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The Importance of Wind Barriers for Insulated Timber Frame Constructions
Abstract
The goal of this research project was to get more information about the influence of wind pressure on the heat transmission through timber frame con structions and to establish a recommended limit for air permeance of wind barriers.* The project was divided into three parts: wind pressure measurements on a rotatable test house, hot-box measurements on a wall, and calculations. The theoretical studies as well as the experimental investigations in the hot-box, have been restricted to one specific type of forced convection in the thermal insulation, the interchange of air be tween the insulation and the air gap between the wind barrier and the outer cladding. The results of the project show the importance of protecting the insulation layer with a wind barrier to achieve full effect of the insulation in wind exposed constructions. The measurements indicate that heat loss caused by this type of forced convection can be three to ten times higher than calculated for ideal constructions. Based on the measurements carried out in this project, Norwegian Building Research Institute, NBI, is recommending an upper limit for the air permeance of wind barriers, includ ing joints, of 0.05 m3/m2 h Pa (1.4E-5 m3/m 2 s Pa).
... This bulk movement of air increases heat loss, resulting in increased energy consumption, risk of condensation on cooled surfaces, and increased space-heating loads. Previous research has shown that wind washing of air-permeable insulations is most significant for installations with • higher air velocities along the insulation surface due to higher wind exposure and/or more open cladding systems (Bankvall 1978;Henning 1983;Taylor and Phillips 1983;Uvsløkk 1996); ...
... Figure 1 illustrates the process of wind washing, and Figure 2 illustrates two measures to limit it: air cavity compart-mentalization and the application of a so-called "wind barrier" on the exterior of the insulation. Uvsløkk (1996) recommended a wind barrier air permeance no greater than 1.4E-5 m 3 /m 2 ·s·Pa, based on laboratory tests and field monitoring of a small test house. Mineral wool insulation boards used as sheathing have air permeances greater than Uvsløkk's recommendation, hence some designers choose to place wind barriers over top of them. ...
... psf/ft), respectively. Uvsløkk (1996) measured cavity pressure gradients of 0.1-0.5 Pa/m (0.001-0.003 pcf/ft) for mean wind speeds of 3 m/s (6.7 mph) in a 23 mm (~1 in.) clear air gap behind wood siding of a small test house. To determine cavity pressure coefficients for vinyl siding, the cavity pressure data, which were also available in Cope et al.'s (2012) data set, were analyzed. ...
Wind washing is wind-driven air movement through or behind thermal insulation within enclosures. This bulk movement of air increases heat loss, resulting in increased energy consumption, risk of condensation on cooled surfaces, and increased space-heating loads. Previous studies have investigated airflow bypassing the insulation through gaps, around insulation boards, and through low-density fibrous insulation products. The impact of wind washing on well-installed mineral wool board products used outside the air barrier as continuous insulation has not been previously studied in detail. This paper presents a methodology for predicting wind washing impacts and laboratory measurement of heat loss impacts for insulation products exposed to cavity airflows. It is recommended that average winter wind speeds (or ASHRAE mean coincident wind speed) be used for wind washing analysis. A method is presented for adjusting wind speeds for sheltering and height impacts based on approaches used in structural wind load calculations. Hourly average air speeds as high as 0.1 and 0.7 m/s (0.4 and 2.4 ft/s) were predicted using simple airflow network analysis for well-ventilated brick cladding and slot panels, respectively. Analysis of full-scale wind tunnel pressure tap data for a small house was used to predict vinyl siding cavity air speeds as high as 0.5 m/s (1.5 ft/s). Heat flow measurements were taken for a number of insulation products in a purpose-built apparatus. Of the mineral wool board samples tested, only the 25 and 50 mm (1 and 2 in.) thick 70 kg/m 3 (4.4 pcf) samples showed measurable wind washing impact. This impact was as much as 0.03 RSI (R-0.2) reduction of thermal performance. Hence, wind washing impacts are expected to be small for well-installed mineral wool board continuous insulation.
... were reported by Deseyve and Bednar [5]. Most striking in both studies are not the obtained results as suchsimilar results have been published in the 1990's [6] -, but the fact that the pitched roof constructions tested in these laboratory tests are considered as standard building practice. Deseyve and Bednar [5], for example, studied pitched roofs with low density glass wool (14 kg/m³) with air layers around this insulation of 2 cm. ...
... Austria. Yet this was definitely not in agreement with design recommendations documented in previous research [6][7][8]. The same holds for the BBA-study [4] in which an underlay with two unsealed laps was used. ...
... Already in 1989, Powell et al. [9] published a literature survey on the influence of air movement on the effective thermal resistance of porous insulations. Ever since, several studies investigated the effect of different air movement patterns on the thermal performance and durability of (mainly lightweight) building components [2,6,7,8]. Several of those studies, resulted in guidelines and performance requirements of the composing material layers [10][11][12]. ...
This paper reviews previous research work on the effect of air movement on the heat transmission losses in light weight building components in general and pitched roofs in particular. Air movement in pitched roofs is typically a mixture of three phenomena: (1) in/exfiltration, (2) natural convection and (3) wind-washing. While the importance of air barrier systems to guarantee the thermal and hygric performance by the exclusion of exfiltration is widely acknowledged today, less emphasis is put on the necessity of a continuous wind barrier to prevent wind-washing of the insulation layer. Also the effects of small air leakages around the insulation layer which can trigger natural convection is currently underestimated in practice. Based on findings in the literature, guidelines are put forward on how to construct a pitched roof with a robust high thermal performance.
... Similar differences between calculated and measured U-value in laboratory conditions were reported by Deseyve and Bednar [5]. Most striking in both studies are not the obtained results as suchsimilar results have been published in the 1990's [6] -, but the fact that the pitched roof constructions tested in these laboratory tests are considered as standard building practice. Deseyve and Bednar [5], for example, studied pitched roofs with low density glass wool (14 kg/m³) with air layers around this insulation of 2 cm. ...
... They claim this is according to the current building practice in Austria. Yet this was definitely not in agreement with design recommendations documented in previous research [6][7][8]. The same holds for the BBA-study [4] in which an underlay with two unsealed laps was used. ...
... Already in 1989, Powell et al. [9] published a literature survey on the influence of air movement on the effective thermal resistance of porous insulations. Ever since, several studies investigated the effect of different air movement patterns on the thermal performance and durability of (mainly lightweight) building components [2,6,7,8]. Several of those studies, resulted in guidelines and performance requirements of the composing material layers [10][11][12]. However, it seems that these (fragmented) recommendations did not lead to a roof construction resilient to air flow effects in day to day building practice, and this notwithstanding the rapid development in technology on improved air and wind barrier systems 1 . ...
The influence of the emissivity of a roof underlay on the global thermal behaviour of sloped roofs is investigated. Five well-insulated pitched roofs have been constructed in a test building. The five roofs have a south-west and north-east-oriented pitch and differ in long wave emissivity of the underlay. All roofs are equipped with thermocouples and heat fluxes sensors to evaluate the thermal response of the roofs to the climatic conditions. Both summer and winter conditions have been measured. In addition to the in situ evaluation, a laboratory experiment was set-up to evaluate the influence of the emissivity of the underlay on the summer behaviour of a sloped roof under fixed boundary conditions. With thermocouples and heat flux sensors at different heights in the roof the effect of the reflective foil on the heat gain to the inside could be evaluated. The measured data are compared with a simple numerical model that accounts for the buoyancy effects in the ventilated cavity between tiles and underlay. Laboratory experiments and simulations revealed that a low emissivity of the underlay decreases the heat gain to the indoor environment, but that due to the thermal stack flow in the air cavity underneath the tiles, the advantage of a reflective foil mainly plays a role in the bottom part of the roof. In the in situ measurements it was found that workmanship, airtightness and wind and thermal stack effects are much more important and disturb the possible benefits of using a reflective underlay.
... Measurements also showed that a 40 mm wide gap provides 20-25 times higher drying potential compared to a 5 mm wide gap. In [40], the effect of wind on air ...
... Measurements also showed that a 40 mm wide gap provides 20-25 times higher drying potential compared to a 5 mm wide gap. In [40], the effect of wind on air pressure in the air gap was considered. Field experiments, laboratory experiments, and calculations were carried out. ...
Precise meanings of thermophysical processes taking place in air gaps have decisive importance in composite cladding structure systems’ calculation and modeling. The climatic load conditions in Kazakhstan can significantly affect the microclimate of premises in general. In this work, a review study is carried out to obtain the relevant scientific literature on enclosing structures with air gaps under various climatic conditions. The review mainly covers research institutes from Sweden, Norway, France, Saudi Arabia, Russia, and China. On the issue of the air gap parameter’s influence on thermophysical processes, 16 papers were analyzed, and on the issue of air infiltration, 12 papers were analyzed. However, the review shows a lack of research in this area under various climatic conditions. At the same time, experience has shown that the principle of multilayer protection from climatic influences creates a favorable microclimate in buildings, but due to a possible temperature drop, wall structures made of composite building materials can be quite favorable under some conditions, and under others they may be less favorable. Therefore, working out a new energy-saving design with air gaps for climatic conditions with large temperature fluctuations during summer and winter is an urgent task.
... Field measurements are not typically used to determine peak net loads on air-permeable multilayer cladding for design. Rather, these studies tend to use differential pressure measurements through cladding to calculate ventilation for moisture and frost accumulation studies (e.g., Uvslokk, 1996;Straube, 1999). These types of studies are not used for design wind loads since they are not concerned with capturing high wind conditions. ...
Air-permeable multilayer cladding (vinyl siding, roof pavers, discontinuous metal roofing, solar panels, etc.) are one of the most common types of building components in North America. Their defining aerodynamic feature is that they have an air cavity separating the component from the sheathing, studs, or interior layer. Due to air-permeability, external wind loads can transfer into the air cavity between the layers. Although these cladding systems have similar geometries in many ways, design loads are not generally available for such systems. This study aims to synthesize the available literature on the pressure equalization factor, which is the proportion of external load acting on the cladding and provide a framework for design wind loads on air-permeable multilayer cladding systems. To accomplish this, the many factors that affect the pressure equalization factor, such as the gap-to-cavity-depth ratio, panel size, and exposure are discussed. Then, the pressure equalization factors from multiple studies are combined to examine the effect of effective area on the pressure equalization factor. Finally, recommendations for implementing these guidelines into design standards are provided.
... The parameters studied in this research have already been applied in existing research based on laboratory measurements or numerical simulations. Depending on their individual research purposes, the considered parameters for wood-frame enclosures can be different, including the selection and placement of insulation materials (Geving et al., 2015;Jelle et al., 2013;Jerman et al., 2019;Latif et al., 2014;Levin and Gudmundsson, 1999), airtightness and air control layer (Bunkholt et al., 2021;Langmans et al., 2010Langmans et al., , 2012Uvsløkk, 1996), vapor permeance and vapor control layer (Geving and Holme, 2013;Langmans et al., 2012;Lee et al., 2020;Vinha, 2007), the impact of wind driving rain on the building envelope (Lee et al., 2020), and the interaction with indoor climate conditions (Alev and Kalamees, 2016). ...
According to existing measurements and simulation results, the indoor thermal comfort in traditional wooden buildings (still remaining in a large amount) in the Chinese Hot-Summer-Cold-Winter zone is very poor in winter. However, few studies can be found regarding the energy retrofitting of their wooden enclosures, which is increasingly regarded as essential for improving indoor thermal comfort and maintaining built heritage. Therefore, this study demonstrates a method based on parametric study applying the widely validated WUFI ® Plus software to help design hygrothermally functional insulation systems for this area. The parametric study was conducted on the example of traditional exterior wooden walls in Tongren in southern China. Five parameters were investigated, including internal and external insulation systems, vapor-open (mineral wool) and vapor-tight (XPS) insulation materials, a U-value of 0.8 W/(m ² K) as well as a lower U-value of 0.24 W/(m ² K) for the insulated walls, different capabilities and positions of an additional vapor control layer, as well as different cooling/dehumidification conditions in the warm period of a year. It has been found in this study that, if possible, a lower U-value than the current Chinese design standard for energy efficiency of buildings (0.8 W/(m ² K) should be preferred for energy retrofitting. This can limit the yearly duration of a high internal surface relative humidity over 80% shorter than 30 days without any dehumidification devices. Besides, this study provides some feasible wall configurations with instructions on their limitations to guide future work regarding the design of insulated building components and the operation of renovated traditional wooden buildings.
... Uvsløkk [41] presented information on how wind affects the air pressure in an air cavity. Field experiments, laboratory experiments, and calculations were conducted. ...
Accurate values for the climatic conditions in an air cavity, hereby called the microclimate, are crucial when calculating and simulating the performance of a ventilated roof and façade system. The climatic stress of its components and their mould and rot potential influence the long-term durability of the roof or façade. A scoping study is conducted to gain an overview on research and the scientific literature on the microclimate of air cavities in ventilated roofing and claddings in Nordic climates. From the body of the research literature, 21 scientific works were of particular interest, and their findings are summarized. The review shows that only a limited number of studies discuss the microclimate of air cavities. Roofs are discussed to a greater and more varied degree compared to façades and air cavities behind solar panels. However, the results cannot be compared and validated against each other to generally describe the microclimate of air cavities, as the surveyed papers approach the subject differently. This knowledge gap indicates that calculations and simulations can be performed without knowing whether the results represent reality. If the structure of ventilated roof and façade systems are only designed based on experience, it can be difficult to be proactive and adapt to future climate changes. Further studies are needed to determine the relation between the exterior climate and the air cavity microclimate, so that future climate predictions can be used to simulate the long-term performance of ventilated roof and façade systems.
... In their research and measurements, Timusk et al. [26] and Uvsløkk [27] demonstrated that there is a clear impact of the wind on the increase of the heat loss through the walls, especially in the corners. Wind washing is a serious problem in the corners of the building because the pressure gradient force of the wind, which drives this phenomenon, is considerable in these areas [28]. ...
The paper presents the results of research concerning three fiber materials-mineral wool, hemp fiber and wood wool-as loose-fill thermal insulation materials. The analysis used the material parameters determined in previous works conducted by the authors, such as thermal conductivity and air permeability in relation to bulk density. These materials exhibit open porosity; thus, convection is an essential phenomenon in the heat transfer process. The paper aimed at conducting thermal simulations of various frame wall variants which were filled with the above-mentioned insulation materials. The simulations were performed with the Control Volume Method using the Delphin 5.8 software. The studies accounted for the effect of wind pressure and the time of its influence on a wall insulated by means of fiber material with a thickness of 150 as well as 250 mm. The simulation enabled us to obtain such data as maximal R-value reduction and time to return to equilibrium after filtration for the analyzed materials. The study proved that heat transfer in these insulations strongly depends on the bulk density, thickness of the insulation and wind pressure. The decrease in R is reduced as the density increases. This results from the decreased air permeability characterizing the material. Wind washing causes lower R reduction than air filtration in all models. The greater the thickness, the longer it takes for the models to return to the equilibrium state following air filtration (and wind washing). This period is comparable for air filtration and wind washing. Hemp fibers were characterized with the strongest susceptibility to air filtration; in the case of wood wool, it was also high, but lower than for hemp fibers, while mineral wool was characterized with the lowest.
... However, given the stricter requirements regarding airtightness of buildings, recent field and laboratory measurements are questioning if these solutions can guarantee the airtightness of a building. A promising solution seems to be the application of self-adhesive tapes [11][12][13]. ...
The harsh Norwegian climate requires buildings designed to high standards. An airtight building envelope is crucial to achieve an energy efficient building and to avoid moisture problems. Results from the SINTEF Building defects archive show that a considerable part of the building defects is related to air leakages. In addition, air leakages increase the energy demand of buildings. A literature study has been conducted in order to map typical air leakage paths of Norwegian wooden houses. In order to increase the performance, different sealing methods including the use of tape has been reviewed. The results show that the most common air leakages reported from field measurements in the literature are in the connections between external wall and ceiling or floor, external wall and window or door, and external wall and penetrations in the barrier layers. Results from laboratory investigations showed that the traditional solutions can be further improved by introduction of modern foil materials in combination with sealing tapes. However, questions can be raised regarding the necessity of tape sealing all available joints.
... As stated by Van Straaten et al. (2016) and Straube and Smegal (2018), if the airflow velocity rises too high, or if the material properties or assembly are improper, the airflow can pass through or behind the insulation and result in increased heat loss than assumed in design due to forced convection in the thermal insulation. The measurements undertaken by Uvsløkk (1996) on full scale models of timber walls, indicate that heat loss caused by wind washing can be three to ten times higher than calculated for ideal constructions. Deseyve and Bednar (2008) have studied the impact of air flow through the ventilation gap on heat transfer in a roof insulated with mineral wool. ...
The article concerns the wind washing of the cavity insulation in leaky walls. The measurements in lightweight timber frame buildings have shown a big contribution of air filtration on heat transfer through envelope. This article presents results of the laboratory study on wind washing of structures insulated with loose mineral wool with partial and without wind protection (corresponding to met existing ones) under different wind load. Wind washing leads to 85% reduction of thermal resistance of examined structures. Long-term measurements show also the problems of subsidence of loose fibrous material in the construction which leads to increased convection heat transfer.
... More than a decade after this work, Uvslokk (1996) presented further detailed field measurements of wind washing for similar wall construction: ventilated cladding, wood framing, interior air barrier and leaky or non-existent exterior sheathing. The research quantified the amount of airtightness required to protect cavity insulation, and reported on extensive field pressure measurements within ventilated gaps behind claddings. ...
Review of wind washing and convection of exterior insulation, both fibrous and rigid
... The construction is of special interest regarding the dry-out capacity in roofs with load-bearing systems of wood. When built in line with the guidelines given by Roels and Langmans (2016), Uvsløkk (1996), and Edvardsen and Ramstad (2014), ventilated pitched wooden roofs can be considered a robust roof design. However, there are well-known degradation issues related to snow melt and mould growth. ...
Pitched roofs with a ventilated air cavity to avoid snow melt and ensure dry conditions beneath the roofing are a widely used construction in northern parts of Europe and America. The purpose of this study has been to determine pressure losses at the inlet (eaves) and inside the air cavity consisting of friction losses and passing of tile battens. These results are necessary to increase the accuracy of ventilation calculations of pitched roofs. Laboratory measurements, numerical analysis as well as calculations by use of empirical expressions have been used in the study. A large difference in the local loss coefficients depending on the edge design and height of the tile batten was found. The local loss coefficients of the round-edged tile battens were approximately 40% lower than the local loss coefficients of the sharp-edged tile battens. Furthermore, the local loss factor increased by increasing height of the tile batten. The numerical analysis was found to reliably reproduce the results from the measurements.
... However, the few findings on the impact of wind flow on heat losses reveal a strong influence. The effect of wind-washing on exterior walls, respectively wall corners, was studied first by Timusk et al. [2] and Uvsløkk [3] in laboratory tests. They point out the importance of windprotection of the cavity. ...
For a reduction of heat energy losses much effort has been undertaken during the last years to improve insulation, heating and ventilation systems. Despite, wind-tightness is often disregarded, even though the importance of heat losses through wind-washing has already been shown by research findings. Especially the realization of the wind-tightness at the intersection of wall and rafters proves to be challenging due to difficult geometrics at this position. The presented study clarifies the influence of different wind-tightness qualities at the wall-roof intersection on the heat flow through a rafter roof. For that purpose laboratory tests were carried out. The gained data shows that the insulation material has a very strong effect on the increased heat flow through the roof caused by wind-washing. The results also point out that a certain duration and value of the pressure difference (wind load) between eaves and attic is necessary to cause an effect. The influence of the pressure difference is more pronounced compared to the influence of the gap geometry at the wall-roof intersections. While the projecting roof and the fixation of the roof underlay have a very strong effect, the inclination of the roof has no significant influence on the additional heat flow due to low wind-tightness.
... The measured air-tightness of a construction assembly is not comparable with the envelope leakage measured under the field conditions as it does not include joints and 3 This term is used in Scandinavia for membranes used to provide external protection from air ingress; in the United States the term WRB represents the class of materials that accomplish this function. (Uvslokk, 1996). ...
... In cold and moderate climates, such as North-West European areas, this air barrier function is often combined with that of the 'vapour retarder'. In order to protect the insulation layer from infiltration of outside cold air, a 'wind barrier' is often provided outside the insulation (Uvslokk 1996). In addition, this exterior layer serves as drainage plane to prevent water infiltration into the structure. ...
Algae are very ancient living organisms. Their presence on earth came about some 3.5 billion years ago. They are considered “pioneer organisms” of outdoor environment, and it is actually possible to find different varieties of algae on the ground, in the air, in ice and even in anthropogenic elements such as the facades of buildings since they are able to survive through frequent freeze–thaw and dehydration cycles. The aesthetic quality and durability of an external building envelope could be seriously impaired by the development of algae which will colonise building materials whenever a suitable combination of humidity, warmth and light occurs. The fundamental role of water for algal growth is clear which, for several reasons, is found in large quantities on building facades. External sources of water here include rain, snow, ground moisture, airborne humidity and condensation of vapour from outdoor air. In addition to environmental conditions, the rate of stain development largely depends on the “bioreceptivity” of the material, that is, its aptitude to be biologically colonised which is related to the material properties that contribute to the anchorage and development of microorganisms. The facades of the buildings are then fertile substrates for the growth of algae.
... This impacts upon thermal comfort and energy demand. Uvsløkk [10] examined the impact of wind tightness upon thermal performance and proposed a wind tightness standard whereby the infiltration of external air should be limited so that the in situ U-value does not exceed the notional U-value by more than 5%. ...
The Passivhaus (or Passive House) Standard is one of the world's most widely known voluntary energy performance standards. For a dwelling to achieve the Standard and be granted Certification, the building fabric requires careful design and detailing, high levels of thermal insulation, building airtightness, close site supervision and careful workmanship. However, achieving Passivhaus Certification is not a guarantee that the thermal performance of the building fabric as designed will actually be achieved in situ. This paper presents the results obtained from measuring the in situ whole building heat loss coefficient (HLC) of a small number of Certified Passivhaus case study dwellings. They are located on different sites and constructed using different technologies in the UK. Despite the small and non-random nature of the dwelling sample, the results obtained from the in situ measurements revealed that the thermal performance of the building fabric, for all of the dwellings, performed very close to the design predictions. This suggests that in terms of the thermal performance of the building fabric, Passivhaus does exactly what it says on the tin.
... In addition, this exterior layer often serves as drainage plane to prevent water infiltration into the structure. The performance criteria for wind barrier systems regarding air permeance are less severe than for air barriers (Uvsløkk 1996). Therefore, the joints in the wind barrier are usually left unsealed. ...
... In cold and moderate climates, such as North-West European areas, this air barrier function is often combined with that of the 'vapour retarder'. In order to protect the insulation layer from infiltration of outside cold air, a 'wind barrier' is often provided outside the insulation (Uvslokk 1996). In addition, this exterior layer serves as drainage plane to prevent water infiltration into the structure. ...
... In cold and moderate climates, such as North-West European areas, this air barrier function is often combined with that of the 'vapour retarder' [8]. To protect the insulation layer from unwanted infiltration of outside cold air by natural or forced convection a 'wind barrier' is provided at the outside of the insulation [9]. In addition, this exterior layer serves as drainage plane to prevent water infiltration into the structure. ...
... The measured air-tightness of a construction assembly is not comparable with the envelope leakage measured under the field conditions as it does not include joints and 3 This term is used in Scandinavia for membranes used to provide external protection from air ingress; in the United States the term WRB represents the class of materials that accomplish this function. (Uvslokk, 1996). ...
As a result of increased concern with energy consumption in the industrial world, it is only natural to look towards the building sector to seek significant improvements to meet expectations of the society. After all, the building sector consumes more energy than the transportation sector. Yet, the procedures that are used to define the thermal performance of, for example a wall, are typically based on the tests performed on dry materials, without consideration of air and moisture movements. In other words, these tests represent arbitrary rating conditions because we know that the energy performance of materials and building assemblies are affected by moisture and air flows. It is believed that to improve their energy performance one must have more precise means of evaluation of their field performance that would also include the consideration of air and moisture transfer conditions. In the first part of this article a background for the evaluation of thermal performance by traditional testing with calibrated boxes shows that use of these tests is limited. The average heat flow that they measure is sufficient to rate the wall assemblies but insufficient to calculate its thermal performance under field conditions. To include the effect of climate on thermal performance one must use computer models that are capable of simultaneous calculations of heat, air, and moisture transfer. Effectively, to characterize energy performance of the building enclosure one must simultaneously use assembly testing and modeling, i.e., an integrated methodology. In the second part of the article, this integrated testing and modeling methodology is applied to a few selected residential and commercial walls to highlight the magnitude of air flow effects on the steady-state thermal resistance. The integrated methodology proposed by Syracuse University includes several other aspects of hygrothermal performance evaluations. Those aspects will be addressed in later parts of this article series.
... At 75 Pa, the airtightness of the house is predicted to be 1.4 L/(s m 2 ), which is significantly greater than the recommended value in Canada. Nevertheless, the airtightness is in the range of 1 to 3 Â 10 À5 m 3 /(s m 2 Pa) (0.5 to 1.5 L/(s m 2 ) at 50 Pa) as recommended by Uvslokk (1996) and Ojanen (1993). ...
Vapor-permeable building envelopes have received renewed interest because they can moderate indoor humidity levels and improve the drying of the envelope during summer condensation conditions. In this paper, the moisture performance of a vapor-permeable building envelope is presented with field measurements and numerical simulations. The results show that the diffusion resistance of the internal surface should be greater than that of the external surface (typically recommended ratio of 3: 1 or 5: 1), but that the vapor resistance of the vapor retarder can be significantly below that provided by polyethylene and still result in a safe structure, even in a cold climate.
... Field measurements of well-vented wall systems (i.e., vent areas of more than 1% of wall area) show that such systems typically experience flow velocities of 0.05 to 0.2 m/s (Jung 1985, Popp et al. 1980, and Kuenzel et al. 1983). Schwarz (1973) and Uvsløkk (1988) both found higher average velocities behind well-vented cladding panels with continuous slotted vents. ...
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 approaches that rely solely on drainage to remove moisture from behind the outer layers or cladding ignore the significant quantities of moisture that can be stored in the outer layers of most enclosure walls. Most cladding systems have relatively low vapor permeability and therefore tend to restrict diffusive drying. Moisture trapped in or behind the cladding can be transported into the enclosure by solar-driven diffusion, especially in air-condition ed buildings. Rather than control vapor diffusion, a 6 mille vapor retarder close to the interior may, in many instances, exacerbate wetting and greatly retard drying.
... Providing that an air barrier system is in place, a semi-permeable water vapor barrier should remain as the primary choice for construction in most of the North American climates. Research from Finland (Ojnanen 1993, Uvslokk, 1996, Simonson et al 2004, 2004a, 2004b) and Canada (Karagiosis and Kumaran, 1993; Ojnanen & Kumaran, 1996) supports this choice. Currently, a Class 1 vapor barrier is typically used. ...
Recent failures of exterior stucco on walls were discussed in the first part of this paper. After assessing those problems, the authors postulate that providing stucco with a capillary breaking layer combined with exterior thermal insulation and improved ability of the wall to dry both outwards and inwards will restore the excellent track record for which exterior stucco walls were known forcenturies. This 3E wall system is designed for energy efficiency, environmental control, and ecological responsibility. The authors highlight that exterior stucco should be applied on exterior insulation in both hot and cold climates. The system should incorporate water resistive barriers (WRB) and cellulose fiber insulation in the framing cavities. Such a wall can be one of the most economical and ecologically-justified systems that will perform well in most climates. However, such a wall should also be designed with a different paradigm for heat, air and moisture control of the building envelope (BE). A new paradigm should be based on moisture balance in relation to climatic and service conditions. In this context, modern lime-cement rendering placed on thermal insulation and provided with proper architectural detailing may successfully compete with all other cladding systems. * © Copyright NLA Building Lime Group 2005 The views presented in this paper are solely those of the authors. The National Lime Association (NLA) and the Building Lime Group assume no liability or responsibility for any errors, omissions, or other limitations in this paper or for any products, services, or methods presented. This paper is intended for use by professional personnel competent to evaluate the significance and limitations of the information provided and who will accept full responsibility for the application of this information. NLA and the Building Lime Group do not intend to infringe on any patent or other intellectual property right or induce any other party to do so, and thus users of this document are responsible for determining whether any method, technique, or technology described herein is protected by patent or other legal restriction.
... Estimated increase in heat transmission through the wood frame wall in percent, for different wind barriers (see text) applied to a frame with 150 mm thick MFI (Uvslokk, 1996 ...
In the study, according to the data of the national hydrometeorological service Kazhydromet, over the past 30 years, engineering and climatic calculations of Shymkent have been carried out in the context of annual, monthly and daily values, where the main purpose according to the data obtained was to determine favorable, unfavorable, permissible and unacceptable orientations, which were calculated based on the values of solar radiation and wind regime of the specified territory. As a result of the engineering and climatic calculation, a final comprehensive assessment of the climate analysis was compiled, where the south-eastern direction was set as the sector of favorable orientation for Shymkent between 140–200°, the sector of unacceptable orientation was set to the northern direction between 320–40°, the sector of permissible orientation was set to the north-western direction between 270–320°, the sector of unfavorable orientation was set to the south-western The direction is between 200–270°, and the optimal orientation is set to the east direction between 40–140°. It is noted that the obtained results of this study are relevant and can be used further in the study of the heat transfer process in external wall enclosing structures, taking into account the influence of solar radiation in the hot climate of the Republic of Kazakhstan.
In recent building practice, obligations to legislation on energy saving are carried out mainly by a high thermal resistance and a global airtightness of the envelope, aiming to minimise heat dispersions by conduction and infiltration as much as possible. These measures determine new ways of heat and moisture exchange in the building envelope and are likely to exacerbate the growth of microorganisms. New poorly permeable buildings are in fact more subject to high internal moisture load, in combination with an unsuitable ventilation strategy. Modern exterior insulation finish systems do not have much thermal inertia and are more subject to undercooling phenomena, condensation and a consequent higher biological growth risk. Renovation techniques, such as the replacement of single glazed windows by new very tight double or triple glazed windows or the addition of interior insulation, induce condensation phenomena on the unavoidable thermal bridges (frames, subframes, structure). The NZEB of the future must be able to give a concrete answer to these problems, since, although no changes occur in the thermal performance of the buildings, biological defacement has an enormous aesthetic, health and economic impact, which gathers the disapproval of building’s dwellers. This chapter will explore these topics, by describing the major consequences of the ‘sealing action’ and ‘overinsulation’ on the proliferation of microorganisms in NZEB.
The research presented in this report demonstrates the moisture, thermal and ventilation performance of a recently built ecological house in the Tapanila district of Helsinki, Finland. The single-family house (gross floor area of 237 m2 including the basement and porch) has a well-insulated (250 mm in the walls and 425 mm in the roof) wooden frame with no plastic vapour retarder. A natural ventilation system provides outdoor ventilation and district heating and a wood-burning fireplace provide space heating. The space heating energy consumption was measured to be 76 kWh/(m2·a) of which 29% was provided by wood. For comparison, Finnish houses typically consume 120 kWh/(m2·a) or nearly 60% more energy for space heating. If the building envelope of Tapanila ecological house had been insulated according to the building code, the space heating energy consumption is expected to be 40% higher. The total energy consumption (121 kWh/(m2·a)) and electricity consumption (28 kWh/(m2·a)) were quite low. As a result, the total primary energy consumption was only 162 kWh/(m2·a), while the primary energy consumption in typical Finnish houses is over 40% higher. However, the outdoor ventilation rate provided by the natural ventilation system tended to be lacking (i.e., less than the required value of 0.5 ach) even though the measured CO2 concentrations were generally below 1000 ppm when the bedroom doors were open. Extrapolating the measured ventilation data shows that the ventilation rate is expected to be about 0.45 ach (10% below the required value) in the winter and about 0.25 ach (50% of required value) in the summer when the windows are closed. When the windows are open in the summer, the outdoor ventilation rate will be higher. The moisture performance of the building envelope was good and the risk of mould growth low. In addition, the moisture transfer between the envelope and indoor air was measured to significantly influence the indoor humidity. At a ventilation rate of 0.5 ach, the results show that a porous building envelope can decrease the maximum humidity in a bedroom during the night by up to 20% RH, which may double the number of occupants satisfied with thermal comfort and perceived air quality. Furthermore, the minimum indoor humidity in the winter can be increased by about 10% RH, which is also important in cold climates. These results show that it is possible to build a house with a porous and vapour permeable envelope that is moisture physically safe and improves the indoor climate.
Moisture-induced damage is one of the major causes of degradations and reduced thermal performance in wood frame buildings. It is therefore crucial to incorporate the hygrothermal assessment of new timber frame building envelopes systems from the early development phase onwards. The article at hand presents the simulation results studying the hygrothermal performance of various timber frame wall configurations with exterior air barrier systems. A parameter analysis explores the impact of different European climates, insulation materials, exterior air barrier materials and verifies in addition to the impact of bad workmanship in the installation of the insulation layer. This study reveals that the application of mineral wool (MW) insulated timber frame walls in combination with exterior air barriers results in increased moisture loads. Moreover, small air gap channels between the MW and the adjacent exterior air barrier significantly increase natural convection and add up to harmful moisture levels. Yet the simulations indicate that the use of blown-in cellulose insulation can avoid these issues. The study further indicates that the technique of exterior air barrier is more suitable for continental climates rather than for moderate sea climates in Europe.
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.
The air permeability represents that feature of the building playing a major role in both the building energy performance and the indoor environment quality, therefore its prediction is very important. The statistical prediction models which are used today on a very large scale present large errors. The experimental measurements correct this deficit, but they are impossible to be carried out for large apartment building due to technical concerns. In this study we propose an intermediate approach “the prediction of average permeability as a weighted mean of the different measured permeabilities characteristic to the different types of joinery”. The article presents the mathematical models and the adapted experimental protocol for four different parameters that describes the permeability. The experimental work was carried out for an apartment placed at the ground level of a two storey house in Romania. The proposed approach presents smaller errors: 5% for the overall leakage airflow and 15% for the average permeability. The study presents interesting data being among the first permeability measurements in Romania. The originality of the study is also given by the proposed model which is oriented towards large dimensions blocks of apartments.
ABSTRACT The hygrothermal performance,of wood siding and exterior sheathing and the need of cavity ventilation for wood frame wall systems has been investigated. The aim of this preliminary study was to investigate various design strategies to improve the drying performance,and capabilities of wood frame wall systems. A moisture engineering approach was undertaken by conducting a combination of laboratory experiments and advanced computer simulations. The intent of the work was to investigate the heat and moisture performance,as affected by variations in the wall design. Several different walls systems with wood siding were constructed that included different material layers as exterior sheathing, different insulation materials, and wall systems that incorporated air cavity ventilation and others that did not incorporate a cavity between the wood siding and the exterior sheathing. Laboratory experiments were carried out to examine the hygrothermal,perfor- mance of the walls exposed to different exterior and interior boundary conditions. The drying capabilities of the walls and their ability to recover from moisture loads caused by vapor convection and diffusion were investigated. The information generated from the laboratory experiments was subsequently analyzed by advanced computer modeling, and additional simulations were performed,to determine the response of various wall systems using realistic environmental condi- tions. The experiments were also numerically simulated for the same laboratory conditions. In general, when comparing the numerical and experimental results, good agreement was observed, both indicating similar trends in the hygrothermal behavior. But at the same time, some anomalous results were also found that were initially believed to be due to anomalies within details of the wall structures but were later found to be caused by spurious values in the prescribed boundary conditions. The-different requirements due to actual climatic boundary conditions were addressed by selecting exterior data from a cold climate. The anal- ysis developed preliminary information in terms of guidelines and practices for acceptable thermal and moisture performance of wood frame walls. The hygrothermal performance of exterior sheathing materials, their effect on wall moisture performance, and the ability of the structure to dry out moisture from possible leaks (or initial construction moisture) need further research to establish guidelines applicable for a wider range of climates. Even today, many examples of moisture-related problems exist in the literature, some of which have been attributed to improper design of the cladding system. This paper attempts to shed some light on the issues and concerns of the drying performance,of wood frame wall systems exposed to cold climates.
Vent openings in masonry veneers are commonly specified in modern Canadian wall systems. Several different products are available to screen these vent openings from insects and direct penetration by driving rain. While venting is considered good practise, there is little engineering basis for the size and spacing of vent openings. With the support of a number of research partners, the role of venting in masonry veneer walls has been investigated. The importance of venting to pressure moderation and the control of rainwater, the removal of water vapour from behind the relatively vapour impermeable masonry veneer screen, and to the potential for ventilation drying were studied through a combination of theory, laboratory testing, and field monitoring. Both theory and testing have confirmed the importance of sufficient venting. The four vent inserts tested in our program all restricted airflow to a very high degree; in fact, they practically negated most of the benefits of venting. It was shown that ventilation could remove significant quantities of water from both the back of the veneer and from the back-up wall so long as proper design and construction were provided. Field measurements confirmed that sufficient pressures act over most faces of a building for most of the time to drive ventilation air flow. It was also demonstrated that vent area and vent location are important variables that must be considered in design if the performance and durability benefits of venting and ventilation are to be realised in service.
This article presents measuring results of the thermal performance of duo-pitched tiled woodframe roof designs. The roofs have been monitored in a test building, exposed to the outside climate. The experiment was part of a programme to study the hygrothermal performance of highly insulated envelope parts in situ, in order to investigate whether a good thermal quality (U = 0.2 W/(m2 K)) is achievable with current residential construction practices in Belgium. The results show the effect of wind on the thermal performance of duo-pitched roofs. The measured thermal properties of the roof components are compared to the design values, and related to the wind speeds and directions registered near the test building. The established thermal effects are explained using tracer gas tests to show the pattern of wind driven air flow in the roofs.
In this paper, the thermal and ventilation performance of an ecological house in Helsinki, Finland are presented. The single-family dwelling has a well-insulated, wooden frame construction with no plastic vapour retarder. The measured and simulated results show that the energy consumption of the house is low and that the outdoor ventilation rate is generally satisfactory based on the measured CO2 concentrations. Extrapolating the measured ventilation data shows that, when the operable windows are closed, the ventilation rate is expected to be about 0.45 air-changes-per-hour (ach) in the winter and about 0.25 ach in the summer. The consumption of total primary energy and space heating energy were measured to be 30% less (162 kWh/(m2 a)) and 36% less (76 kWh/(m2 a)) than in typical Finnish houses, respectively. The paper also uses a numerical model to investigate the sensitivity of energy consumption to the insulation level, household electricity and domestic hot water consumption, window area, ventilation rate and heat recovery effectiveness.
Thesis (Ph. D.)--University of Waterloo, 1998. Includes bibliographical references.
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