Building Envelope Design
The basics start with environmental loads
Building envelopes must be designed to perform under all weather conditions. The BC Cancer Agency Research Centre in Vancouver by IBI Group ans Henriquez Architects.Jeong-sik Jeong and Gilbert Larocque .
Modern building systems consist of structural, service and envelope components that can be respectively compared to the bones, organs and skin of the human body. The skin protects the body from harmful exterior environments and maintains comfortable body conditions. In the same manner, the building envelope aims to regulate indoor environmental conditions for human use or occupancy.
Clothing has been specially designed to assist the skin to protect the body more effectively. In the building envelope industry, new products that control one or other aspect of envelope performance are being introduced each year.
A building envelope, however, differs from the skin in that it cannot yet detect problems and adjust for new environmental conditions. This inability for self-detection and self-adjustment requires us to design building envelopes that will perform under all types of weather conditions.
The Modern Movement in architecture introduced the idea that a well designed system could be applied equally effectively in any climate. Time has proven this to be an invalid assumption. As shown in Figure 1, each region in Canada has different climates where the human comfort zone is relatively narrow.
The differences between the exterior environment and what constitute comfortable interior conditions generate environmental loads on the building envelope. The control of interior conditions is dependent on both envelope systems and mechanical systems - the latter being usually referred to as heating, ventilating, and air conditioning or HVAC - which require energy input.
Using appropriate envelope systems for each building type, use and climatic region, can dramatically reduce the overall energy input to the mechanical systems. Hence, better envelope designs can improve performance while reducing energy consumption.
The cost of rehabilitation and repairs, and health concerns due to the poor performance of building envelopes have become a big issue. The cost of repairing and maintaining envelopes is estimated at billions of dollars annually in Canada. For example, the total value of the building envelope repairs [related to the so-called 'Leaky condo' crisis] in British Columbia, is estimated at $792 million from 1999 to July 2008.
The cost for health care due to chemical or biological envelope failures is directly and indirectly much more than that of material deterioration. An understanding of building envelope physics, external environments, envelope systems and their interactions is necessary to design energy-efficient and durable building envelopes
A building envelope separates the interior environment from the exterior environment. Differences in the two environments generate environmental loads.The most important of these can be categorized as: temperature, moisture, and air pressure.
The temperature load is generated by exterior temperature factors [i.e., exterior air temperature, ground temperature, solar radiation, and wind], and interior temperature factors [i.e., occupant activities, ventilation, and heating equipment].
The moisture load is generated by exterior moisture factors [i.e., exterior ambient relative humidity, precipitation, and ground water] and interior moisture factors [i.e., ventilation, occupant activities, and humidification]. The air pressure load is generated by wind, stack effects, and air handling equipment.
All environmental factors are interrelated in a complicated way with the environmental loads as shown in Figure 2.
Temperature and Relative Humidity
Local climate data are accessible through worldwide weather stations. To utilize measured ambient temperature and relative humidity, weather data measured in country areas [airports, parks, etc.] must be modified for micro-climatic effects.
Since urban areas have sources of heat generation [i.e., cars, people, buildings, etc.], absorb more solar radiation, and consist of large thermal masses of concrete and asphalt, the temperature measured in rural areas tends to be lower than that in urban areas. This phenomenon is called the ‘heat island’ effect. If the available weather data were obtained from a rural weather station, it would be reasonable to add 2 or 3 degrees in temperature for buildings in nearby urban areas.
Buildings in a valley may experience a sharp drop in temperature at night because cold air sinks. Design exterior temperatures for a building in a valley should be reduced by several degrees depending on the geometry of a valley. For example, Toronto may experience a large difference in temperature over a short distance as shown in Figure 3. Unfortunately, there is no simple method to predict heat islands and the heat sink effects.
Solar radiation greatly influences building envelope. It induces high surface temperatures, which cause high drying rates and inward vapour flows in building envelopes. Radiation travels in a straight line between two surfaces. This fact enables the prediction of the location, period, and intensity of solar radiation based on sun path and building orientation.
The ASHRAE Handbook of Fundamentals provides detailed equations for these predictions that require data such as the sun’s declination angle, solar altitude, in addition to latitude and longitude for the building under consideration. However it is more usual to use approximations based on monthly or seasonally averaged climate data.
It is often important to determine wind-induced pressures on a specific spot across an envelope for airflow analysis - and commonly now to optimize natural ventilation effects. The reference mean dynamic pressure is based on long-term records of mean hourly wind velocities measured at 10m above grade. However, the National Building Code of Canada presents a simplified equation [NBCC 1996, Commentary B] to account for localized and intermittent environmental effects on the reference mean pressure.
Most building envelope design is based on average long term wind pressure data, however, to account for short-duration random wind gusts, the gust factor can be employed. The gust factor is the ratio of maximum wind pressure to mean wind pressure, and is most often used to estimate the accidental moisture gains due to wind blown rain.
Calculations can give only a prediction of actual performance, and the detailed wind effects on building envelope are most generally determined using empirical data from wind tunnel testing of scale models.
The consideration of rain is critical for building envelope design since rain can cause a large amount of water entry into systems in a short period of time. Different envelope systems require different kinds of consideration with respect to water infiltration.
Perfect barrier systems [i.e., the concept that no driving rain infiltrates into the system] in theory do not need to account for rain entry because these systems are based on the assumption that the rain water does not enter the system. The face-sealed cladding system was common for residential and commercial buildings in Canada before 1996. Such systems rely heavily on the integrity of the cladding and failure, particularly in wood frame walls that cannot tolerate sustained exposure to moisture, can have major cost and health implications.
In recent history, the performance of such construction in weather-exposed conditions [e.g. no overhangs, increased height, proximity to ocean, elevation of the site] has been very poor. Consequently, the Canadian building codes and by-laws have gradually adopted the rainscreen cladding system for the exterior building walls that are exposed to certain levels of precipitation.
Rain screen systems [i.e., the concept that a layer screens and drains driving rain] may require one to account for rain screen wetting since rainwater entering the cavity and absorbed by the rain screen influence the cavity humidity for a long period after the initial wetting.
The mass wall system (i.e., the concept of the provision of a sufficient mass to endure rainwater entry until drying occurs) requires one to account for the balance of wetting and drying by rainwater.
Driving rain is deposited on the exterior surface until the amount of the deposition reaches about 100 to 500 g/m2 [Kuenzel et al. 2001], which is the initial point of gravity drainage. After the outer layer reaches capillary saturation, all additional rainwater will be drained off the surface. The rain deposition and entry processes are illustrated in Figure 4 for a rain screen system.
Rainwater striking the exterior envelope surfaces may be absorbed into the outer envelope layer. Rainwater absorption rate can be estimated by using water absorption coefficients such as those listed in Table 1.
For example, assume a brick layer of a thickness of 100 mm and a density of about 1950 kg/m3 has a water storage capacity of 9% by weight. The maximum amount of water that can be stored in the brick layer is calculated as: 1950 kg/m3 x 0.1 m x 9% = 17.55 kg/m2
If the rainwater absorption rate is lower than the vertical driving rain rate, the rainwater absorption rate can be used to estimate the time to reach capillary saturation. A large amount of rainwater can enter through the joints of brick and mortar and cracks by capillary action. The method above does not take into account for this type of rainwater entry.
Snow is an important environmental consideration in envelope design because of its insulation effect, water source, and solar reflectance.
Snow Insulation Effect
The thermal resistance of fresh snow accumulating on a roof, or drifting against a wall provides additional insulation to a structure. As snow becomes compacted, however,
its thermal resistance decreases although the relationship between density and conductivity is not a linear one.
Solar Radiation Reflectance of Snow Surface
A clean snow surface reflects nearly 90% of incoming solar radiation. In winter, building envelopes experience the radiation reflected from surrounding snow. The aamount of reflected solar radiation depends on time of day and solar altitude.
Building envelopes below grade requires special care to determine boundary conditions for hygrothermal analysis. The relative humidity below grade is likely to be 100% RH because highly porous materials [i.e., soils] are in direct contact with and transmit groundwater. Temperatures below grade can be found from functions of ambient temperature and thermal mass.
Thermal Conductivity of Soil
Heat transfer through basement walls and floors depends on the temperature difference between the inside air and the ground. It is necessary to know both the insulation characteristics of the wall and the thermal properties of the earth for energy loss calculations. The conductivity of soil is typically in the range of 0.6 to 2.3 W/m•K.
Soil temperatures documented in the literature can be used for the exterior temperature within acceptable margins of error. The errors are generated from the heating effects of the energy emitted from basements
adjacent to the soil. A highly accurate calculation of energy exchange effects between the soil and the basements requires the use of a dynamic thermal analysis and site specific soil tests.
Nevertheless, using measured soil temperatures can be an acceptable way to estimate interstitial temperature of basement walls. The soil temperatures associated with various depths for Canadian cities are presented
in Figure 5.
Building envelope design has evolved from a ‘one size fits all’ approach, to a complex science that optimizes a multitude of environmental factors in response to the specific requirements of each project. As we strive for improved energy and environmental performance in our buildings, a basic understanding of how building physics can be used to mitigate the impacts of our varied climates will assist design teams in making sound strategic decisions.Jeong-sik Jeong, M.A.Sc., P.Eng., LEED AP has provided consulting engineering services for building envelope projects in British Columbia since 2002. Gilbert J. Larocque, CD, P.Eng., Ll.B. is Manager of the Building Science Division of Levelton. He is also a qualified solicitor specializing in construction and professional services contracts. .
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