Journal of Building Physics recent issues

A Differential-difference Approach for the Modeling of Electrical Floor Heating Systems

Pakanen, J.E., Saastamoinen, J.J. — Mon, 05 Oct 2009 04:15:47 PDT

During recent years, many papers have been published concerning dimensioning, analysis, and control of floor heating. A central issue in all these activities is to create a suitable model, which explains the thermal behavior of the floor and its environment. The article introduces a differential-difference approach, which is believed to be new in floor heating modeling. The proposed method is applied in a slab, a semi-infinite and infinite solid, which are connected to electrical heating. The results are compared with the corresponding analytical solution. This also concerns the proportional heating control applied in the infinite solid. The comparison reveals the characteristic features of the differential-difference solution, which are briefly discussed. The first results are encouraging and show good model behavior.

Uncertainty and Sensitivity Analysis for Building Energy Rating

Corrado, V., Mechri, H. E. — Mon, 05 Oct 2009 04:15:47 PDT

One of the main tasks of the European Energy Performance of Buildings Directive 2002/91/EC (EPBD) pertains to energy building certification. For this purpose, recent European standards have defined a methodology for the assessment of the energy use of buildings through the calculation of energy ratings. The accuracy of the energy rating depends both on the detail level of the model and on the confidence of the input data. In this work, the quasi-steady simplified monthly method, presented in the ISO 13790: 2008 standard, has been applied to a dwelling in Turin, Italy, in order to calculate its energy rating. A widespread list of 129 input data was identified and grouped into three sets: climatic data, envelope data, and building use data. Probability density functions were assigned after a detailed bibliographic research. The Monte Carlo Latin hypercube sampling technique has been used to assess the building energy needs confidence interval and the energy performance class uncertainty. A sensitivity analysis based on the Morris method, has also been performed for the different building heat balance terms, in order to identify the most important parameter set that takes into account for the uncertainty in the model output. The main results demonstrate that the asset energy rating is only slightly affected by data uncertainties; moreover, the analysis highlights the importance of associating an operational energy rating to the asset rating in order to show the influence of user behavior on building energy performance. On the other hand, the results show that less than 10 input data, among the considered 129 ones, have a significant influence on the energy rating uncertainties.

An Experimental Device for the Measurement of Hygroscopic Inertia Influence on RH Variation

Ramos, N.M.M., De Freitas, V.P. — Mon, 05 Oct 2009 04:15:48 PDT

This article reports the development of an innovative flux chamber that is specifically developed for the support of experimental work that is aiming at a systematic measurement of the hygroscopic inertia influence on relative humidity (RH) variation inside rooms. This flux chamber is a flexible infrastructure where different test configurations can be run. The precise control of vapor production, inside air renovation, and system temperature allow for the reproduction, in a small-scale room, of hygrothermal scenarios that can be fully repeated in an unlimited number of cycles. Results obtained are presented in this article, relating especially to the testing and calibration procedures and highlighting the limitations and possibilities of the facility. The hygroscopic inertia effect on RH variation is demonstrated. A discussion on the perfect mixing of the air inside a room is also addressed, based on the obtained results.

Thermal Benefits of Tiled Roofs with Above-sheathing Ventilation

De With, G., Cherry, N., Haig, J. — Mon, 05 Oct 2009 04:15:48 PDT

Computer simulations are conducted to demonstrate the thermal benefits of good roofing design in hot climates. The work is aimed to quantify the thermal benefits from batten-space ventilation, commonly referred to as above sheathing ventilation (ASV), and thermal mass. Both roof features are effective in reducing the heat transmission into the habitable space. Nevertheless, the use of roof ventilation is often misunderstood and most procedures for energy assessment of building structures do not recognize the contribution from ASV and thermal mass.

As part of this work, a computer algorithm is developed to predict the heat, air and moisture (HAM) transport in the building envelope. The algorithm predicts the transient temperature and moisture behavior in the building envelope and takes account of the air movement from bouyancy and wind driven airflows.

On the Application of Uncertainty and Sensitivity Analysis with Abstract Building Performance Simulation Tools

Struck, C., Jan Hensen, , Kotek, P. — Wed, 15 Jul 2009 03:44:09 PDT

Building performance simulation (BPS) is a powerful measure to educate the building design process. However, its use in practice is to a large extent limited to the detailed design stage dedicated to the code compliance analysis of worked out design proposals. BPS is not much used to support the conceptual design stage. To date BPS tools are regarded as pure analysis tools, which do not provide design information. It is hypothesized that, when integrating uncertainty analysis techniques to existing BPS tools, following an incremental research approach, valuable design information can be provided. The article gives an update on the process to extend the capabilities of a tool specifically developed to support the conceptual design stage about uncertainty assessment. The resulting prototype should be capable of providing information about the variation of specific building performance metrics (simulation output) based on the propagated uncertainties of the building specification (simulation input). The main focus was herby the representation of material properties and their impact on the performance uncertainty of the building concept. The results also indicate the degree of impact, sensitivity, of the building specification parameters on the variation of the performance metrics as a result of their uncertainty.

Free Water Intake as Means of Material Characterization

Vejmelkova, E., Pavlikova, M., Jerman, M., Cerny, R. — Wed, 15 Jul 2009 03:44:09 PDT

Free water intake experiment is analyzed from the point of view of the applicability of its output for material characterization. Several experimental setups are presented and discussed. Characteristic examples of sorptivity plots for different materials are given and their main features have been described. The application of the experiment for characterization of material damage and for explaining the water transport mechanisms in two-layer systems is presented. It is concluded that the free water intake experiment basically can be used as a means of material characterization but its strong and weak points should always be considered if conclusions on the behavior of a particular material are to be drawn.

Examination of Stability of Boundary Conditions in Water Vapor Transmission Tests

Pazera, M., Salonvaara, M. — Wed, 15 Jul 2009 03:44:09 PDT

The diffusion of water vapor through construction materials is driven by the gradient of partial water vapor pressure. Traditionally, water vapor transmission tests (WVT), the `dry cup', and the `wet cup' tests have been conducted with 0 to 50% RH, and from 50% to 100% RH, respectively. Often, desiccants, saturated salt solutions or distilled water are used to generate the required RH. The stability of vapor pressure in WVT tests is of paramount significance because these values are used in calculating permeance. Dry cup tests performed with high permeance materials (>25 Perms) indicated a transient behavior for the two types of adsorbing desiccants; calcium sulfate and silica gel. Monitoring RH inside the dry cup was performed to examine the effect of low and high WVT on the performance of moisture sinks. The results indicate that for highly permeable construction materials, the use of vapor pressures corresponding to 0% RH in calculation of permeance is not appropriate.

Envelope Thermal Design Optimization of Buildings with Intermittent Occupancy

Al-Homoud, M. S. — Wed, 15 Jul 2009 03:44:09 PDT

Buildings with intermittent occupancy may not perform thermally the same as typical commercial and residential facilities. Thermal comfort requirements require careful envelope design coupled with the appropriate air-conditioning system operation strategies. One of the most prominent examples of such buildings is mosques. Mosques are usually occupied five intermittent times day and night all year round. Like any other building, they have to be mechanically air-conditioned to achieve the required thermal comfort for worshippers especially in harsh climatic regions. This paper describes the physical and operating characteristics typical for the intermittently occupied mosques as well as the results of the thermal optimization of a medium size mosque in the two hot-dry and hot-humid Saudi Arabian cities of Riyadh and Jeddah. The analysis utilizes a direct search optimization technique that is coupled to an hourly energy simulation program. Based on that, design guidelines are presented for the optimum thermal performance of mosques in these two cities in addition to other design and operating factors that need to be considered for mosques in general.

Sampling and Analysis of Natural Isotopes in Moisture Transport from Porous Materials: Applications to Capillary Suction

Koniorczyk, M., Gudmundsson, K., Johannesson, G. — Wed, 15 Jul 2009 03:44:09 PDT

Apart from the most common ¹H and ¹⁶O we have the sister isotopes: deuterium D and ¹⁸O in water. Isotope fractionation (the change in its concentration) occurs in any thermodynamic reaction. The isotopic composition serves as a distinct mark for each water sample. To carry out the isotope analysis of pore water, which may be used to reveal the source of excessive water in building elements, firstly one has to extrude water from voids. For this we tested three different methods of retrieving water from the porous material: squeezing and evaporation - the direct methods, dilution - the indirect method. The influence of capillary suction on the hydrogen and oxygen isotopes abundance ratio was also analysed. Materials whose substantial part of voids are gel pores (cementitious materials) act as a membrane during isotope transport separating lighter from heavier isotopes and therefore cause the fractionation of isotopes. The results of performed experiments indicate that moisture transport (capillary suction) has an influence on the isotopic composition of water. Therefore the isotope analysis can be useful in the investigation of moisture behaviour of porous building materials.

Editorial

Bomberg, M. — Wed, 25 Mar 2009 04:41:44 PDT

Ventilation Measured in the Wall Cavities of High Moisture Risk Buildings

Bassett, M., McNeil, S. — Wed, 25 Mar 2009 04:41:44 PDT

New building designs following the New Zealand Building Code Approved Documents complete a 'risk matrix' of climate and building design factors with associated 'risk scores' to guide the selection of wall cladding and cavity design. The risk matrix is briefly described in this paper, along with cavity ventilation results measured with a continuous emission carbon dioxide tracer method. Ventilation rates in a selection of wall cavities are compared with ventilation rates calculated from wind pressures and vent opening sizes. Fan pressurization methods are used to measure the effective leakage areas of infiltration paths because these contribute significantly to ventilation in some cavity types. Acceptable agreement between calculated and measured day average ventilation rates has been achieved in a range of water-managed cavities from drainage planes to drained and ventilated brick veneer cavities with results spanning three orders of magnitude.

Ventilating Wall Cavities Above Windows

Bassett, M., McNeil, S. — Wed, 25 Mar 2009 04:41:44 PDT

This paper shows that the cavities above two-part head flashings can be designed to borrow ventilation from cavities flanking the window through vented battens. It is part of a program of research designed to extend the range of flashing options for windows used in temperate New Zealand. Ventilation rates are measured in the cavities above window heads with a constant and uniform emission tracer method. These are similar to ventilation rates calculated from wind and stack pressures at openings representing engineered vents and infiltration paths into the head cavities. The leakage characteristics of these openings are measured with a pressurization technique, and together with temperature and wind data, formed the data set used to calculate equivalent cavity ventilation rates. As with earlier studies of ventilation in water-managed cavities, infiltration turned out to be an important source of cavity ventilation. Turning off the infiltration paths (possible in the numerical model) shows that the vented batten solution is ultimately less dependent on infiltration than the traditional head flashing ventilated case, and that ventilation through vented battens can be engineered to replace ventilation over the head flashing.

Drainage and Evaporation from Window Sill Trays

Bassett, M., McNeil, S., Bennett, J. — Wed, 25 Mar 2009 04:41:44 PDT

Flat sill trays can be attractive where trim cavity depths are too narrow for a sloped sill tray, but their effectiveness in managing water leaks is sometimes questioned. This research contributes to the debate on sloped versus flat trays in windows used in temperate New Zealand. Ventilation rates are measured in each trim cavity with a constant and uniform emission tracer method and are found to fall in the range 0.04—0.13 L/s. Equivalent ventilation rates calculated using a duct flow model and local climate data were in the same range (0.09—0.13 L/s) and this allows the model to be used to estimate ventilation rates and consequential evaporation over a longer period. During artificial irrigation with water, the sloped trays are found to drain effectively but the flat trays only intermittently drain to outside. The evaporated loss from the two sloped sill trays (30 and 41 g/day) is comparable to calculated evaporation rate for the period, but both flat sill trays lose water into the framing below the windows due to installation faults.

Heat, Air, and Moisture Control in Slab-on-ground Structures

Rantala, J., Leivo, V. — Wed, 25 Mar 2009 04:41:44 PDT

The objective of this research is to determine the heat, air, and moisture conditions of slab-on-ground of a heated building. Measurements show that the coarse-grained fill or drainage layers beneath heated slab-on-ground structures are warm and moist throughout the year. Moisture migration: gravitational water, ground water table, capillary action and diffusion, keep these layer moist throughout the year. The water vapor content of the warm and moist fill layer is usually higher than the content of the indoor air. Thus, the diffusion flow is directed into the indoor air, despite the existing heat flow into the opposite direction, and the ground is a moisture source for the structures above. Therefore, thermal and moisture conditions of the fill layers have the major effect on the overall moisture behavior of ground slabs: on the drying times of the fresh in situ cast concrete slab, and on the thermal and moisture behavior of the structure in normal long-term conditions. These conditions are also favorable for microbe growth, which is very common in fill layers. Fungal or bacterial growth was detected in 98% of the test specimens taken beneath the ground slabs of heated buildings. These conditions: warm fill layers, high moisture level (RH near 100%) and microbe growth, are the normal conditions for the well functioning slab-on-ground structure. They are not a sign of moisture damage, but the existing boundary conditions for the design of ground slabs. Therefore, it is important to prevent air infiltration between the ground and the indoor air, as effective as possible. The adequate sealing of the joints and lead-ins of the floor assure that the impurities do not penetrate the indoor air.

Experimental Investigation of Hygrothermal Parameters of Building Materials Under Isothermal Conditions

Li Kuishan, , Zhang Xu, , Gao Jun, — Wed, 25 Mar 2009 04:41:44 PDT

To determine thermal parameters of porous building materials for building thermal and energy analysis, an experimental study is presently carried out, which applies constant relative humidity controlled by the saturated salt-water solution under isothermal conditions. The principle of isothermal vapor sorption is discussed and the method for temperature and humidity control is proposed. Experiments are first performed using the routine materials of concrete, mortars, and clay brick whose experimental data widely existed in the previous studies, to confirm the present experimental work. Insulation materials of expanded polystyrene, extruded polystyrene, and polyurethane are tested for determining their water vapor permeability and isothermal sorption. These materials are considered due to the fact that they are mostly used as building insulation materials in China but lack thermal-hygro parameters for actual applications. The humidity-bound ranges 11.3—96.7%, which basically covers the whole range of building thermal-hygro environment is encountered. The least square method is used to fit the experimental data obtained, and finally to present regression equations for the three materials. Results of this study are expected to provide key parameters for the model of heat and moisture transfer in the materials for real buildings in hot-summer and cold-winter regions of China, where the transfer of moisture is exhibited significantly in building energy.

Editorial

Bomberg, M. — Wed, 14 Jan 2009 08:21:06 PST

Combined Heat, Air and Moisture (HAM) Transfer Model for Porous Building Materials

Dos Santos, G. H., Mendes, N. — Wed, 14 Jan 2009 08:21:06 PST

In the building science area, mathematical models are developed to provide better indoor thermal comfort with lower energy consumption. Although the fact moisture and air transfer can strongly affect the temperature distribution within constructions, whole-building simulation codes do not take into account the convective air transport in porous materials. In this way, this article presents a heat, air, and moisture (HAM) transfer model based on driving potentials of temperature, air pressure, and water vapor pressure gradients for consolidated porous material in both pendular and funicular states. The solution of the set of governing equations has been simultaneously obtained using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three potentials. To conclude, results are presented showing the impact of convective terms on the HAM transfer through a two-layer porous building envelope.

Modeling of Indoor Thermally Stratified Flows on the Basis of Eddy Viscosity/Diffusivity Model: State of the Art Review

Jun Gao, , Jia Ning Zhao, , Fu Sheng Gao, , Xu Zhang, — Wed, 14 Jan 2009 08:21:06 PST

Modeling of indoor air flow is one subject of significance in the area of building physics. As a multidisciplinary issue, the physics of air flow has been insufficiently investigated, especially for the thermally stratified flows that is widely encountered in indoor environment, and for the perimeter zones where exterior environment contributes much to the indoor thermal environment through envelops. This article tries to do an extensive survey about how to model thermally stratified flows within the RANS framework based on a literature review. First, turbulence phenomenon with the indoor thermally stratified flow is briefly interpreted. Second, the nonisotropic modification of Reynolds stress and heat flux is analyzed and the direct vertical correction is suggested to keep the simplicity of traditional EVM/EDM. Finally, a simpler nonisotropic correction is discussed, which adapts turbulent diffusion to the attenuated turbulence through modifying the isotropic turbulent viscosity and/or turbulent Prandtl number. This study, based on a limited but in-depth literature review, intends to sum up some useful suggestion on the modeling of indoor thermally stratified flows on the principles of simplicity and reliability. The motivation and objective of the present work is to achieve some fundamental guidelines for the evaluation of existing models and the exploration of new models.

Determination of the Air/Fiber Conductivity of Mineral Wool Insulations in Building Applications Using Nonlinear Estimation Methods

Veiseh, S., Hakkaki-Fard, A., Kowsary, F. — Wed, 14 Jan 2009 08:21:06 PST

The limitation of the experimental methods in determining the thermal conductivity due to the air/fiber conduction in mineral wools leads the use of inverse parameter estimation techniques. However, in some circumstances, the convergence of the inverse solution is impossible due to the correlation of the involving parameters and the existing noises in measurement data. In this article two relations are obtained to model thermal conductivity of the air/fiber conduction k(T, ), for glass fiber and rock wool insulations in temperatures between 0°C and 20°C, commonly encountered in building applications. Levenberg Marquardt's (LM) method is used for estimating of the unknown parameters in this article. Considering the nonlinear nature of the inverse problem and the correlation of the involving parameters and the existing noises in measurement data, the convergence of the problem is acceptable. The results obtained by the estimated parameters agree extremely well with experimental measurements of the Heat Flow Meter (HFM) apparatus conducted at the Building and Housing Research Center (BHRC).

Measurement of Return Leaks in Residential Buildings in North Louisiana

Erinjeri, J. J., Witriol, N. M., Katz, M. — Wed, 14 Jan 2009 08:21:06 PST

Heating ventilation and air conditioning (HVAC) duct leaks in residential buildings can occur both at the supply side and the return side of the duct system. The objective of this study is to develop a methodology to measure return duct leakage at the actual HVAC system operating pressure. Two techniques are developed and field studies are performed to measure return leakage in residential homes. The weighted-average return leakage in residential homes in North Louisiana is determined to be 58L/s (116 ft³/min) and the weighted-average equivalent orifice leakage area is determined to be 0.018 m² (28.5 in.²). The weighted flow exponent of return leakage is determined to be 0.55.