As codes and standards evolve toward low- or net zero–energy buildings, the practicality of achieving these targets in high-rise concrete construction gets increasingly challenging. High-rise residential buildings are becoming more common as cities redevelop and add density. Current design and construction practice for high-rise multi-unit residential buildings present a number of constraints with regard to achieving high levels of energy performance. These practice issues typically include (a) the desire to maximize glass to enhance marketability, daylight, and views; (b) the desire to provide access to the outdoors via extended balconies; (c) the need for Code-mandated non-combustibility and life safety requirements; (d) a preference for building systems that minimize exterior construction access and streamlines construction sequencing; (e) the adoption of increased structural load requirements; and/or (f) the drive to minimize initial capital costs. The outcome of these combined constraints is often poor energy efficiency, with the burden of higher operating costs deferred to future owners. There has been significant industry discussion on the poor energy performance of this class of building, but there is very little guidance or long-term factual strategic information beyond broad principles of minimizing glazing areas, maximizing glazing performance, increasing airtightness, and adding more insulation to opaque areas. This article explores the prospect of energy use becoming a primary consideration in high-rise residential buildings and what that will likely mean for the typical competing constraints mentioned earlier. This article utilizes the current common construction practices for concrete-framed, high-rise residential buildings in heating dominated climates (ASHRAE Zones 4 to 7) as a baseline to evaluate the impact of the interconnected variables related to reducing overall heating energy use. The objective is to weigh the impact of individual improvements against integrated bundles of measures to develop a roadmap and a better understanding of a practical path toward low-energy, high-rise residential buildings. The article focuses on solutions related to building envelope performance but from a holistic perspective that recognizes the interaction and contribution of mechanical systems typical of this construction type. The building envelope parameters covered includes glazing performance (for both conventional and innovative technologies) and opaque wall performance (with a focus on specific details to reduce thermal bridging rather than increasing insulation levels). The analysis presented draws upon three-dimensional thermal modeling, whole building energy analysis, field testing and monitoring, and typical construction costs. The goal is to develop realistic targets for high-rise buildings and identify improvements that can be arrived at by market forces rather than those that can only be realized through more stringent and enforceable codes and standards.
Purpose The purpose of this paper is to propose a methodology for evaluating the hygrothermal performance of framed wall assemblies based on design limits. This methodology allows designers to evaluate wall assemblies based on their absolute performance rather than relative performance which is typically done for most hygrothermal analysis. Design/methodology/approach The approach in developing this methodology was to evaluate wall assemblies against three typical design loads (e.g. air leakage, construction moisture, rain penetration) and determine limits in minimum insulation ratio, maximum indoor humidity and maximum rain penetration rates. This analysis was performed at both the field area of the wall and at framing junctions such as window sills. Findings The findings in this paper shows example design limits for various wall assemblies in heating-dominated climates in North America. Design limits for wall assemblies with moisture membranes of different vapour permeance are provided for both the field area of the wall and at window sills. Discussions about the importance of 2D hygrothermal simulation and performance of vapour permeable sub-sill membranes are also provided. Originality/value This framework of hygrothermal analysis will enable designers to make better decisions when designing framed wall assemblies suitable to the local climate and interior specifications for their projects. It will also enable the development of a design tool that will allow designers to visually see the implications of certain design decisions and filter out designs that do not meet their design conditions.
Indoor relative humidity (RH) is commonly used to characterize the indoor environment for heat-air-moisture (HAM) simulations, chamber studies, analysis of monitoring data, or test hut studies of buildings without recognition that indoor RH and condensation potential depend on concurrent outdoor temperature and RH. This can lead to the use of unrealistic boundary conditions for HAM simulations and test programs, which may result in misleading conclusions. In buildings operating without mechanical dehumidification, the indoor air moisture level (vapor pressure) is directly related to the outdoor vapor pressure, moisture sources in the space, and the level of ventilation. Mathematics suggests that one can expect buildings with similar operation, occupancy, and construction, but affected by different weather conditions, to have a similar difference between indoor and outdoor vapor pressures. This paper provides a foundation for selecting appropriate and realistic boundary conditions for the design of residential buildings that are based on vapor pressure difference with the aim to eliminate any significant bias for a particular climate. The paper will present the following: (1) Discussion of current standards that provide some guidance to selecting appropriate indoor moisture levels based on vapor pressure difference; (2) Moisture balance equations will be used to show the impact of ventilation and moisture generation rates on the vapor pressure difference; (3) Monitoring data for six multi-unit residential buildings in two Canadian climates (Toronto and Vancouver) showing the relationship between the outdoor temperatures and vapor pressure difference; (4) Analysis of seasonal indoor moisture conditions and their impact on HAM modeling based on assumed indoor RH and conditions derived by a constant vapor pressure difference; and (5) Exploration of the concept that vapor pressure difference and indoor RH are limited by moisture removal on windows.
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