Kim D. Pressnail

Air Pressure and Building Envelopes

Control of airflow is essential to several important performance aspects of the building system. Air carries moisture which impacts a material’s longterm performance (serviceability) and structural integrity (durability), behavior in fire (smoke spread), indoor air quality (distribution of pollutants and microbial reservoirs) and thermal energy. Typical case studies are presented to illustrate how each of the above characteristics is affected when unintended airflow occurs as a result of poor construction. In some cases, there was simply a lack of understanding of the consequences of ignoring potential leakage paths and the interaction of the mechanical conditioning systems with the building structure. Rehabilitation of a troubled building requires that these interactions be understood. In general, the approach to developing that understanding is not involved.

Hour‐By‐Hour Analysis for Increased Accuracy of Greenhouse Gas Emissions for a Low‐Energy Condominium Design

The seasonal and hourly variation of electricity grid emissions and building operational energy use are generally not accounted for in carbon footprint analyses of buildings. This work presents a technique for and results of such an analysis and quantifies the errors that can be encountered when these variations are not appropriately addressed. The study consists of an hour‐by‐hour analysis of the energy used by four different variations of a five‐story condominium building, with a gross floor area of approximately 9,290 square meters (m2), planned for construction in Markham, Ontario, Canada. The results of the case studied indicate that failure to account for variation can, for example, cause a 4% error in the carbon footprint of a building where ground source heat pumps are used and a 6% and 8% error in accounting for the carbon savings of wind and photovoltaic systems, respectively. After the building envelope was enhanced and sources of alternative energy were incorporated, the embodied greenhouse gas (GHG) emissions were more than 50% of the buildings operational emissions. This work illustrates the importance of short‐time‐scale GHG analysis for buildings.

Evaluating the Air Pressure Response of Multizonal Buildings

Air flow in buildings is a complex flow and pressure distribution problem that makes quantification difficult. However, certain parameters have recently become easy to quantify – specifically the air pressure relationships within buildings. The measured building air pressure field can be used with network analysis to solve the building flow and leakage regime creating an analytical macro model of the building flow and leakage regime. The response of the analytical model can be further tuned by perturbing both the building air pressure field and the analytical model. Building analysis typically focuses on flows and requires that all flow paths into and out of a control volume be defined. The flow path resistances need to be characterized. Determining all air flow paths and determining the flow path resistances directly is difficult. As such, estimates of these flow path resistances are commonly used. These estimates are based on limited field data and laboratory measurements. The literature provides some component values that vary by orders of magnitude and their application is often unable to predict building flow fields (ASHRAE, 1997). Standard building analysis develops the building pressure field from the flow field. This paper argues that developing the flow field from the building pressure field is more powerful. Determining the characteristics of the building pressure field directly is considerably easier than determining flow path resistances. It allows closing of the gap between the mathematical sophistication of available multi-cell air flow models and the necessary input information defining the building boundary conditions. This approach allows the pressure response of the building to be used to ‘‘tune’’ the models extending the range of their applicability and accuracy.

Transient Interaction of Buildings with HVAC Systems— Updating the State of the Art

This paper reviews work that has led to the development of a number of multi-zonal airflow models (network models). At present, the network analysis and perturbation methods cannot be used to solve the interstitial flow, pressure and resistance regimes. Network analysis and perturbation may suggest that such flows exist, but the complexity and workmanship dependence of the interstitial flow, pressure and resistance regime requires direct measurement. In other words, at present, the boundary conditions of the interstitial regime can be defined analytically using traditional methods, but the pressures and flows within the interstitial spaces cannot be predicted with certainty using analytical means. Difficulties in obtaining the detailed information on mechanical systems and the leakage areas of building assemblies make the traditional approach of measuring airflows and constructing models using leakage areas impractical for diagnostic purposes. Additionally, a pressure difference across an assembly alone, where interstitial pressures are not considered, is not enough to describe performance of the building envelope. The interstitial air pressures are usually small and until recently have been beyond measurement. Research on building envelope durability and indoor air quality has shown the significance of these small, but persisting interstitial air pressure fields. To enhance the capability of network models, a relational model was developed. This approach permits the measured building air pressure field to be used with the network analysis. Furthermore, the response of the analytical model is calibrated by comparing the effect of a specific perturbation on both the building air pressure field and the analytical model. This paper is written in two parts, the first reviewing issues in prediction of airflow in the buildings and building envelope, the second analyzing the actual data of measured pressure response of buildings.

More Sustainable Masonry Façades: Preheating Ventilation Air Using a Dynamic Buffer Zone:

During sunny conditions, surface temperatures on masonry façades can rise to over 40°C above the ambient temperatures. Conventional wall designs minimize the benefits of this solar heat through the use of thermal insulation. However, air that is drawn from the outdoors, between the façade and sheathing, can be used to recover heat from the masonry. The system, which utilizes a dynamic buffer zone (DBZ), acts as a solar air collector. This system can provide an effective way to preheat ventilation air at little to no extra cost, while not compromising the architectural features of the masonry wall system. A numerical model was developed to predict the amount of heat recovery possible using a DBZ. The numerical model was verified by comparing results with a commercial computational fluid dynamics software package and by conducting laboratory experiments. Preliminary results indicate that the DBZ as a solar air collector can achieve as high as 33% daily solar efficiency and seasonal solar efficiencies of up to 27%. Since this system is low-cost, yet effective, it may offer designers an opportunity to build more sustainable masonry wall systems.

The Reduced Gradient Approach (RGA): An Alternate Method to Optimizing Humidity Conditions in House Museums in Cold Climates

The modern approach to artifact preservation comprises an aggressive interior environment with high relative humidity levels. Maintaining high interior humidity coupled with typical interior temperatures in house museums has proved to be detrimental to their building envelopes. Such envelopes are typically un-insulated and often experience excessive, uncontrolled air leakage. Thus, the potential is high for interstitial condensation and the deleterious effects that typically accompany it. For house museums, solutions to mitigate this problem are limited because the degree of intrusiveness on original structure and associated costs are primary concerns. This article presents research on a simple, low-cost, non-destructive preservation control technique applicable to house museums and similar designated historical structures. The reduced gradient approach involves manipulation of the interior temperature regime to minimize the water vapor pressure differential across the building envelope throughout the year while maintaining the desired indoor relative humidity. Such an approach will minimize the amount of condensation due to air leakage. Two separate studies present the design and application of the reduced gradient approach when applied to a house museum in the cold climate of Toronto, Ontario, Canada. The results show that this approach as a successful preservation strategy for house museums with an interior environment that does not jeopardize artifacts nor the building structure.