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The energy consumption analyses of high-rise buildings have some fundamental limitations that include the treatment of building size, changes in microclimate parameters with altitude, and the uncertainties associated with the existing building facade convective heat transfer coefficients correlations (CHTC). This study investigates the effects of these parameters on the energy consumption by individual rooms at a different location as part of a 100 m high-rise building, exposed to different weather conditions, having a different window-to-wall ratio. In the first part of the study, representative new-CHTC at the windward facade of the building is generated by using CFD simulations. In the second part of the study, comparative energy consumption assessment is carried out using the newly generated CHTC and other commonly used correlations by using EnergyPlus. The result shows that for high-rise building with 100% WWR exposed to a windy microclimate (such as Boston, MA), a deviation of 11.2% and 4.7% on annual heating and cooling energy consumption, respectively, have been observed. Further, the energy consumption of each room throughout the building height was also investigated to investigate the effect of building height. Compared to a room located at the mid-height (15th floor), the annual heating consumption on a room located at the 5th floor room was 7.8% lower and a room located on the 25th floor room the consumption was 7.6% higher. In summary, this study highlights the importance of accurate local CHTC generation to enhance thermal comfort in individual rooms and to optimize overall building energy consumption.

The initial high moisture content of concrete and the low vapor permeability of insulation layers on both sides of the concrete complicate the drying process of Insulated Concrete Forms (ICF). In order to facilitate the moisture transport and enhance the drying process, different moisture control strategies and wall designs can be implemented. The application of an air and vapor barrier is one of the most common moisture control strategies. In this paper, the impact of vapor permeance of an air and vapor barriers on the hygrothermal performance of an ICF wall in three different cold and wet climates is examined using a validated Heat-Air-Moisture transfer model. The hygrothermal performance of an ICF wall assembly with different types of barriers and locations in the wall system for several wall designs is investigated. Results indicate that a smaller thickness of insulation on the outside facilitates removing the moisture towards the outside and installing low permeance air/vapor barrier systems on the outside prohibits drying and drives the moisture to the inside. Our findings also show that with the proper selection of insulation thickness and vapor control strategy moisture-related problems can be avoided.

Window sizing and configuration can have a significant influence on building energy consumption. Window selection often has a conflicting objective on heating, cooling, and lighting performance. The smaller window performs better on controlling heat loss in winter and solar heat gain in summer, while the larger window performs better on providing views, daylight, and solar heat gains in winter. Also, the energy consumption analyses of high-rise buildings have some fundamental limitations that include the changes in microclimate parameters with altitude, the treatment of building size, uncertainties associated with the existing convective heat transfer coefficients correlations (CHTC). This study provides a framework for simulation-based optimization of window configuration for a high-rise building to minimize its energy consumption. The technique involves CFD modeling to validate and develop new-CHTCs, a Building Energy Simulation used to assess the energy consumption using the newly developed CHTC, and a numerical optimizer for iterative optimal window configuration selection. The decision parameters are window size and room location. The thermal comfort temperature set points and daylight illuminance are taken as constraints. The proposed approach is implemented as a case study on a single by single room model basis positioned at different heights in an isolated 100 m tall building exposed to Boston, MA microclimate. For a room located on the 2nd, 15th, and 29th floor, an optimum window configuration of 30%, 48%, and 30%, window-to-wall ratio, respectively, are obtained.

The impact of the ventilation cavity design on the energy performance of the rainscreen wall assembly was numerically investigated in CFD. The ventilated cavity design parameters considered were the cladding material type (i.e. Fibre cement, brick, and metal cladding), the sheathing membrane emissivity coefficient (i.e. 0.1–0.9), the ventilated cavity height (i.e. 1H and 2H) and the air gap width (i.e. 13 mm–50 mm). The CFD model was validated with experimental data and a dynamic simulation for a typical hot day and cold day is performed for a timestep interval of 5 min. The heat flux through the wall assembly was compared amongst the design alternatives. For the range of parameters considered, the ventilated cavity construct with brick cladding, sheathing membrane emissivity of 0.1, one-storey ventilated cavity, and 13 mm air gap width minimized the heat loss for a typical cold day. However, ventilated cavity construct with brick cladding, sheathing membrane emissivity of 0.1, one-storey ventilated cavity, and 50 mm air gap width minimized the heat gain for a typical hot day.

To mitigate the overheating problem of high-performance buildings in the summertime, a ventilative cooling method behind the cladding has been widely employed. The commonly used airspace behind cladding ranges between 9.5 mm (3/8″) to 19 mm (3/4″) and it is set solely considering moisture performance in North America. Although the literature indicates that higher thermal performance can be achieved by implementing bigger airspaces, the advantage of higher air gaps in comparison to the current way of construction is not clear. Therefore, in this study, an experimental investigation is carried out by comparing cavity depths of 19 mm (3/4″), 51 mm (2″), 102 mm (4″), and 152 mm (6”) under a mild marine climate of Burnaby, British Columbia, Canada. The test walls are instrumented for temperature, heat flux, and cavity airflow. The results indicate that the commonly used 19 mm cavity depth has the lowest airflow rate and therefore gets warm and results in the highest heat-gain to the building. Compared to the 19 mm cavity, the 50 mm cavity has 5x more total airflow and 25% lower heat-gain, while the 152 mm cavity has the biggest heat-gain drop of 40%.

The current residential buildings are of light weight construction. As such, they tend to frequent indoor air temperatures fluctuations and have been proven detrimental for thermal comfort and mechanical system energy consumption. This is reflected in the energy consumption statistics for residential buildings. More than 62% of the building energy use is towards maintaining comfortable indoor conditions. Phase change materials (PCM); a latent heat thermal storage material, have the potential to increase the thermal mass of these buildings without drastically affecting the current construction techniques. In this paper, the potential of phase change materials is investigated through numerical and experimental studies. The field experimental study is conducted using twin side-by-side buildings exposed to the same interior and exterior boundary conditions, and EnergyPlus, after being benchmarked with the experimental results, is used for the numerical study. The numerical study is carried out for an existing residential apartment unit with particular emphasis on the effects of different design parameters such as orientation and window to wall ratio. Preliminary analyses of experimental data show that phase change materials are effective in stabilizing the indoor air by reversing the heat flow direction. In fact, the indoor air and wall temperature fluctuations are reduced by 1.4 °C and 2.7 °C respectively. Following, benchmarking of the numerical simulation shows confidence levels in predicting the interior conditions since discrepancies between experimental data and numerical data are within tolerance limits of the measuring device. Further, from the analysis of the numerical data, phase change material is effective in moderating the operative temperature but does not translate to significant thermal comfort improvement when evaluated over a night time occupancy regime in the summer. On the contrary, PCM is effective in lowering the heating energy demand by up to 57% during the winter condition.

ASHRAE Standard 90.1 provides a useful guideline for the Window-to-Wall Ratio (WWR), stating that “the total vertical fenestration area shall be less than 40% of the gross wall area” However, different window configurations with the same WWR can perform thermally differently. In this paper, the effect of window configurations on the convective heat transfer rate of a window and downdraft velocity is investigated numerically at different Rayleigh numbers ranging from 1.7 × 106 to 1.7 × 1011. Parameters investigated in this study include window shape, heater location, and heater temperature. A convective heat transfer rate of each of the window models is evaluated using high-resolution 3-D steady Reynolds Averaged Navier-Stokes (RANS) based computational fluid dynamics (CFD) simulations. The heater and the window are modeled as isothermal plane boundaries, where the window possesses a cold temperature, and the natural convective heater is hotter than room temperature. The sensitivity of the window Nusselt number and downdraft velocity of the windows have been examined. The thermal performance ranking, from higher to lower, is horizontal rectangular, square, circular, and vertical rectangular window configuration, respectively. This implies that proper window geometry configuration selection can be used as one of the passive strategies for saving energy in buildings.

The application of exterior insulation in both new construction and retrofits is a common practice to enhance the energy efficiency of buildings. In addition to increased thermal performance, the rigid insulation can serve to keep the sheathing board warm and serve as a water-resistive barrier to keep moisture-related problems due to condensation and wind-driven rain. Polyisocyanurate (PIR) rigid boards have a higher thermal resistance in comparison to other commonly used exterior insulation boards. However, because of its perceived lower permeance, its use as exterior insulation is not very common. In this study, the hygrothermal property of PIR boards with different facer types and thicknesses is characterized. The material data obtained through experimental test and extrapolation is used in a long term hygrothermal performance assessment of a wood frame wall with PIR boards as exterior insulation. Results show that PIR with no facer has the smallest accumulated moisture on the sheathing board in comparison to other insulation boards. Walls with a bigger thickness of exterior insulation perform better when no vapor barrier is used. The PIR exterior insulation supports the moisture control strategy well in colder climates in perfect wall scenarios, where there is no air leakage and moisture intrusion. In cases where there is trapped moisture, the sheathing board has a higher moisture content with PIR boards with both aluminum or fiberglass type facers. An innovative facer material development for PIR boards can help efforts targeting improved energy-efficient and durable wall systems.

Current external convective heat transfer coefficient estimation methods are known to introduce significant uncertainty in energy consumption evaluations for building with complex surrounding conditions. In this study, the impact of built area density (aggregation or rarefaction) on convective heat transfer from buildings is numerically investigated in an urban-like setting. Arrays of cubical buildings with twelve different packing density in different flow regimes including three isolated, five wake interference and four skimming flow regimes, and an isolated cube case for comparison have been investigated. Reynolds stress turbulence model (RSM) is used for closure of the steady Reynolds averaged momentum and energy equations. The results indicate that the behavior of convective heat transfer varies from one flow regime to another. The convective heat transfer coefficient (CHTC) trends in the isolated flow regimes are characterized by sharp changes with density; whereas in the interference and skimming flow regimes the CHTC gently decreases with the increase of built area density. New correlations for estimating CHTC in these three regimes are proposed. The proposed correlations are expected to enable estimation of CHTC for buildings located in urban neighborhoods based on the built area density.

The overall U-factor values for an attic assembly are usually computed with the ANSI/ASHRAE/IES based R-value (thermal resistance) conversion. In the ANSI/ASHRAE/IES Standard 90.1 (2010), the effects of attic air resistance, roof pitch and attic width are not taken into account while calculating the U-Factor values. In addition, the R-value is estimated using a one dimensional thermal resistance model. In ventilated attics, where the insulation near the roof sheathing is tapered, it is difficult to find the correct R-value of the attic system as the heat transfer becomes two dimensional. In this paper, a 2-dimensional CFD model is developed for various insulation R-values and insulation taper angles near roof decks. COMSOL Multiphysics 4.4 is used to model and analyse the attic structure. Results show that a discrepancy in overall U-factor for entire attic assembly between the developed model and the existing standard estimation. These results are pronounced for lower slope roofs with high insulation thickness.

Attic air ventilation can be influenced by various vent considerations. In addition to vent ratio and location of roof vents, attic insulation thickness can be considered as an influential factor in attic air flow and temperature distribution. Most existing building codes do have a minimum requirement for venting parameters and type and thickness of the insulation used. In this paper, the effect of insulation thickness in attic ventilation rate, attic air temperature and heating and cooling loads in a mild climatic zone is studied. A typical mild climate summer and winter temperatures and solar radiations data are used for 24 hours transient conjugate heat transfer simulations. Results show that solar radiation has significant impact on the amount and the pattern of airflow in attic. An increase in attic insulation yields a decrease in attic ventilation during winter period, but has no effect in summer period for the climate considered. In general, the higher the attic insulation thickness is the lower the building takes advantage of solar gain during winter period, but higher insulation levels tend to be advantageous during summer cooling period.

This paper aims to study the effects of wind-driven rain load and vapor diffusion on the hygrothermal performance of wall systems in a wet and mild climate through a field experimental study. In the study, four test panels with a combination of vapor barrier and capillary break are manufactured, instrumented and installed in a field experimental facility. The wetting and drying potentials of the test panels in response to a predominately vapor diffusion and a wind-driven rain load are discussed based on the analysis of 15 months of measurement data. The experimental result shows that, in a yearly basis, the wetting and drying rates of a wall without a capillary break are about two times higher than that of the wall with a capillary break. While the wetting and drying rates are comparable in a wall system with a vapor barrier, the drying rate is 38% higher than the wetting rate in a wall with no vapor barrier. In general, a wall with no vapor barrier wets and also dries faster than a wall with a vapor barrier. For the wall types and climate considered in this paper, the wetting rates of walls with a predominately wetting mechanism of vapor diffusion and wind-drive rain load are comparable. In general, the experimental data suggest that even in a mild climate, vapor diffusion is a critical moisture load with comparable effect that wind-driven load induces.

The international residential code (IRC) and most building codes in North America provide attic ventilation codes which allow a certain minimum venting area with an unblocked space by the ceiling insulation. Most of these codes have similar minimum venting ratio, minimum space gap between the roof sheathing and ceiling insulation and vent area location for similar climatic conditions. In this paper, the effects of varying the gap between roof sheathing and ceiling insulation (baffle size) and the locations of vent area under both summer and winter conditions are investigated. Three different baffle sizes and three different locations of the attic vent are used to study their effect on the air distribution and temperature profile inside the attic space. A CFD model is developed and validated using existing experimental measurements. Results show that increasing baffle size hugely affects the air distribution when the air flow is majorly driven by wind. The upper side roof vents have been located at three different locations and our findings show when the upper vent is placed the furthest from the ridge the Air Change per Hour (ACH) value in the attic increases but the air circulation is minimal in the top parts of the attic space and structural elements.

Performing accurate hourly building energy modeling requires the presence of reliable boundary conditions. The required data for energy simulation model entries are exterior air temperature, exteri...

Understanding temperature and airflow patterns in an attic roof helps to identify the sections of the roof where moisture damage and roofing material durability problems can be expected. In this paper, the temperature distribution and the airflow patterns inside an attic space in a climate that can be characterized with cold winter and hot summer periods are discussed. In the study, transient boundary conditions were applied, and the dynamic responses of a typical attic roof, including the flow patterns, attic air and roof sheathing temperatures, ventilation rates, and heating and cooling loads, are presented. These findings can be used to understand the airflow pattern and how it can be affected by an incoming wind speed and solar gain.

A building shall be classified as high performance building if it is energy efficient and durable and at the same time provides comfortable and healthy indoor environment for occupants. To achieve this objective, the hygrothermal performance of alternative building designs should be evaluated based on the simultaneous analysis of these three functional requirements rather than separately. In this article, a Whole-Building Hygrothermal model is used for evaluation of various retrofit design parameters that potentially enhance the overall performance of an existing residential house. The retrofit options considered in this study include changes to the reference house’s ventilation rate and operation, windows, insulation level, and various combinations of these options. Energy efficiency, building envelope and moisture management potential, indoor humidity control, and window condensation potentials are considered to be the four performance indicators in searching for a retrofit option that delivers an optimal performance. The hygrothermal simulation results indicate that changing a design parameter to improve one of the design goals may result in less optimal results in the other one or both goals, or even in some cases result in severe negative consequences.

Many high-rise buildings have intricate facade appurtenances such as balconies, mullions, or egg-crates shading elements. These facade systems interact with the external microclimate in a complex manner that affects the convective heat transfer coefficient (CHTC) significantly. In this study, CFD and heat transfer simulations are carried out for buildings with balconies, vertical shading element, egg-crates, and a smooth facade. Comparisons of local-CHTC distribution between buildings with and without the facade appurtenances for rooms on different floor heights and located at a corner or a center region of the building are performed. Two equation SST κ-ω turbulence modeling is used for momentum closure. The result of this study shows that the local and surface-averaged CHTC values at the surfaces of each building are dependent on building aerodynamics and forms of the facade appurtenances. For instance, for a 100 m tall building case study with rectangular floor plan, having an egg-crate shading with a depth of 1 m decreased the local-CHTC of a room around the corner-zone by 35%, 37%, and 38% on the 1st, 15th, and 30th floors, respectively compared to a room in building a smooth facade. Thus, necessitating detail local-CHTC variations and wind directionality effect assessments especially on glazed buildings characterized by weak thermal performances.

Precipitation is one of the most common moisture sources on which building designers focus. Water comes from both top down and bottom up. Although foundations are sometimes constructed out of pressure-treated lumber, generally they are constructed from poured concrete. In a wet climate zone, the foundation of a house is often under continuous contact with moisture, which is mainly caused by rundown rainwater, wet soil, a high water table, or a combination of all these factors. This causes rot growth and decay of the wood-frame structure as it sits constantly on the damp foundation concrete. In this research, moisture transfer between concrete and wood is investigated under three different scenarios: a case with direct wood and concrete contact and two cases with different moisture barriers between the two materials. The moisture barrier materials considered in this study are the damp-proofing layer and sill plate gasket. The moisture transfer processes in these three cases are investigated in a fi...

There is a continuous dynamic heat, air and moisture (HAM) interaction between the indoor environment, building envelope and mechanical systems. In spite of these interdependences, the current indoor, building envelope and energy analysis tools are used independently. In this paper a holistic HAM model that integrates building envelope enclosures, indoor environment, HVAC systems, and indoor heat and moisture generation mechanisms, and solves simultaneously for the respective design parameters is developed. The model is benchmarked with internationally published test cases that require simultaneous prediction of indoor environmental conditions, building envelope moisture performance and energy efficiency of a building.

Most building materials are porous, composed of solid matrix and pores. The time varying indoor and outdoor climatic conditions result heat, air and moisture (HAM) transfer across building enclosures. In this paper, a transient model that solves the coupled heat, air and moisture transfer through multilayered porous media is developed and benchmarked using internationally published analytical, numerical and experimental test cases. The good agreements obtained with the respective test cases suggest that the model can be used to assess the hygrothermal performance of building envelope components as well as to simulate the dynamic moisture absorption and release of moisture buffering materials.

It is essential to design and operate buildings with good indoor air quality because people spend most of their time indoors, and their productivity, comfort, and health depend on the quality of the indoor air. In addition to other indoor-air-quality parameters, the indoor humidity and temperature need to be controlled and maintained within acceptable ranges. Elevated indoor humidity creates favorable conditions for mold growth and building-envelope damage. To minimize such problems, it is important that designers have insight into the level of indoor humidity that will be expected in a building operating under a set of conditions and weather variation. In this paper, the results of monitoring the indoor temperature and humidity of four apartment suites with different occupancy levels are reported. Along with the indoor-air conditions, the local outdoor temperature and relative humidity were continuously measured for 17 months. The indoor humidities in the suites were correlated with the outdoor a...

The moisture design of exterior walls in a building envelope is an important task that needs to be carried out systematically to generate a sustainable and healthy built environment. Many conventional methods or practice guidelines are available for this purpose, based primarily on local traditions and with limited performance assessment records. In recent years, with the rapid development of global free trade and economy, new wall systems and unconventional materials have been introduced in every part of the world for reasons such as aesthetic appeal, cost effectiveness and so on. However, neither the long-term moisture management performance of these new wall systems nor the uses of unconventional materials have been assessed in a systematic way. The primary reason for this lack of assessment is the absence of a design-oriented methodology to perform the task. This paper presents selected results from a recently completed research project that demonstrate that it is indeed possible to assess the moisture management performance of exterior walls in a systematic way, using a hygrothermal modeling tool together with key inputs from a limited number of laboratory and field investigations. In this project the hygrothermal responses of exterior walls and their components were assessed with a novel moisture response indicator, called the RHT index, which is derived from relative humidity and temperature data over a time period. The results and discussion presented in this paper clearly show the need and usefulness of the application of hygrothermal simulation tool for the optimum moisture design of exterior wall systems in various geographic locations, when sufficient information is available from laboratory and field experiments.

In many mechanical systems, the tendency of sliding components to intermittently stick and slip leads to undesirable performance, vibration, and control behaviors. Computer simulations of mechanical systems with friction are difficult because of the strongly nonlinear behavior of the friction force near zero sliding velocity. In this paper, two improved friction models are proposed. One model is based on the force-balance method and the other model uses a spring-damper during sticking. The models are tested on hundreds of lumped mass-spring-damper systems with time-varying excitation and normal contact forces for both one-dimensional and two-dimensional stick-slip motions on a planar surface. Piece-wise continuous analytical solutions are compared with solutions using other published force-balance and spring-damper friction models. A method has been developed to set the size of the velocity window for Karnopp’s friction model. The extensive test results show that the new force-balance algorithm gives much lower sticking velocity errors compared to the original method and that the new spring-damper algorithm exhibits no spikes at the beginning of sticking. Weibull distributions of the sticking velocity errors enable maximum errors to be estimated a priori.

In this paper, the development and benchmarking of a newly developed whole building hygrothermal model are presented. The model considers the building as a system and accounts for the dynamic heat, air and moisture (HAM) interaction between building envelope components and indoor environmental conditions including HVAC systems, moisture and heat sources. The methodology adapted in this work is to develop and validate two primary models: building envelope and indoor models independently and couple them to form the whole building hygrothermal model. After successful integration of the models, the whole building hygrothermal model is benchmarked against internationally published numerical and experimental test results. The holistic model is used to assess building enclosures durability, indoor conditions (temperature and relative humidity), occupant comfort, and energy efficiency of a building in an integrated manner.

Using numerical modelling to simulate and predict the hygrothermal (i.e., combined thermal and moisture)performance of building envelopes is very recent. Key questions include: how to model accurately coupled heat-air and capillary moisture transports in building envelope components; a satisfactory definition of a set of representative environmental boundary conditions to be used for long-term hygrothermal calculations; how to characterize the moisture- and temperature-dependent properties; the effect of aging and cyclic environmental conditions on porous building materials; and how to develop sound criteria to predict the moisture durability of building envelope components. This paper presents the findings of a research project involving detailed hygrothermal modelling. The heat, air and moisture results demonstrated that the in-house model could be adapted successfully for high-rise building calculations. The findings also show how the long-term hygrothermal performance of typical wall systems can be assessed using numerical modelling. A short description of an advanced in-house heat, air and moisture model, hygIRC, is also presented., 12th International Heat Transfer Conference: 18 August 2002, Grenoble, France

Heat, air, and moisture transfer models that are used as practical building design tools require reliable inputs to provide meaningful results. One of these inputs is the set of heat, air, and moisture transport properties of materials. For any given class of building materials the properties may vary within a broad range. This paper reports the porosity, density, matrix density, thermal conductivity, equilibrium moisture content, water vapor permeability, water absorption coefficient, liquid diffusivity, and air permeability of regular lime stucco, regular portland cement stucco, and acrylic stucco which are commonly used in North America. The experimental and analytical procedures follow either international standards or well established methodologies.