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An approach to determining the water vapour transport properties of buildingmaterials
KTH, Superseded Departments, Building Sciences and Engineering.ORCID iD: 0000-0003-0615-4505
2003 (English)In: Nordic Journal of Building Physics: Acta Physica Aedificiorum, ISSN 1402-5728, Vol. 3Article in journal (Refereed) Published
Abstract [en]

An analytical model, based on the literature, is used to show how the material parameters that rule the transport of water vapour in porous building materials can be evaluated by tracer gas measurements. A distinction is made between viscous flow, Knudsen diffusion and continuum diffusion. Each transportmechanism is formulated by explicit functions of the textural properties of the material and the thermodynamic properties of the gas. Tracer gas measurements can therefore be used to reveal the textural properties of amaterial, from which the potential water vapour transport, in gas phase, can be derived from the thermodynamic properties of water vapour.The well known concentration dependency of the diffusion coefficient is explained with the random hoppingmodel. The random hopping describes how the surface diffusion of adsorbed molecules relates to the amount adsorbed and the energies involved in adsorption. It is also described how the activation energies of surfacemigration can be obtained from sorption measurements and how the constant parameters of the surface diffusioncoefficient can be determined by stationary or transient wet-cup measurements.

Place, publisher, year, edition, pages
2003. Vol. 3
Keyword [en]
Diffusion, tracer gas, surface diffusion, Random Hopping Model, surface diffusion, porous building materials, time lag method.
National Category
Building Technologies
Identifiers
URN: urn:nbn:se:kth:diva-13308OAI: oai:DiVA.org:kth-13308DiVA: diva2:323515
Note
QC 20100611Available from: 2010-06-11 Created: 2010-06-11 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Alternative methods for analysing moisture transport in buildings: Utilisation of tracer gas and natural stable isotopes
Open this publication in new window or tab >>Alternative methods for analysing moisture transport in buildings: Utilisation of tracer gas and natural stable isotopes
2003 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

New methods, based on tracer gas measurements and isotopicanalysis can be used to evaluate the moisture properties ofbuilding materials and provide the means for forensic analysisof the origins and history of excessive water in buildings, theimmediate practical consequences of which will be the abilityto improve the moisture performance of constructions.

It is shown, in theory and through measurements how thewater vapour permeability of porous building materials can witha good degree of accuracy be estimated with tracer gasmeasurements that provide an efficient alternative to the cupmethod. Complementary measurements may be carried out in orderto evaluate the contribution of surface diffusion and theeventual enhancing effects of moisture content on the diffusioncoefficient. The Random Hopping Model is used to illustrate howthe surface diffusion coefficient depends on the amountadsorbed and the activation energy of migration that can beevaluated from the sorption isotherms.

It is explained how the abundance ratios of two of the mostordinary isotopes of hydrogen and oxygen in water can be usedto determine its history. These isotopes are stable and givethe water a distinct signature that can be used to reveal itssource as shown in a case study. In a contrary manner themeasured isotopic separation can be used to determine therelevance of different transport processes and reactions. It isof central importance that not only does the magnitude ofisotopic separation for the reactions vary for deuterium andoxygen-18 but even the ratio thereof. One of the challenges hasbeen to construct an experimental method for retrieving samplesof water for comparison.

Furthermore this thesis includes an evaluation of a new typeof a light weight construction with loose-fill cellulose fibre,in which the conventional polyethylene vapour barrier has beenreplaced with polypropylene fabric. With a verified model ithas been investigated how the construction would perform fordifferent internal moisture loads and reference climate fromthe literature. The results suggest that this type ofconstruction is not to be recommended.

KEYWORDS:tracer gas, water vapour permeability,diffusion, surface diffusion, isotopic analysis, deuterium,oxygen-18, fractionation, vapour barrier, transient numericalmodelling of diffusion.

Place, publisher, year, edition, pages
Stockholm: KTH, 2003. xii, 57 p.
Series
Trita-BYT, 191
Keyword
tracer gas, water vapour permeability, diffusion, surface diffusion, isotopic analysis, deuterium, oxygen-18, fractionation, vapour barrier, transient numerical modelling of diffusion
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-3509 (URN)
Public defence
2003-05-09, 00:00
Note
QC 20100611Available from: 2003-04-28 Created: 2003-04-28 Last updated: 2010-06-11Bibliographically approved

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Gudmundsson, Kjartan

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