Is the moisture buffer value a reliable material property to characterise the hygric buffering capacities of building materials? Staf Roels and Hans Janssen Laboratory of Building Physics, K.U.Leuven, Belgium ABSTRACT. The Moisture Buffer Value and its corresponding test protocol are analysed in detail by a numerical study on three materials: wood fibreboard, plywood and gypsum plaster. First the reliability of the test protocol proposed by the NORDTEST-project is investigated by varying influencing parameters as thickness of the specimen, surface film coefficient and accuracy of the imposed signal. The obtained moisture buffer values are then compared with calculated moisture buffer values based on an analytic solution of the problem. Finally, the practical applicability of the MBV as characteristic value for hygric buffering is investigated by confronting the MBV of the different materials with the dynamic moisture response of a small room with each of the materials used in turns as finishing material. 1. Introduction Since the start of the workshop on moisture buffer capacity in the Nordic countries and the corresponding Nordtest-project a new material property slipped into the building physics vocabulary: the moisture buffer value, abbreviated to MBV. With the moisture buffer value a simple characterisation of building materials with respect to moisture buffering performance is aimed at. The need for such a characterisation results from the increasing interest to use the capability of finishing materials like gypsum, wood, to buffer hygroscopic moisture and hence reduce possible variations in the indoor relative humidity. In the NORDTEST-project a definition of the moisture buffer value is proposed as well as a test protocol for the experimental determination of the moisture buffer value. The definition of the moisture buffer value sounds (Rode et al., 25): The Moisture Buffer Value (MBV) indicates the amount of moisture uptake or release by a material when it is exposed to repeated daily variations in relative humidity between two given levels. When the moisture uptake from beginning to end of the exposure to high relative humidity is reported per open surface area and per % RH variation, the result is the MBV. The unit for MBV is kg/(m².%rh).. The corresponding proposed test protocol from the NORDTEST-project is based on climatic chamber tests, where a specimen is subjected to environmental changes following a square wave signal. Beforehand, the specimen have to be sealed on all but one or two surfaces, with the thickness of the specimen at least 1 mm or the moisture penetration depth for daily humidity variations, if larger. The test protocol proposes 8h + 16h cycles: 8 hours of high relative humidity (proposed value: 75% RH) followed intermittently by 16 hours of low humidity (proposed to be 33% RH). The initial moisture content of the specimen should be in equilibrium with 5% RH. Since this value is not necessary corresponding to the mean RH inside the specimen during the test, it is proposed to continue the test until the change in weight over the cycle varies by less than 5% from day to day, with a minimum of three cycles. Due to the supposed link between the MBV and the moisture effusivity b m, the moisture buffer value can also be calculated based on the sorption isotherm and cup test data. Rode et al. (25), however, draw attention to the fact that a discrepancy between calculated and measured moisture buffer value can be observed because the moisture effusivity is based on material properties determined under steady state conditions, whereas the proposed MBV-test protocol is a dynamic experiment. Peuhkuri and Rode (24) mention such a difference when reporting measurements on gypsum board in the framework of the Annex 41-project of the IEA. They found that the MBV calculated from steady state material properties was almost three times higher than the MBV measured in dynamic tests. A possible explanation for the observed difference was not given. Aim of this paper is to investigate the reliability of the proposed test protocol and the practical applicability of the MBV as characterising value for moisture buffering performance. Both issues are studied numerically on three different materials. First the sensitivity of the obtained MBV is investigated by varying influencing parameters of the test protocol as accuracy of the imposed signal, surface film coefficient, thickness of the material layer. In the next paragraph the MBV measured according to the test protocol is compared with the MBV calculated from steady state material data. Finally, the MBV of the three materials investigated, is confronted with the dynamic moisture response of a small room both for daily as for short term variations with each of the materials used in turns as finishing material for the walls.
2. Materials investigated The test protocol proposed by the NORDTEST-project and the practical applicability is numerically investigated for three materials: wood fibreboard (WFB), plywood (PW) and gypsum plaster (GP). Those three materials were chosen because of their differences in water vapour permeability and sorption isotherm in a way that a different penetration depth and moisture buffering value could be expected. Material data of plywood and wood fibreboard were obtained from Kumaran (1996). For the gypsum plaster the sorption isotherm and cup tests results determined by Goossens (23) were used. Figure 1 shows both the sorption isotherm in moisture content w (kg/m³) and water vapour resistance factor µ (-) as a function of relative humidity for the three materials. moisture content (kg/m³) 15 1 5 WOOD FIBRE BOARD vapour resistance factor (-) 3 2 1 WOOD FIBRE BOARD.2.4.6.8 1.2.4.6.8 1 moisture content (kg/m³) 2 15 1 5 PLYWOOD vapour resistance factor (-) 25 2 15 1 5 PLYWOOD.2.4.6.8 1.2.4.6.8 1 moisture content (kg/m³) 4 3 2 1 GYPSUM PLASTER vapour resistance factor (-) 3 2 1 GYPSUM PLASTER..2.4.6.8 1. Figure 1.-..2.4.6.8 1. Sorption isotherm (left) and water vapour resistance factor (right) for the three different materials investigated: wood fibreboard (top), plywood (middle) and gypsum plaster (bottom). The dots correspond to the measured values, the continuous lines to the analytic fits used in the numerical simulations. To use the measured data in the numerical modelling, sorption isotherm and vapour resistance factor are described by an analytic function of the form: (1 n) n w = w ( m ) n sat 1+.ln( Φ) (1) 1 µ = (2) a + be cφ
with m, n and a, b and c parameters. The parameters of the analytic fit for the different materials are given in Table 1. The fitted curves are also presented as the continuous lines on Figure 1. In Table 1 also the penetration depth (for daily variations) of the three materials is given at the mean RH of the test (5%). Note the important difference in penetration depth between the three materials: PW around 1 mm, WFB almost 7 mm and GP more than 3 mm. Table 1.- The parameters for the analytic fit of the sorption isotherm and water vapour resistance factor for the different materials. In addition the calculated penetration depth at 5%RH is given. Wood fibre board (WFB) Plywood (PW) Gypsum plaster (GP) sorption isotherm w sat 15 2 3.36 m -91.41-8.58-68.6 N 1.416 1.337 1.8 vapour resistance factor A.85.5.268 B 2.47*1-3 5.46*1-6. C 5. 1.5 - penetration depth (cm) at 5% RH.687.125 3.332 3. Sensitivity analysis of the test protocol The proposed test protocol of the moisture buffer value has some weak points: 1) as stated by Rode et al. (25) the MBV is only a true material property if the convective surface resistance during the test is negligible compared to the vapour resistance of the material, 2) depending on the equipment, the desired step in the boundary condition may take some time to realise, which may influence the obtained MBV, and 3) the test protocol states that the thickness of the specimen has to be taken thicker than the moisture penetration depth for daily humidity variations. However, since the MBV is just introduced as an easy to measure value (without having to measuring all hygric properties), how can the penetration depth and hence, the necessary thickness be known? Therefore, the influence of the surface transfer coefficient, the time needed to achieve a step in RH in the chamber, as well as the thickness of the specimen is numerically investigated for the three materials. 3.1. Influence of surface film coefficient In the numerical simulations the surface transfer coefficient is varied between 3.1-6 s/m (an extremely high value in a way that the effect of the boundary layer can be neglected) towards 1.1-8 s/m (a low value even for indoor conditions corresponding to hardly any air flow above the specimen). Simulations are performed for all three materials and for five different values of the surface coefficient. Ten cycles have been simulated and for all three materials the MBV is determined on the results for the last day. To be far above the moisture penetration depth for daily humidity variations, all specimens were supposed to be 1 cm thick. Table 2 gives an overview of the obtained MBV s for the three materials. Figure 2 shows the predicted influence of the surface film coefficient on the hygric response of a sample of wood fibre board. For the wood fibre board the difference in surface film coefficient results in a MBV 33-75% varying between 2.52 and 1.74 g/m²%rh: a relative difference of 31%! Note however, that the WFB is a rather vapour open material with a high moisture capacity. As indicated in Table 2 the influence on the other two materials (plywood and gypsum plaster) is less, though far from negligible. As will be shown in paragraph 4, the surface film resistance may be one of the reasons for the deviation between measured MBV and MBV calculated from moisture effusivity. Table 2.- Influence of the surface film coefficient on the obtained MBV (in g/m².%rh) for the three materials investigated. β x 1-8 (s/m) Wood fibre board (WFB) Plywood (PW) Gypsum plaster (GP) 1 1.74.7.95 3 2.19.77 1.6 1 2.42.81 1.11 3 2.49.82 1.12 3 2.52.82 1.13
2.72 2.7 WOOD FIBRE BOARD moisture content (kg/m²) 2.68 2.66 2.64 2.62 3.1-6 3.1-7 3.1-8 1.1-8 decreasing β 2.6 2.58 6 12 18 24 time (hours) Figure 2.- Influence of the surface transfer coefficient on the hygric response of the sample (and hence on the MBV) as numerically predicted for wood fibre board.. 3.2. Influence of the time needed to achieve a step in RH The test protocol supposes a square wave signal with sudden steps from low to high relative humidity and reverse. In reality and depending on the equipment, increasing and decreasing the relative humidity inside the test chamber can take some time. To investigate the effect of an imperfect control of the step in RH on the obtained MBV, numerical simulation were performed in which increasing and decreasing the RH inside the chamber was supposed to take some time t. During this period t the boundary condition was assumed to evolve linearly with time. The time needed to reach the required RH was taken as a variable: from t= hours (perfect control of RH in the climatic chamber) to t=1.5 hours (a rather poor adjustment of the RH inside the climatic chamber). For all simulations the surface film coefficient was kept constant at 3.1-8 s/m. Figure 3 shows the imposed boundary condition for the two extreme values of t and the corresponding moisture response for wood fibre board. The influence on the obtained MBV is presented in Table 3. As can be seen, the influence is limited: even when it takes one hour and a half to achieve the required RH inside the chamber, the error on the obtained moisture buffer values is for all three materials less than 5%. 2.7 WOOD FIBRE BOARD.8 moisture content (kg/m²) 2.68 2.66 2.64 2.62 2.6 2.58 t.7.6.5.4 imposed RH (-) 2.56.3 18 24 3 36 42 square root of time (Vs) Figure 3.- Influence of the time needed to achieve the required RH inside the climatic chamber on the hygric response of the sample. The thin lines represent the RH in the climatic chamber, the corresponding response of the specimen is plotted as thick lines. In case of the dotted line the required RH inside the chamber is only reached after 9 minutes. This results in a systematically lower moisture content inside the sample and a slightly lower MBV.
Table 3.- Influence of the time needed ( t) to achieve the required RH inside the climatic chamber on the obtained MBV (g/m².%rh) for the three materials. t (hours) Wood fibre board (WFB) Plywood (PW) Gypsum plaster (GP) 2.19.77 1.6.5 2.17 1 2.12 1.5 2.9.73 1.2 3.3. Influence of thickness of the sample Although the test protocol requires the thickness of the specimen to be above the moisture penetration depth for daily humidity variations, one can expect that in reality this requirement will not be checked for the simple reason that in most cases the moisture penetration depth will not be known. Furthermore for some materials as board products, there will be no other possibility but measuring the commercially available thicknesses. Therefore, to investigate the influence of the thickness of the specimen on the obtained MBV, the prescribed test protocol was simulated with the thickness of the specimens as parameter. Again, the surface film coefficient was kept constant for all simulations at 3.1-8 s/m. Table 4 shows the results. As soon as the thickness of the specimen is less than the moisture penetration depth for daily humidity variations (respectively.69 and 3.33 cm for WFB and GP), the obtained MBV s drop. This is most pronounced for gypsum plaster where when going to thin samples the measured values are only 2% of the theoretical value. Comparing these results on gypsum plaster with the measurements and calculations performed by Peuhkuri and Rode (25) on gypsum board a very good agreement is found. They measured a moisture buffer value of.49 g/(m².%rh) on uncoated gypsum board, while they calculated a value of 1.43 g/(m².%rh) based on the moisture capacity and vapour permeability. Their conclusion however that the MBV determined from steady state material properties is significantly higher than the MBV determined form the dynamic tests is incorrect. It has all to do with the thickness of the specimen as provided by the manufacturer, which is for gypsum board below the moisture penetration depth for daily humidity variations. Note in Table 4, that also some small differences are found in the obtained MBV s even when the thickness of the specimens is larger than the penetration depth. These result from the fact that the thickness of the sample will influence the mean relative humidity over the sample. Table 4.- Influence of the thickness of the specimen on the obtained MBV for the three materials investigated. The values marked in light grey are those determined on specimen for which the thickness is less than the penetration depth. thickness (cm) Wood fibre board (WFB) Plywood (PW) 1 2.19.77 1.6 3.5 2.19 1.3 1.5 2.21.63 1 2.26.43.5 1.8.77.22 Gypsum plaster (GP) 4. Theoretically determined MBV versus dynamically measured MBV Instead of measuring the MBV in dynamic tests, the value can also be calculated based on the sorption isotherm and cup test data as there is a (conditional) link between the MBV and the moisture effusivity b m expressed in kg/(m².pa.s 1/2 ) w b m = δ p (3) p v where δ p is the water vapour permeability, w is the moisture content and p v the partial vapour pressure. To calculate the MBV from the moisture effusivity, the analytic solution for a step response on a semi-infinite sample is used (Peuhkuri and Rode, 25). It should be emphasised that the test protocol for the MBV does not really correspond to a step response on a semi-infinite sample and that a good agreement can only be expected if:
1) the thickness of the specimen is much larger than the moisture penetration depth and can be seen as semi-infinite for the imposed signal 2) the non-linearity of the material properties is negligible in the RH-range under consideration 3) the initial moisture content is uniform. Looking at the test protocol this means that it is assumed that after 16 hours of low relative humidity a uniform moisture content (in equilibrium with the low RH) is achieved and this at least in the relevant part of the specimen 4) the surface film resistance is negligible It has to be kept in mind that the analytic solution is derived for thermal problems, and, though there exists a similarity between the thermal behaviour and moisture behaviour, mainly the second requirement is for moisture problems mostly not fulfilled, making in contradiction to the thermal effusivity the moisture effusivity (and hence the MBV) not a constant value, but a continuous function of relative humidity. If we assume that all four requirements are fulfilled, the moisture buffer value can be determined as (Hagentoft, 1999): MBV =.1 2 p b t sat m (4) π This means that if we plot the change in moisture content (kg/m²) as a function of square root of time, a linear relationship can be expected with as slope the moisture effusivity multiplied with a constant. Figure 4 plots the data of the upload curve for WFB for the two extreme values of the surface film coefficient: 3.1-6 s/m and 1.1-8 s/m (see Figure 2). As can be seen an almost perfect linear relationship is found if the surface resistance is negligible. However, for lower values of the surface film coefficient, no constant linear relationship is found and the theoretical relationship between MBV and moisture effusivity will no longer be valid. Due to the third requirement (uniform initial moisture content), one can expect that even for a high value of the surface film coefficient the square-rootof-time behaviour of the descending branch will be less pronounced..1 WOOD FIBRE BOARD moisture uptake (kg/m²).8.6.4.2 3.1-6 1.1-8 decreasing β. 2 4 6 8 1 12 14 16 square root of time (s 1/2 ) Figure 4.- Moisture uptake plotted vs. square root of time for wood fibreboard. Only if the surface film resistance is negligible a linear relationship is obtained and the direct link between MBV and moisture effusivity can be calculated. In Figure 5 the evolution of the calculated MBV as a function of relative humidity is plotted for the three materials. On the same figure the values of the dynamically measured MBV in case of a very high surface film coefficient (Table 2, β = 3x 1-8 s/m) are plotted as straight lines between 33 and 75% RH. As can be seen, for all three materials a rather good agreement is found around 54% RH (mean value of low and high RH of the test protocol) between the two ways to determine the MBV if the surface film resistance is negligible and the thickness of the specimen is large enough as in the case presented. Though, the exact intersections of the dynamically determined MBV with the continuous lines of the MBV determined from the moisture effusivity are situated at different RH for the three materials (indicated by the arrows on Figure 5). This can probably be attributed to the violation of the second requirement and the fact that also the requirement of a uniform initial moisture content in
agreement with the low relative humidity will probably not be met due to the dynamic nature of the test protocol. As a result, comparing the measured MBV with the calculated one is less straightforward as often presented since it will not be clear at which RH the moisture effusivity (and MBV) has to be calculated. MBV (g/(m².%rh)) 1 5 wood fibreboard plywood gypsum plaster mean RH of test protocol.2.4.6.8 1 Figure 5.- Calculated MBV as a function of relative humidity compared with the dynamically determined MBV (straight lines) for the three materials. A rather good agreement is found around 54%RH between the two ways to determine the MBV, however the exact agreement between both methods is situated at different RH for all three materials. 5. Reliability of the MBV in practical applications To investigate whether the moisture buffer value as determined with the proposed test protocol is indeed a reliable material property to quantifiy the hygric buffering capacity of materials, the MBV of the different materials are confronted with the dynamic hygric response of a small room with each of the materials used in turns as available hygric buffering material. Therefore, numerical simulations are performed both for long term (daily) variations as for short term variations of the RH inside the room. 5.1. Numerical simulations of the moisture balance of a room with hygric buffering In the numerical simulations the hygric response of a small room of 9 m³ is analysed. In the room the walls are assumed to be finished with one of the three materials investigated, with a total absorbing area of 6 m². The calculation is performed for a period of ten days. Each day a constant scheme of moisture production is imposed. For the long term variations a moisture production of 3 g/h is assumed every first eight hours of the day, followed by a period of 16 hours without moisture production. For the short term variations, a scheme of one hour with a moisture production of 6 g/h is followed by five hours without moisture production, and this four times a day. Assuming that no other hygroscopic materials are present and that the only coupling between indoor climate and outside climate is the ventilation of the room with outside air, the moisture balance of the room can be written as: ρ viv nv = ( ρ ve ρ vi ) + G t 36 vp Aq buf with ρ vi the water vapour concentration inside, ρ ve the water vapour concentration outside, V the volume of the room, n the ventilation rate taken at.5 per hour, G vp the vapour production inside the room, A the surface available for hygroscopic buffering and q buf the water vapour exchange with this absorbing surface. The latter is calculated with a finite element model to analyse the coupled heat and moisture transport in building enclosures, with the relative humidity inside the room as boundary condition. Each time step the inside relative humidity is updated by solving Equation (5) implicitly. To avoid that outside climatic conditions would disguise the hygric response of the room, no real climatic data are used, but instead the outside boundary conditions are kept constant at 1 C and 65% RH. The interior surface transfer coefficients are taken at 8 W/m²K for heat transfer and 3.1-8 s/m for vapour transfer. The inside temperature is kept constant at 2 C. (5)
5.2. Results for long term (daily) variations In the simulations with long term variations, the periods with and without vapour production have the same length as the steps of the signal in the test protocol to determine the MBV. Though, the relative humidity inside is now no longer imposed as a square wave signal, but a gradually increase of the indoor RH will be observed during the eight hours of moisture production followed by a gradually decrease during the intermediate 16 hours without moisture production. To compare the influence of the type of finishing material on the inside RH, a reference simulation has been performed in which no absorbing material was present. Furthermore for each of the three materials, two simulations were performed. Difference between both was the thickness of the material layer: 1 or 1 cm. Figure 6 shows for the last day the response of relative humidity inside the room for the different configurations in case of a 1 cm thick layer. Figure 7 shows the same results if only a 1 cm thick layer is applied..8 relative humidity inside (-).7.6.5.4 without hygroscopic material Wood Fibreboard 1 cm Plywood 1 cm Gypsum Plaster 1 cm.3 21 216 222 228 234 24 time (hours) Figure 6.- Evolution of the relative humidity in the room with time for the daily variations in case no absorbing material is present (bold continuous line) and if 6m² of the walls is finished with 1 cm of wood fibreboard (circles), plywood (triangles) or gypsum plaster (diamonds)..8 relative humidity inside (-).7.6.5.4 without hygroscopic material Wood Fibreboard 1 cm Plywood 1 cm Gypsum Plaster 1 cm.3 21 216 222 228 234 24 time (hours) Figure 7.- Evolution of the relative humidity in the room for the daily variations, with compared to Figure 5 the thickness of the absorbing materials limited to 1 cm.
If the materials are applied with a thickness of 1 cm (Figure 6), one can notice that all three materials are significantly reducing the relative humidity variation inside the room compared to the reference case when no hygroscopic material is present. The strongest reduction is found for wood fibreboard, which is able to bring back the original RH-amplitude of 18.5% to a value of 6.8%. Comparing the amplitudes in the room found for the three materials with the moisture buffer values (see Table 5), a good correlation is found. Looking at the evolution of the relative humidity inside the room in case the materials are applied with a thickness of only 1 cm (Figure 7), hardly any difference is found for the room with wood fibreboard or plywood. For the room with gypsum plaster, however, the hygric buffering in case of a 1 cm layer is much less than with a 1 cm layer and this room now shows the highest amplitude instead of the one with plywood as finishing material. This results from the fact that for gypsum plaster 1 cm is below the moisture penetration depth for daily humidity variations. Because of that, the MBV determined according to the test protocol (on samples with a thickness larger than the penetration depth) is no good indication for the buffering capacities of the finishing layer. This can clearly be illustrated if we plot the amplitude of the relative humidity in the room as a function of the moisture buffer value (Figure 8, left side). If the moisture penetration depth is larger than the applied thickness, as is the present case for gypsum plaster, different points will be obtained without a correlation between MBV and amplitude. However, when we plot the amplitudes found with the 1 cm layers, as a function of the moisture buffer values determined on specimen of the same thickness (see Figure 8, right side and Table 4), a nice descending curve is found between moisture buffer values and indoor amplitude. 2% 2% RH amplitude (%) 15% 1% 5% WFB, 1 cm WFB, 1 cm PW, 1cm PW, 1cm GP, 1cm GP, 1 cm RH amplitude (%) 15% 1% 5% WFB, 1 cm WFB, 1 cm PW, 1cm PW, 1cm GP, 1cm GP, 1 cm %.5 1 1.5 2 2.5 Moisture Buffer Value (kg/(m².%rh)) %.5 1 1.5 2 2.5 Moisture Buffer Value (kg/(m².%rh)) Figure 8.- Amplitude of the relative humidity in the room in case of daily variations plotted as a function of the moisture buffer value as determined according to the test protocol (left) and as determined on specimen with the same thickness as applied in the room (right). Table 5.- Amplitude of the relative humidity in the room in the case of long term variations and variable thickness of the finishing layer (1cm or 1cm). As a reference also the MBV determined according to the test protocol is given for the different materials. no absorbing with WFB with PW with GP material MBV - 2.19.77 1.6 Amplitude by a material 18.5 6.8 11.9 9.9 layer thickness of 1 cm Amplitude by a material layer thickness of 1 cm 18.5 6.7 11.9 14.7 5.3. Results for short term variations Compared to the previous simulations the vapour production rate is doubled (6 g/h instead of 3 g/h), but during a shorter period. Every day shows now four peaks of vapour production, each of one hour followed by five hours without vapour production. Again two simulation rounds are performed with the thickness of the absorbing material layer as variable. Figure 9 shows the results if the materials are applied with a thickness of 1 cm, Figure 1 if applied with a
thickness of 1 cm. The continuous lines without dots on the figures correspond to the reference case when no hygroscopic material is present. Table 6 compares again the moisture buffer values of the materials with the amplitudes in the room..7 relative humidity inside (-).6.5.4 without hygroscopic material Wood Fibreboard 1 cm Plywood 1 cm Gypsum Plaster 1 cm.3 36 42 48 54 6 time (hours) Figure 9.- Evolution of the relative humidity in the room with time for the short time variations in case no absorbing material is present (bold continuous line) and if 6m² of the walls is finished with 1 cm of wood fibreboard (circles), plywood (triangles) or gypsum plaster (diamonds)..7 relative humidity inside (-).6.5.4 Figure 1.-.3 36 42 48 54 6 time (hours) without hygroscopic material Wood Fibreboard 1 cm Plywood 1 cm Gypsum Plaster 1 cm Evolution of the relative humidity in the room for the short time variations, with compared to Figure 7 the thickness of the absorbing materials limited to 1 cm. Also for the short term variations wood fibreboard (with the highest MBV) gives the strongest reduction in indoor RH amplitude, followed by gypsum plaster and plywood. Figure 11 plots, as for the daily variations, the obtained RH-amplitude inside the room as a function of the moisture buffer value of the materials. Both for the moisture buffer values determined according to the test protocol as for the moisture buffer values determined on specimen with the same thickness as applied in the room, a rather poor relationship is found. So it seems that the test protocol,
based on daily variations, is especially useful as a characterisation of the materials when looking at hygric buffering also on daily variations. The proposed test protocol with the 8-16 hours seems less appropriate to appreciate the hygric buffering capacities for short term variations. 2% 2% RH amplitude (%) 15% 1% 5% WFB, 1 cm WFB, 1 cm PW, 1cm PW, 1cm GP, 1cm GP, 1 cm RH amplitude (%) 15% 1% 5% WFB, 1 cm WFB, 1 cm PW, 1cm PW, 1cm GP, 1cm GP, 1 cm %.5 1 1.5 2 2.5 Moisture Buffer Value (kg/(m².%rh)) %.5 1 1.5 2 2.5 Moisture Buffer Value (kg/(m².%rh)) Figure 11.- Amplitude of the relative humidity in the room in case of short term variations plotted as a function of the moisture buffer value as determined according to the test protocol (left) and as determined on specimen with the same thickness as applied in the room (right). Table 6.- Amplitude of the relative humidity in the room in the case of short term variations and variable thickness of the finishing layer (1cm or 1cm). As a reference also the MBV determined according to the test protocol is given for the different materials. no absorbing with WFB with PW with GP material MBV - 2.19.77 1.6 Amplitude by a material 14. 5.9 9.3 7.8 layer thickness of 1 cm Amplitude by a material layer thickness of 1 cm 14. 5.8 9.3 8.2 6. Conclusions and discussion The Moisture Buffer Value, proposed as an easy-to-measure material property to characterise the hygric buffering capacities of building materials, and its corresponding test protocol were analysed in detail by a numerical study on three materials: wood fibreboard, plywood and gypsum plaster. Aim of the study was to verify the reliability of the proposed test protocol as well as the practical applicability of the MBV as characterising value. Concerning the test protocol, three weak points were identified: the influence of the surface film coefficient, the time needed to impose a step in RH and the thickness of the specimen. Whereas, the influence of the time needed to achieve a step in RH on the obtained MBV showed to be almost negligible, the surface film coefficient and the thickness of the specimen were found to have a significant influence on the obtained MBV. Both factors were also found to be responsible for the discrepancy between the MBV determined form steady state material properties and the MBV determined by dynamic tests as mentioned in some previous studies. The thickness of the specimen showed to be very important once it dropped below the moisture penetration depth for daily humidity variations. And although the test protocol requires the thickness of the specimen to be larger than this value, one has to take into account that in reality this requirement will not be checked for the simple reason that the moisture penetration depth is unknown without additional measurements. When comparing the theoretically determined MBV based on the moisture effusivity with the dynamically determined value according to the test protocol, a rather good agreement was found at the mean RH of the test protocol. The exact agreement between both values, however, was found to be situated at different RH for all three materials investigated. This was attributed to the fact that the proposed test protocol fulfils not all requirements for the analytic solution to be valid.
The practical applicability of the MBV as characterising value for hygric buffering was investigated by comparing the dynamic moisture balance of a small room with each of the materials used in turns as finishing material. The MBV showed to be a good indication of the buffering capacities in the case of long term variations of the vapour production (daily variation comparable with the test protocol) and if the MBV was determined on specimen with the same thickness as applied in the room. For short term variations, or for the MBV determined on specimen with a different thickness the correlation between MBV and dynamic response was less. Note that the requirement of the thickness is in contradiction with the proposed test protocol. However, since the thickness influences the hygric buffering capacities of finishing materials and the MBV is meant as an appreciation of this behaviour it seems logic to measure the MBV on the applied thickness and hence, not to see the MBV as a material property, but more as a product property. Advantage of this approach is that also board products (as gypsum board, calcium silicate board, wood fibreboard, ) can be characterised by their own MBV typical for the material as provided by the manufacturer. Though, the limits of the proposed test protocol remain (minimum and maximum values of the square wave signal and the duration of the low and high relative humidity) and the closer the agreement between indoor conditions and test protocol the more reliable the MBV will be. Acknowledgement The results in this paper have been obtained within KUL OT/4/28 Towards a reliable prediction of the moisture stress on building enclosures, funded by the KULEUVEN. This financial support is gratefully acknowledged. References Goossens E, 23. Moisture transfer properties of coated gypsum. PhD-thesis TU/e, Eindhoven, The Netherlands. Hagentoft CE, 1999. Building Physics Fundamentals. Chalmers Tekniska Högskola, Göteborg, Report R- 97:1. Kumaran MK, 1996. IEA- Annex 24: Heat, air and moisture transfer in insulated envelope parts. Final report, volume 3. Task 3: Material properties. Acco Leuven, Belgium. Peuhkuri R, Rode C, 25. Using dynamic moisture loading tests for determination of moisture buffer value. Paper presented in Subtask 2 of IEA, Annex 41-meeting, 16-18 may, Montreal Canada. Rode C, Peuhkuri R, Hansen KK, Time B, Svennberg K, Arfvidsson J, Ojanen T, 25. Moisture buffer value of materials in buildings. In Proceedings of the 7 th Symposium on Building Physics in the Nordic Countries, June 13-15 25. Reykjavik Iceland.