Achilles Karagiozis

Wind-driven rain distributions on two buildings

Abstract Wind-driven rain is an important consideration in the hygrothermal performance of building envelope parts. Wind-driven rain (in liquid form) can increase the amount of moisture present in the structure by more than 100 times that due to vapor diffusion. To date, very little work that provides field or laboratory wind driven rain data to moisture transport models is available. This information is a definite requirement as a boundary condition by the more sophisticated hygrothermal models such as LATENITE and WUFIZ which consider both vapor and liquid moisture flows. In this paper, the wind driven rain striking the exterior facade of two buildings (one twice the size of the other) is generated using a three-dimensional computational fluid dynamics (CFD) model that solves the air flow and particle tracking of the rain droplets around these two buildings. These simulations were carried out for a city center region. Four factors which govern wind-driven rain are investigated in this work: (a) upstream unobstructed wind conditions, (b) the rainfall intensity, (c) the probability distribution of raindrop sizes, and (d) the local flow patterns around the building. All four of these governing factors make wind-driven rain on a building facade very distinct. Simulations were carried out for three wind speeds of 5, 10 and 25 m/s, three rainfall intensities of 10, 25 and 50 mm/h and three wind directions 0°, 30° and 45° from the west face of the buildings. In this paper, only the results of the 0° wind direction are discussed. The results show distinct wetting patterns on the top of the building of both the two buildings which is most concentrated at the corners when the wind was normal to the facade surface. For the tallest building a distinct wetting pattern is displayed in the mid-height of the building. This information from wind engineering is directly employed for the design of building envelope moisture control. Results on a series of simulations are presented to demonstrate the effect of wind conditions, rain intensities, the interaction between the two buildings, and the droplet sizes on the wetting patterns on the faces of the short and tall building.

Hygrothermal system-performance of a whole building

Abstract In this paper the full house hygrothermal performance of an aerated concrete wall system is examined for the hot and humid climate of Miami. The results clearly demonstrated the limited drying potential for the wall system in that climate. The selected exterior thermal insulation strategies and interior vapor control strategies in this study show the critical behavior of the full house with respect to drying initial construction moisture. Moisture related problems such as mold growth are simulated and discussed. From these results moisture control strategies are identified for the whole house hygrothermal performance.

The Effect of Air Cavity Convection on the Wetting and Drying Behavior of Wood-Frame Walls Using a Multi-Physics Approach

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.

Barrier EIFS Clad Walls: Results from a Moisture Engineering Study

This paper presents the results of work performed to enhance understanding of water penetration and moisture characteristics of barrier Exterior Insulation Finish Systems (EIFS) clad walls. The investigation was prompted by recent failures of barrier EIFS clad walls on houses in the Wilmington, North Caro lina area. The objective was to determine the cause of the problem and gain a better understanding of water penetration and moisture transport in these systems. The study included a field investigation, laboratory experiments, and computer simulation. The experimental results indicate that barrier EIFS clad walls are prone to leak in areas around penetrations and allow water to enter into the wall cavity. A drainage cavity wall was also tested, and observed to perform well, managing water around penetrations and preventing leakage into the wall cavity. The computer simulation indicates that barrier EIFS clad walls in Wilmington have low moisture tolerance due to slow drying rates. In conclusion, barrier EIFS clad walls do not provide effective management of rain penetration. As such, in-service performance is unpredictable and unreliable. In con trast to barrier EIFS clad walls, walls using a drained cavity approach were shown to provide good control of rain penetration.

Measurements and Two-Dimensional Computer Simulations of the Hygrothermal Performance of a Wood Frame Wall

Knowledge of the expected long-term performance of building envelopes subjected to simultaneous heat and moisture transport is critical during the design stage. In the past thirty years researchers have concentrated their efforts in extensive laboratory experiments. These experiments have been expensive as well as time consuming to conduct due to the slow moisture transport phenomena. This paper critically investigates a set of experimental results generated from laboratory controlled measurements on a wood frame wall construction, by employing a state of the art hygrothermal model. The analysis was carried out using the LATENITE model, a three-dimensional heat and moisture transport program tailored specifically for building envelope investigations. For the present simulations this model was adapted for two-dimensional conditions and hourly hygrothermal performances were predicted for a laboratory instrumented wood frame wall section. The investigation showed three main advantages of combining measurements and simulations. By carrying out simulations early in the design stage of laboratory experiments the experimental design will probably yield better quantification of data, placement and types of sensors, and assessment of workmanship influences, etc. Measurements can calibrate, adapt, or check calculated results. Finally, simulations can be performed to explain and interpret experimental results. Marrying experi ments and modeling allows researchers to generate effective hygrothermal perfor mance guidelines.