hygrothermal simulation is increasingly accepted as a practical and reliable tool for enclosure wall design and analysis. However, one-dimensional hygrothermal simulation cannot directly model the physics of drainage, storage and ventilation behind claddings. This paper investigates the use of enhancements to one dimensional models that might be used to simulate the hygrothermal performance of drained and ventilated wall systems.
The paper will document the experimental methodology, details, and results and discuss how this information can be applied to modeling drained wall systems. Practical applications and research questions arising from the work are presented.
... Although the penetrations were caulked, the mold growth pattern indicated there was some air leakage. Straube and Burnett (1998) on center) filled with low-density batt insulation, a 6-mil (0.15-mm) polyethylene vapor retarder, and painted gypsum board. The combination of interior drywall and polyethylene was tested and confirmed to be airtight. ...
This literature review reports in-service moisture and temperature conditions of floor, wall, and roof members of wood-frame buildings and exposed wood decks and permanent wood foundations. A wide variation exists in reported wood moisture content, spanning a range from as low as 2% to well above 30%. Relevant studies are summarized, and measured values of wood moisture content and temperature
are tabulated. Trends are discussed that relate moisture conditions to climate and season, moisture sources and transport mechanisms, and building design and construction.
... Hence to avoid moisture problems, some drying should be provided. (Straube & Burnett 1998) Depending on the type of storage, several mechanisms are available for drying ( Figure 1). Undrained moisture can be removed from the drainage gap by ventilation or diffusion. ...
A common rain control strategy to help minimize moisture damage is the rain screen wall system. This approach requires a rainscreen, a drainage plane, flashing, weep holes, and a drainage gap. Although this approach to rain control is becoming the most common, and is sometimes even mandated by codes and standards, very little research has been undertaken to define the minimum or optimal gap size required for either drainage or ventilation drying. A test method and experimental program were developed to investigate the gap size required to ensure drainage, and the role of small gaps on ventilation drying. Because of the importance of small geometrical details, full-scale wall systems were tested. The experimental method was able to gravimetrically determine the amount of drainage, storage and drying during and after a simulated wetting event. The test apparatus and method developed were shown to provide repeatable results over multiple tests as well as in an independent laboratory. The experiments to date have conclusively shown that even small gaps (less than 1 mm) can drain more water than would normally be found in a drainage gap. It was also found that in some cases small gaps will store less water than a large drainage gap. It was also found that ventilation drying can play a role in very small gaps of approximately 1 mm, at a pressure difference of only 1 Pa. More research is required to further analyze optimal ventilation gap sizes and compare the laboratory results to hygrothermal modeling.
... Drainage aids drying by removing bulk liquid water-especially from parts of saturated or nonabsorptive materials-and can be a powerful mechanism. However, most damage mechanisms (e.g., corrosion, rot and mold, freeze-thaw) require much less moisture than the maximum that can be removed by drainage (Straube 1998). Hence, if moisture damage is to be avoided, additional drying is not only necessary-it is critical to good moisture performance. ...
... Field measurements of walls by many researchers (Sandin 1993;Straube and Burnett 1998;Wilson 1965) have shown that damaging inward vapor drives do occur in the summer even in "cold" climates. As expected, others have confirmed that they occur in warm climates (Tenwolde and Mei 1985;Tobiasson and Harrington 1989). ...
Low-permeance vapor barriers are widely used on the interior of wall and roof systems in large parts of North America. Many codes and standards imply or even state that low-permeance vapor barriers should be used in all cold regions as well as many moderate climate zones. The influence of vapor barriers on the hygrothermal performance of wall and roof systems is a function of exterior climate, interior climate, solar absorptance, rainwater absorption, and the vapor and thermal resistance of all of the layers in the system. In many practical situations, a low-permeance vapor barrier will not improve hygrothermal performance and may in fact increase the likelihood of damaging condensation or trap moisture in the system. This paper will examine the role of vapor barriers on hygrothermal performance with the aid of simple and transparent diffu-sion calculations supported by measurements from full-scale natural exposure monitoring. The phenomenon of summertime condensation, the drying of roofs and walls, and multiple vapor barrier layers will be explored. The importance of properly assess-ing both the interior and exterior climate will be discussed. Vapor diffusion control strategies will be presented.
... Ventilation, or air flow through a space behind the cladding, uses the drier outdoor air to transport water vapour out of the wall. A recent study [2] suggests that ventilation drying can be useful, but more research is required to quantify its benefits. ...
... Lacy's simple model of driving rain in the free wind can now be extended to predict driving rain deposition on the vertical face of a building using: r vb = RAF · DRF(r h ) · cos ) ·V(h) · r h = RAF· cos ) · r v (2) where r bv is the rain deposition rate on a vertical building surface (l/m 2 /h), ...
Building enclosure rain penetration control in a holistic framework of wetting storage and drying. The moisture balance introduced.
... Drainage aids drying by removing bulk liquid water-especially from parts of saturated or nonabsorptive materials-and can be a powerful mechanism. However, most damage mechanisms (e.g., corrosion, rot and mold, freeze-thaw) require much less moisture than the maximum that can be removed by drainage (Straube 1998). Hence, if moisture damage is to be avoided, additional drying is not only necessary-it is critical to good moisture performance. ...
... Field measurements of walls by many researchers (Sandin 1993;Straube and Burnett 1998;Wilson 1965) have shown that damaging inward vapor drives do occur in the summer even in "cold" climates. As expected, others have confirmed that they occur in warm climates (Tenwolde and Mei 1985;Tobiasson and Harrington 1989). ...
Low-permeance vapor barriers are widely used on the interior of wall and roof systems in large parts of North America. Many codes and standards imply or even state that low-permeance vapor barriers should be used in all cold regions as well as many moderate climate zones. The influence of vapor barriers on the hygrothermal performance of wall and roof systems is a function of exterior climate, interior climate, solar absorptance, rainwater absorption, and the vapor and thermal resistance of all of the layers in the system. In many practical situations, a low-permeance vapor barrier will not improve hygrothermal performance and may in fact increase the likelihood of damaging condensation or trap moisture in the system. This paper will examine the role of vapor barriers on hygrothermal performance with the aid of simple and transparent diffu- sion calculations supported by measurements from full-scale natural exposure monitoring. The phenomenon of summertime condensation, the drying of roofs and walls, and multiple vapor barrier layers will be explored. The importance of properly assess- ing both the interior and exterior climate will be discussed. Vapor diffusion control strategies will be presented.
Review of wind washing and convection of exterior insulation, both fibrous and rigid
The moisture performance of building envelope systems are strongly dependent on the materials used, the workmanship, and the exposure loads from the interior and exterior environments. The authors have long recognized the need to include the effects of exterior cladding ventilation in the predictive capability of software tools used for hygrothermal analysis. Exterior cladding ventilation has been studied, but no conclusive recommendations have been generated until recently (Burnett, E., Straube, J., and Karagiozis, A., “Synthesis Report and Guidelines,” ASHRAE TRP-1091 Report No. 12, Nov. 2004). While the physics describing the thermal and moisture transport in the presence of air convection is understood, the pressure dynamics is still somewhat qualitatively known. With the addition of new literature data and available field generated monitored data, a simplified model for the wall air cavity ventilation was developed. The scientific approach followed initially included the benchmarking of multi-dimensional advanced hygrothermal model with laboratory and field data. The flow was understood for a wide range of exterior loadings, and once this was completed, an attempt to reduce the complex three-dimensional air flow characteristics into a simple one-dimensional analogue was made. The paper describes how this important feature was included into the WUFI-4.1 software. The paper also describes how users may employ this feature in hygrothermal designs to investigate the advantages and disadvantages of cavity ventilation. Results are also presented on the hygrothermal performance of two walls, one ventilated and the other is unvented. Results show that major differences were predicted and the wall with the ventilation cavity dried out nearly five times faster than the wall without the ventilation. Field monitored stucco wall systems with and without cavity ventilation are also included compared to the prediction provided by the hygrothermal model. Good agreement is shown between the field and WUFI 4.1 model.
