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ARTICLE IN PRESS640J. Kwiatkowski et al. / Building and Environment 44 (2009) 633–6420.0080.007<strong>hysteresis</strong>adsorptiondesorptionaverageWater content [kg/kg]0.0060.0050.0040.0030.0020 24 48 72 96 120 144 168 192 216 240 264 288 312 336Time [h]Fig. 13. Moisture mass content in the gypsum board at the depth <strong>of</strong> 6.25 mm, for four calculation variants, with changes in air relative humidity.Fig. 14. Scheme <strong>of</strong> the Humi-mur implementation into the TRNSYS simulation tool.It has been shown that the Humi-mur model implemented intoTRNSYS environment allows to predict indoor relative humidity <strong>of</strong>the considered zone [27]. Also, the model <strong>of</strong> moisture bufferingwas used to examine the <strong>influence</strong> <strong>of</strong> materials water vapourstorage on moulds growth risk on the internal wall surfaces. It wasshown that materials that buffer water vapour can significantlydecrease the risk <strong>of</strong> mould growth [28].In order to check if <strong>hysteresis</strong> effect <strong>influence</strong>s moisturebuffering <strong>of</strong> the materials at the building level, some additionalcalculations in the TRNSYS programme have been performed.In the simulations two models have been used: the firstwithout <strong>hysteresis</strong> and with average sorption isotherm, and thesecond with <strong>hysteresis</strong> <strong>of</strong> sorption curves. The calculations <strong>of</strong>indoor absolute humidity were made for a small room(2.5 3.5 2.6 m 3 ) with the gypsum board (30 m 2 ) as a bufferingmaterial. The considered room had two external walls and onewindow. It was assumed that the rest <strong>of</strong> the zone’s envelopes wereadiabatic. Additional gains <strong>of</strong> heat and water vapour were alsoadded. The periods with heat and humidity production wereestablished each morning (from 6 to 8 a.m.), afternoon (from 12 to14 p.m.) and evening (from 18 to 21 p.m.). Also a ventilation rate<strong>of</strong> 22.75 m 3 /h was assumed. The simulations were performed for 1year, using weather file Warsaw (Poland). In order to avoid too lowtemperatures in the zone, the heating season with a constanttemperature <strong>of</strong> 20 1C was set from the 1 October till 15 May. In thesimulation the time step <strong>of</strong> 600 s was used for the model without<strong>hysteresis</strong> effect and <strong>of</strong> 72 s for the model with <strong>hysteresis</strong> effect.Hourly results were compared.7.2. Results and discussionIn Fig. 15 the relative difference <strong>of</strong> water content <strong>of</strong> the indoorair between the models is presented. Although most <strong>of</strong> the timethe difference is smaller than 5% and the mean difference equals3.1%, it can be noticed that sometimes the difference reaches highvalues. It was calculated that for 23% <strong>of</strong> the time, the differencepasses the limit <strong>of</strong> 5% and for 6% <strong>of</strong> the time is higher than 10%.The difference between results <strong>of</strong> the absolute humidity obtainedusing the model without <strong>hysteresis</strong> and with <strong>hysteresis</strong> can reachas much as 20%. The highest values <strong>of</strong> the difference wereobtained when the absolute humidity in the zone had the highestor the lowest values. It was also noticed that for the maximumvalues <strong>of</strong> the zone absolute humidity, the model without<strong>hysteresis</strong> was decreasing water content in the air moresignificantly than the model with the <strong>hysteresis</strong> effect. Similarsituations occurred for the minimum values <strong>of</strong> the zone absolutehumidity. The model only with the average sorption curve wasincreasing the water content <strong>of</strong> the air more significantly than themodel with <strong>hysteresis</strong>. This phenomenon shows that using in thecalculation model only with the average sorption curve reducesthe amplitude <strong>of</strong> indoor absolute humidity more than the morerealistic model with <strong>hysteresis</strong>. Neglecting <strong>hysteresis</strong> in thesorption curve leads to overestimations <strong>of</strong> the moisture bufferingcapacity <strong>of</strong> materials. For some materials those differences willnot provide significant errors but for others (such as wood) theymay lead to underestimations <strong>of</strong> risk <strong>of</strong> condensation or <strong>of</strong> mouldgrowth.
ARTICLE IN PRESSJ. Kwiatkowski et al. / Building and Environment 44 (2009) 633–642 64110000Number <strong>of</strong> hours10001001012.512.5 7.517.522.5Relative difference in water content between two models [%]Fig. 15. Relative difference in water content <strong>of</strong> the air between results obtained using the model without <strong>hysteresis</strong> and with <strong>hysteresis</strong>.Table 5Sensitivity <strong>of</strong> relative humidity calculations for different number <strong>of</strong> layers and timestep for the 150 mm thick sampleParameter Value Deviation(depth 50.0 mm)8. Conclusions and perspectivesDeviation(depth 100.0 mm)Number <strong>of</strong> layers 48 0.01% 0.00%72 Divergence DivergenceTime step (s) 30 0.01% 0.01%120 Divergence DivergenceTable 6Sensitivity <strong>of</strong> relative humidity calculations for changes in sorption isotherm andvapour permeability for the 37.5 mm thick sampleProperty Adjusted Deviation(depth 12.5 mm)Sorption isotherm ( ) 3.50% 6.15%(+) 3.61% 6.19%Deviation(depth 25.0 mm)Vapour permeability ( ) Divergence Divergence(+) 2.70% 4.94%Table 7Sensitivity <strong>of</strong> relative humidity calculations for changes in sorption isotherm andvapour permeability for the 150 mm thick sampleProperty Adjusted Deviation(depth 50.0 mm)Sorption isotherm ( ) 0.48% 3.42%(+) 12.11% 10.67%Deviation(depth 100.0 mm)Vapour permeability ( ) Divergence Divergence(+) 4.37% 3.49%A new module for the precise representation <strong>of</strong> mass transferin materials in contact with indoor air, called Humi-mur, waselaborated and validated in this work. It was then applied toestimate the sensitivity <strong>of</strong> the results to uncertainty in measuredmaterial properties and the impact <strong>of</strong> <strong>hysteresis</strong> effect. Humi-murwas also successfully implemented in a whole-building simulationcode, TRNSYS. The new model allows considering severaldifferent materials, and for precise definition <strong>of</strong> properties formoisture transfer.Reasonable estimation <strong>of</strong> experimental uncertainty resulted inthe deviation <strong>of</strong> approximately 6% in calculated results. It isimportant to know better the accuracy <strong>of</strong> predictions. The simulationtool cannot give results more precisely than the input data. Somekind <strong>of</strong> uncertainty or error-bar should then complement thesimulation results. For some variation <strong>of</strong> parameters the divergenceproblems occur (see Tables 5–7). The difficulty in obtaining resultsfrom numerical calculation is related mainly with too long time stepor too thin layer in the material. The stability <strong>of</strong> numericalsimulation will be investigated in the near future.Concerning <strong>hysteresis</strong> in the sorption isotherm, we showedthat using only one <strong>of</strong> the sorption isotherm equations (adsorptionor desorption) leads to significant differences. More preciseresults were achieved if the average <strong>of</strong> the adsorption anddesorption equations was used in the model. For less-precisecalculations it appeared to be a reasonable approximation <strong>of</strong>mean behaviour. However, for strong variations in boundaryconditions, it is not well suited. Indeed, convergence to some kind<strong>of</strong> quasi-permanent state is much slower if <strong>hysteresis</strong> isconsidered. This effect <strong>influence</strong>s the dynamic behaviour <strong>of</strong>materials. It was also shown that in realistic conditions (a roomunder variable climate and hygrothermal loads), neglecting<strong>hysteresis</strong> leads to overestimation <strong>of</strong> moisture buffering properties<strong>of</strong> materials in contact with the indoor air. In some cases suchoverestimation may conduct to the underestimation <strong>of</strong> risks <strong>of</strong>mould growth and/or condensation.References[1] Padfield T. Humidity buffering <strong>of</strong> the indoor climate by absorbent walls [online]. In: Proceedings <strong>of</strong> the fifth symposium on building physics inNordic countries, vol. 2. Goteborg: Chalmers University <strong>of</strong> Technology;1999. p. 637–44 Pdf available at: /http://www.padfield.org/tim/cfys/appx/pubs.phpS.[2] Osanyintola OF, Simonson CJ. Moisture buffering capacity <strong>of</strong> hygroscopicbuilding materials: experimental facilities and energy impact. Energy andBuildings 2006;38:1270–82.[3] Hens H. Final report, vol. 1, Task 1: modelling, IEA Annex 24. ACCO Leuven;1996.[4] Canada Mortgage and Housing Corporation (CMHC). Review <strong>of</strong>hygrothermal models for building envelope retr<strong>of</strong>it analysis. Researchhighlights, technical series 03-128./http://www.cmhc-schl.gc.ca/publication/en/rh-pr/tech/03-128-e.htlmS.[5] Künzel HM. Simultaneous heat and moisture transport in building components.One- and two-dimensional calculation using simple parameters. IRBVerlag; 1995.
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