For isotropic materials with a significant fraction of micro-pores, the cumulative water intake per unit of inflow surface area typically yields a linear function of the square root of time elapse. The authors postulate that this dependence has a limited range of validity. The validity of this approximation starts from an initial period that is inversely proportional to the rate of water intake and ends much before the material reaches the capillary moisture content. Experimental investigation presented here uses a differential presentation of the cumulative water inflow and clearly indicates that material such as calcium silicate or brick belong to a broad class of materials characterized by a constant water absorption coefficient (A-coefficient). Initial period varies from ½ to 4 minutes. On the other hand, materials with a multiple pore-system such as an Aerated Autoclaved Concrete (AAC) may display a systematically varying A-coefficient. Authors propose a test procedure limited to 1-hour duration that can be used to derive a practical and reproducible value of A-coefficient.
Vapor-permeable building envelopes have received renewed interest because they can moderate indoor humidity levels and improve the drying of the envelope during summer condensation conditions. In this paper, the moisture performance of a vapor-permeable building envelope is presented with field measurements and numerical simulations. The results show that the diffusion resistance of the internal surface should be greater than that of the external surface (typically recommended ratio of 3: 1 or 5: 1), but that the vapor resistance of the vapor retarder can be significantly below that provided by polyethylene and still result in a safe structure, even in a cold climate.
The hygrothermal performance of assemblies can be assessed by testing and monitoring large-scale specimens exposed to realistic conditions under controlled laboratory settings. Moisture transfer cannot be measured directly, but moisture content can be monitored. A large-scale test of two flat roof assemblies filled with cellulose insulation is performed. The six-and-a-half-month test simulated a complete wetting and drying cycle from winter to summer. The wood structure and the cellulose insulation are the object of an extensive moisture content monitoring which included 102 resistance-type electronic moisture probes and 120 gravimetric specimens. The objective of this paper is to report on the changes in moisture content in the cellulose insulation as a function of the different air leakage paths.
Vacuum insulation panels (VIPs) are distinguished by their outstandingly low thermal conductivity. In the evacuated state, the VIPs being examined in this study (which have fumed silica as a core material) have a thermal conductivity of 4 10 3 W/(m K). Gases (N2, O2, H2O,...), which penetrate the foil cover cause an increase in pressure and water content and hence, an increase in the thermal conductivity. To determine these increases, VIPs have been manufactured with laminated aluminum foils (AlF) and aluminum-coated multilayer foils (MFs). The pressure and mass increases are determined at various temperatures, humidity, and with various panel formats. Large differences in the rates of pressure increases (1 -70 mbar/yr) and in the rates of mass increases (0.02-4 mass%/yr) are recorded, depending on the foil type, climatic conditions, and panel formats. From these measurements, the air and vapor transmission rates of the foil covers and their dependence on temperature, relative humidity, and panel size are derived. Using these gas transmission rates, it is possible to estimate which pressure increases are to be expected for panel formats and climatic conditions occurring in building applications. With laminated Al foils and selected Al-coated multilayer foils, rates of pressure increases below 1-2 mbar/yr are achieved. The rates of mass increase for typical climatic conditions for laminated Al foils are significantly below 0.1 mass%/yr, while with Al-coated multilayer foils, depending on the foil quality, mass increases per time of up to 1 mass%/yr are recorded. Increases in gas pressure per time of 1 -2 mbar/yr lead to relatively small increases in thermal conductivity, allowing applications in the construction sector, where service lives of several decades are required. With respect to the humidity-related increase in thermal conductivity, one has to know the climatic conditions, which have a strong influence on the increase in mass, and, above all, the precise dependence of the thermal conductivity on the humidity in the VIP.
The climatic conditions (temperature, relative humidity, and water vapor pressure) on both sides of vacuum insulation panels (VIPs) that were integrated into different building constructions are measured every hour. The influence of these conditions on the increase in air pressure and water content within the VIPs is estimated using a calculation model. The results of these model calculations are correlated with the pressure and mass measurements on VIPs, exposed to actual climate but removed for laboratory measurements. First, we find that upon use of the temperature-dependent air permeation rates for VIPs, the linear increase within the VIPs can be predicted reliably. Thus, it is sufficient to use annual average temperatures for these estimates. Second, the mass increase of VIPs due to infusion of water vapor through the barrier foil can be determined using the calculation model. The ‘driving’ force in this case is the difference in vapor pressure across the foil cover, which decreases with time, once the water vapor pressure within the VIP starts increasing. In effect, the water vapor pressure and the water content within the VIPs reach equilibrium. Depending on the climatic conditions, the maximum water content between 3 and 7 m% can be predicted.
For vacuum insulation panels (VIPs) with fumed silica kernels and foils as cover, a calculation model is developed to predict the service life. It is defined as the period during which the thermal conductivity of the VIP has risen 50% due to infusion of air and moisture. Two panel sizes, 50 ×50 × 1 cm3 and 100 × 100 × 2 cm3 are considered. For VIPs with laminated aluminum foils, calculated service lives of many decades are determined. For VIPs with aluminum-coated multilayer foils, shorter service lives still above 20 are calculated. This is due to the higher water vapor transmission through the Al-coated multilayer foils (compared to laminated Al foil) and the humidity-related increase in thermal conductivity. Overall, our model predicts service lives, which are large enough for applications of VIPs in buildings. An open question that remains is the long-term stability of the foil cover.
The influence of moisture in vacuum insulation panels (VIPs), with fumed silica kernels, on their thermal conductivity has been investigated. The VIPs are produced with different water contents. The thermal conductivities at different water contents are measured under stationary conditions in a hot-plate apparatus with an average temperature of 10°C (plate temperatures are 0 and 20°C). The increase in thermal conductivity is approximately proportional to the water content. The increase is ≈0.5 × 10 -3 W/(m K) per mass% of water. For typical middle European climate, a maximum moisture content of ≈6 mass% can be expected, which corresponds to a maximum increase of thermal conductivity of ≈3 × 10 -3 W/(m K) for VIPs with fumed silica kernels.
In architecture, the outstandingly low thermal conductivity of vacuum insulation panels (VIPs) of 4 103 W/(m K) allows to realize thin thermal insulation layers. Typical U-values are 0.2 W/(m2 K) for a 2 cm-thick VIP. On the other hand, with vacuum-insulated faç ades the relative effect of thermal bridges is much stronger than that for conventionally insulated buildings. In this work, different thermal bridges are investigated. Especially with VIPs with laminated Al foils (here the aluminum foil is 8 mm thick and laminated on both sides with plastic foils of 15 mm PET and 50 mm PE), strong thermal bridges around the perimeter of the VIPs occur. Also the mounting system can have a strong negative effect on the thermal performance of VIP-insulated walls. As our calculations show, the effect of the thermal bridge depends strongly on the thermal contact of the VIPs with the wall. Therefore, it is necessary to optimize every vacuum-insulated construction in order to make the best use of the low thermal conductivity of VIPs. As an example, we describe how VIPs were effectively integrated into a renovated gable faç ade and into a new ultra-low energy timber building.
This paper discusses the prediction of the density necessary for the prevention of the settling of hygroscopic and nonhygroscopic loose-fill insulation in walls, when the criteria for creep in granular materials are applied. The practical use of a theoretical framework presented earlier and the material characteristics used to develop an equation from which the necessary density can be predicted, are described. The empirical framework allows a quantitative approach to the problem of achieving nonsettling of a loose-fill insulation material in a given wall. The study involves the use of empirically derived equations to predict the density of a loose-fill insulation material required to ensure volume stability in a wall exposed to one type of cyclical humidity conditions. An empirical model for predicting the creep of materials has been developed in earlier papers. The creep was determined from the corresponding tests. The use of the method is demonstrated by showing the results from a full-scale wall test. The method is applied for different cavity sizes together with the associated characteristics of four hygroscopic and one nonhygroscopic loose-fill insulation materials exposed to the same climatic conditions. In addition, it is shown that for many repeated cyclical humidity conditions, the creep of the hygroscopic and nonhygroscopic loose-fill insulation materials asymptotically approached equilibrium.
In this paper, measurements are presented which quantify the mass transfer of tracer gases and water vapor between indoor air and a permeable and hygroscopic building envelope. The transfer of tracer gases through the envelope requires the entire envelope to be permeable, while the transfer of moisture requires sufficient hygroscopic mass to be in contact with the indoor air. The results show that mass transfer can improve the indoor air quality and climate. The diffusion of gases through the building envelope significantly increases the effective ventilation rate for poorly ventilated rooms, but only moderately increases the effective ventilation for well-ventilated rooms. Moisture transfer, on the other hand, has a significant influence on the indoor humidity for both poorly and well-ventilated rooms.
This paper presents the results of a research program concerning the thermal and moisture conditions of coarse-grained fill layer underneath a slabon-ground structure in Finnish climate. Conditions were studied by laboratory tests and long-term in situ surveys. Results showed that the temperature behavior of the fill layer underneath a heated building is strongly diverged. The effect of the outdoor seasonal temperature variation has a major effect on the edge of the fill layer, but under the center part of the slab the temperature remains almost constant throughout the year. The measured moisture contents of the fill at the survey building were close to the hygroscopic equilibrium moisture content of coarsegrained materials in relative humidity RH 1/4 100% in prevailing temperatures. The temperature and moisture conditions of the fill material were such that the water vapor diffusion flow is directed from the subsoil towards the drier indoor air.
The purpose of the study was to determine the hygrothermally critical climatic conditions, to assess the heat and moisture performance of building envelope systems, and to design buildings for a longer service life. Estonian climate data have been analyzed based on six weather stations over a 30-year period, from 1970 to 2000. Ten percent-level critical reference years for hygrothermal calculation, for two moisture performance criteria, risk of water vapor condensation and mould growth, have been determined. Air temperature and relative humidity was used for climatic data.
The construction of a building inevitably changes the microclimate in its vicinity. In particular near high-rise buildings, high wind velocities are often introduced at pedestrian level that can be experienced as uncomfortable or even dangerous. Therefore, the design of a building should not only focus on the building envelope and on providing good indoor environment, but should also include the effect of the design on the outdoor environment. The outdoor environment of a building, in particular related to wind, has received relatively little attention in the Building Physics community. The present paper addresses Building Physicists and focuses on the outdoor wind environment for pedestrians. First, a literature review on pedestrian wind studies is provided. The relation between wind effects, wind comfort, wind danger and wind climate is outlined. A brief review on wind tunnel and numerical modeling of building aerodynamics and pedestrian wind is given. The typical wind flow pattern around buildings and the related wind environment at pedestrian level are discussed. Second, these problems are illustrated by means of four practical examples, where the unfavorable pedestrian wind environment has been, is or should be a matter of serious concern for the building designers and the building owner.
While the consideration of the heat transfer resistances on surfaces is natural in practice, the similar effect of the moisture transport is not considered. New investigations showed that in some cases the transition resistance of vapour should not be neglected. Experimental studies have been carried out to determine the rate of vapour transfer at various surfaces. For the transition of vapour at different surfaces specific differences could be detected. A definition by cases concerning the involved surface must be made. This distinction can be met with the aid of the so called vapour transfer resistance, similar to the usual description of heat transfer. Appropriate numerical values are presented and discussed.
Thermal resistivity data are reported for loose-fill cellulose manufactured in Korea and the United States over the temperature range 40 to 100 F. A representative thickness of 1.1 in. for thermal test specimens was determined for cellulose insulation in the density range that was studied. Measurements for insulations with known amounts of fire retardant chemical showed a negligible effect of the fire retardant on the thermal resistivity of the insulation. The test data show that the U.S.-made loose-fill cellulose that was studied had a 5% greater thermal resistivity than the Korean-made product. This difference in thermal resistivity could be due to differences in the re-cycled paper being used in the two countries to produce cellulose insulation
Buildings are large consumers of energy in all countries. In harsh climatic conditions, a substantial share of energy goes to the air-conditioning of buildings. This air-conditioning load can be reduced through many means; notable among them is the proper design and selection of building envelope and its components. The use of thermal insulation in building walls and roof does not only contribute in reducing the required air-conditioning system size but also in reducing the annual energy cost. Additionally, it helps in extending the periods of thermal comfort without reliance on mechanical air-conditioning especially during interseasons periods. Therefore, proper use of thermal insulation in buildings enhances thermal comfort at less operating cost. However, the magnitude of energy savings as a result of using thermal insulation vary according to the building type, the climatic conditions at which the building is located as well as the type, thickness, and location of the insulating material used. The question now is no longer should insulation be used but rather which type and how much. The objective of this paper is to address the impact of building envelope thermal design on the effectiveness of thermal performance of buildings in hot climates. It emphasizes the role of proper material selection in achieving the desired objectives as a function of building type and climatic conditions.
Application of limit state approach for the reliability analysis of building performance in the context of building physics characteristics is presented. Analysis of the probability of surface condensation on windows carried out on the basis of statistical description of the climatic parameters and the moisture production is shown as an example of application of this technique for reliability analysis of moisture performance of a building component. The first-order reliability method (FORM) is employed to estimate the probability of performance failure. The results of probabilistic approximations are verified with the help of statistical analysis of the results obtained from deterministic simulations.
The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound. To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.
Implications of moisture in building and construction are of interest to the international community because of their huge economical consequences, including effects on health, maintenance and repair, retrofitting and conservation, as well as on common welfare. The present day knowledge offers a potential to tackle such problems, both in the design process and during the service life of building. In 2001, the European Commission initiated the project ‘‘HAMSTAD’’ (Heat Air and Moisture Standards Development) to propose a better modelling methodology than the traditional Glaser method. HAMSTAD focused on the development of draft standardisation procedures on determination methods of moisture transfer properties and a draft methodology for certification of advanced moisture modelling codes. To stimulate competitiveness and progress, the project was carried out following an ‘open methodology’ instead of a system of deterministic and prescriptive (pre-) standards. This paper outlines the project and highlights the main outputs, serving as an introduction to the following more detailed research papers resulting from that work.
Several advanced non-destructive techniques are available to measure the evolution of content profiles with time, allowing the analysis of unsaturated flow and the determination of the moisture diffusivity of porous building materials. The reliability of six different techniques is investigated: the NMR-technique, the MRItechnique, the γ-ray attenuation technique, the capacitance method, the X-ray projection method and the TDR-technique. All of them were applied to measure the moisture content evolution during free uptake experiments on two building materials. Considering the limitations of some of the techniques, a good overall agreement is obtained. The work presented is an outcome of the EU-initiated HAMSTAD-project.
