Predicting the Outdoor Moisture Performance of Wood Based on Laboratory Indicators

The service life of wood in outdoor use under humid conditions is mainly determined by its material resistance and the exposure situation. Different standards such as EN 350 (2016) point on the relevance of wood’s resistance against moisture for its expected service life. Recently, different standardized but also numerous nonstandardized methods were suggested to test the water permeability of wooden materials. In the context of this study, different European-grown softand hardwoods, tropical hardwoods, modified wood and wood treated with waterand oil-borne preservatives were subjected to floating and submersion tests according to CEN/TS 16818 (2018) and different short-term water uptake and release tests. Moisture performance data from field tests with the same materials were analyzed and used to assess the predictive power of different laboratory moisture indicators. The moisture characteristics suggested by CEN/TS 16818 (2018)—rm168 (residual moisture content after water uptake and release processes) and res312 (residue as a percentage of the absorbed moisture)—showed the little potential to predict the outdoor moisture performance of the tested materials. In contrast, the mean moisture content during absorption and desorption (MCmean) predicted well the outdoor moisture performance of the materials under test. Short-term water uptake and release of small specimens also showed high predictive power.


Introduction
The service life of wood in outdoor use or elsewhere under humid conditions is determined by its material resistance and the exposure situation. The latter is represented by macro-, mesoand microlevel climatic parameters and can finally be expressed as the material climate in terms of wood moisture content and temperature [1]. The material resistance of wood is opposed to exposure. In general, toxic and inhibiting wood ingredients and a more or less pronounced ability to withstand moistening increase the material resistance.   From each material, three boards were selected. Nine specimens were prepared per material and specimen dimension (Figure 1), with three specimens coming from one individual board. Specimens of 100 (ax.) × 10 × 5 mm 3 were subjected to 24 h water uptake and release tests [3]. The sides of floating (50 (ax.) × 50 × 25 mm 3 ) and submersion (150 (ax.) × 50 × 25 mm 3 ) specimens were sealed with polyurethane (Sikaflex ® 221i, Sika Austria GmbH, Austria, Figure 1, dark gray) prior testing the moisture performance according to [1]. Laboratory testing was followed by a horizontal outdoor exposure with five replicates per test specimen type and material (test field in Goettingen, Germany, 51 • 33 34.9" N, 9 • 57 18.5" E, elevation: 199 m, oceanic continental climate), to record the moisture performance under field conditions. The remaining (n = 4) submersion test specimens were cut with a notch of 3.2 (ax.) × 50 × 13 mm 3 to detect the impact of end-grain surfaces on the moisture performance of the materials (Figure 1).

Liquid Water Uptake by Submersion
Specimens of 100 (ax.) × 10 × 5 mm 3 were oven-dried at 103 • C until constant mass. The oven-dry mass was determined to the nearest 0.001 g. Oven-dry specimens were submerged in a sealed plastic container with demineralized water and placed in a climate chamber with 20 • C, 65% relative humidity (RH; "normal climate"). Specimens were separated from each other by square-shaped stainless-steel meshes. The specimens were weighed again after 24 h submersion. The liquid water uptake of the specimens (W24 sub. ) was determined, and the resulting moisture content (MC) calculated (Equation (1)).
Water Vapor Uptake in Water-Saturated Atmosphere Specimens of 100 (ax.) × 10 × 5 mm 3 from liquid water uptake testing (W24 sub. ) were again oven-dried at 103 • C until constant mass. The oven-dry mass was determined to the nearest 0.001 g. The bottom of a miniature climate chamber (sealed plastic container with stainless-steel perforated plates) was filled with 5 L demineralized water. Specimens were exposed with approx. 5 mm distance to each other on the stainless-steel plates above water. The containers were stored in a climate chamber with "normal climate" and specimens weighed again after 24 h. The water vapor uptake of the specimens (W24 100%RH ) was determined and the resulting MC calculated (Equation (2)).

Desorption
After water vapor uptake testing, specimens of 100 (ax.) × 10 × 5 mm 3 were stored in sealed containers above water at 20 • C (approximately 100% RH) until constant mass. The mass at approx. cell wall saturation (CWS) was determined to the nearest 0.001 g. Specimens were exposed directly on freshly activated silica gel in sealed boxes (0% RH) and weighed again after 24 h. The water vapor release (desorption) of the specimens during 24 h was determined and expressed as a relative value of the mass at CWS (Equation (3)).

Capillary Water Uptake Tests (CWU)
Short-term water absorption was measured using a Krüss Processor Tensiometer K100MK2 (Krüss GmbH, Hamburg, Germany). Specimens of 60 (ax.) × 10 × 5 mm 3 were stored at "normal climate" until constant mass. The axial specimen surfaces (10 × 5 mm 2 ) were fixed in the tensiometer and positioned to be in contact with water. The specimen's mass was recorded to the nearest 0.0001 g in intervals of 2 s for a period of 200 s. The capillary water uptake (CWU) was determined over time and related to the cross-sectional area of the specimens (Equation (4)).
CWU = Capillary water uptake during 200 s (g/cm 2 ); m 200s = Mass after 200 s in contact with water (g); m 65%RH = Mass at 20 • C, 65% RH (g). Specimens of 50 (ax.) × 50 × 25 mm 3 were dried at 103 • C until constant mass. The oven-dry weight (m 0 ) was measured to the nearest 0.01 g. Prior testing, specimens were conditioned at "normal climate". After conditioning, four sides (including end-grain surfaces) were sealed with a polyurethane sealant (Figure 1), the weight of the sealant and the initial specimen mass (m i ) recorded. The moisture performance was tested during a water uptake (absorption) and release (desorption) cycle according to [5] with nine specimens of each material. Plastic containers were filled with demineralized water and stored at "normal climate". For the water uptake cycle, test specimens were placed approx. 10 mm under the water level to ensure that one specimen surface (50 × 50 mm 2 ) was completely submerged. Test specimens were removed in intervals after 1, 4,8,24,48,72 and 144 h, liquid water on the surface dabbed and the weight determined to the nearest 0.01 g.
After 144 h absorption, all test specimens were put on a stainless-steel grid on their sealed side and stored at "normal climate". The minimum distance between specimens was 10 mm to guarantee adequate air circulation during the desorption cycle (drying). Test specimens were weighed to the nearest 0.01 g after 1, 4, 8, 24, 48, 72, 96 and 168 h.

Submersion Test-Absorption and Desorption Cycle
Specimens of 150 (ax.) × 50 × 25 mm 3 were dried at 103 • C until constant mass. The oven-dry weight (m 0 ) was determined to the nearest 0.01 g. Prior testing, specimens were conditioned at "normal climate", sealed as described above (Figure 1) and the initial specimen mass (m i ) determined. The moisture performance was tested during liquid water uptake and desorption cycles according to [5] with nine specimens of each material. Plastic containers were filled with demineralized water and stored at "normal climate". For the water uptake cycle, test specimens were placed in the water so that they were fully submerged over the entire absorption cycle. Test specimens were removed in intervals after 1, 4, 8, 24, 48, 72 and 144 h, liquid water on the surface dabbed and the weight recorded to the nearest 0.01 g.
After 144 h absorption, a desorption cycle (drying) was performed as described for the floating test procedure (Figure 2).

Submersion Test-Absorption and Desorption Cycle
Specimens of 150 (ax.) × 50 × 25 mm 3 were dried at 103 °C until constant mass. The oven-dry weight (m0) was determined to the nearest 0.01 g. Prior testing, specimens were conditioned at "normal climate", sealed as described above (Figure 1) and the initial specimen mass (mi) determined. The moisture performance was tested during liquid water uptake and desorption cycles according to [5] with nine specimens of each material. Plastic containers were filled with demineralized water and stored at "normal climate". For the water uptake cycle, test specimens were placed in the water so that they were fully submerged over the entire absorption cycle. Test specimens were removed in intervals after 1, 4, 8, 24, 48, 72 and 144 h, liquid water on the surface dabbed and the weight recorded to the nearest 0.01 g.
After 144 h absorption, a desorption cycle (drying) was performed as described for the floating test procedure ( Figure 2).

Indicators and Calculations from Floating and Submersion Tests
For each interval during the absorption and desorption period (Figure 2), the specimens' weight (m) was recorded and the moisture content (MC, Equation (5)) calculated based on the oven-dry mass of the tested specimen without sealant (m0).

Indicators and Calculations from Floating and Submersion Tests
For each interval during the absorption and desorption period (Figure 2), the specimens' weight (m) was recorded and the moisture content (MC, Equation (5)) calculated based on the oven-dry mass of the tested specimen without sealant (m 0 ).
To consider the extra mass (weight percent gain (WPG)) due to the presence of chemicals inside modified specimens, a corrected moisture content (MC corr ) was calculated for modified specimens and presented per cell wall mass (Equation (6)).
MC corr = Material moisture content of the modified specimen related to the oven-dry; cell wall mass (%); MC = Material moisture content of the modified specimen related to the oven-dry; material mass (cell wall + chemical) (%).
The amount of absorbed moisture after 144 h (a 144 ) was calculated following Equation (7).
a 144 = Amount of absorbed moisture after 144 h (g); MC i = Initial moisture content of the specimen (g); MC 144 = Moisture content of the specimen after 144 h of water uptake (g).
The amount of desorbed moisture after 168 h (d 168 ) was calculated following Equation (8). The residual moisture content (rm 168 ), which represents the increase in moisture content after 144 h of absorption followed by 168 h of desorption, compared to the initial moisture content, was calculated according to Equation (9). rm 168 = a 144 − d 168 (%) (9) rm 168 = Residual moisture content of the specimen after 312 h of testing (g); a 144 = Amount of absorbed moisture after 144 h (g); d 168 = Amount of desorbed moisture after 168 h (g). The residue (res 312 ) was calculated (Equation (10)), representing the moisture content left in the test specimen after 168 h desorption (rm 168 ) as a percentage of the absorbed moisture after 144 h (a 144 ). res 312 = rm 168 a 144 × 100 (%) (10) res 312 = Residue in the specimen after 312 h of testing expressed as a percentage (%); rm 168 = Residual moisture content of the specimen after 312 h of testing (g); a 144 = Amount of absorbed moisture after 144 h (g).

Field Exposure
After testing the moisture performance under laboratory conditions (Sections 2.2.1 and 2.2.2), all materials with the different specimen designs were exposed on a test field at the University of Goettingen (51 • 33 34.9 N, 9 • 57 18.5 E, elevation: 199 m, oceanic continental climate). Prior exposure, all specimens were conditioned at "normal climate". Specimens were horizontally exposed with the nonsealed surfaces on wire meshes (mesh size: 13 mm) with a distance of at least 10 mm between the specimens and a distance from the ground of 0.85 m, ensuring free air circulation. Five specimens of each format ( Figure 1) were chosen from originally nine. A notch of 3.2 (ax.) × 50 × 13 mm 3 was cut in the remaining specimens (150 (ax.) × 50 × 25 mm 3 ) from submersion tests (n = 4) and horizontally exposed, to detect the impact of end-grain surfaces on the moisture performance of the materials. Specimens were exposed between 15 April 2019 and 20 June 2019 for a period of 10 weeks. The specimens were weighed to the nearest 0.01 g twice a week with a minimum time between each measurement of two days but not more than four days. MC was calculated for each measurement (MC i ) and the mean MC (MC mean ) calculated over the entire exposure period (Equation (11)).
MC mean = Mean moisture content over 10 weeks of outside exposure (%); MC j = Mean moisture content per material and specimen design at each day of measurement (j) (%); n = Number of moisture content measurements (-).
Local climate parameters like ambient temperature ( • C), RH (%) and daily precipitation (mm) were recorded over the entire exposure by a meteorological station which was located approx. 500 m away from the test field and at an elevation of 240 m.

W24-Tests (24 h Water Uptake and Release Tests (Meyer-Veltrup et al. 2017))
To predict the moisture behavior of wood in service, four different test procedures-liquid water uptake (W24 sub. ), water vapor uptake (W24 100%RH ), water vapor release (W24 0%RH ) and capillary water uptake (CWU)-were applied to test specimens of 100 (ax.) × 10 × 5 mm 3 . Results and factors describing the wetting ability (k wa ) are shown in Table 1. The k wa factors were calculated for each procedure separately. Additionally, all procedures were summarized in one k wa , all factor according to Equation (12) with Norway spruce as the reference material.
The results from short-term W24-tests differed between both, test procedure and wood species. Regarding the kwa factors, they showed the narrowest range of data between the different materials during water vapor release (desorption) tests while the widest range was observed in the water vapor uptake tests. Differences in water uptake and release behavior became evident between wood species, between different chemical modification technologies, and between treatments with water-and/or oil-borne preservatives (Table 1).

Floating Test (CEN/TS 16818 (2018))
In addition to nonstandardized short-term water uptake and release tests (W24-tests [2]), liquid water uptake and water vapor release were investigated in floating tests according to CEN/TS 16818 [1]. The residual moisture content after an absorption period of 144 h followed by a desorption step of 168 h (rm 168 ) and the residue (res 312 ) as the percentage of the rm 168 related to the amount of absorbed water (a 144 ), had been calculated as laboratory moisture resistance indicators. Results are summarized in Table 2, strictly following the terminology of [5] and showing corresponding k wa factors. Hence, all indicators were calculated based on the absolute amount of absorbed water in grams. On the same data basis, the mean moisture content (MC mean ) was calculated over the entire moisture uptake and release procedure as an additional moisture resistance indicator. The moisture performance of the materials during the floating tests differed between both, different moisture resistance indicators and between untreated and treated wood ( Table 2). Table 2. Moisture content at the end of absorption (MC 144 in g) and desorption cycle (MC 312 in g), amount of absorbed moisture after 144 h (a 144 in g), amount of desorbed moisture after 168 h (d 312 in g), residual moisture content (rm 168 in g), residue (res 312 in g) and mean MC (MC mean ) in floating tests (50 (ax.) × 50 × 25 mm 3 ) according to [5].

Material
MC 144 MC 312 a 144 d 168 rm 168 res 312 MC mean The same indicators had been calculated considering the absorbed and released amounts of water as percentage values, related to the oven-dry wood mass. The latter were summarized in Table 3 for all tested materials. Table 3. Moisture content at the end of absorption (MC 144 in %) and desorption cycle (MC 312 in %), amount of absorbed moisture after 144 h (a 144 in %), amount of desorbed moisture after 168 h (d 312 in %), residual moisture content (rm 168 in %), residue (res 312 in %) and mean MC (MC mean ) in floating tests (50 (ax.) × 50 × 25 mm 3 ) according to [5]. Among the softwoods, Radiata pine SW showed the most rapid and highest change in MC during the absorption step of the floating test (a 144 = 62.33%); among the hardwoods, it was European beech (a 144 = 45.66%). Among all materials, the tropical hardwoods and Black locust showed the lowest changes in MC. The other wood species and the treated materials were listed in between (Table 3).

Submersion Test (CEN/TS 16818 (2018))
Liquid water uptake and water vapor release were also investigated in submersion tests according to [5]. Laboratory moisture resistance indicators were calculated as described under Section 3.1.2. Results were summarized in Table 4 plus corresponding k wa factors. All indicators were calculated based on the absolute amount of absorbed water in grams. The moisture performance of the materials during submersion tests differed between both, different moisture resistance indicators and between untreated and treated wood (Table 4). Table 4. Moisture content at the end of absorption (MC 144 in g) and desorption cycle (MC 312 in g), amount of absorbed moisture after 144 h (a 144 in g), amount of desorbed moisture after 168 h (d 312 in g), residual moisture content (rm 168 in g), residue (res 312 in g) and mean MC (MC mean ) in submersion tests (150 (ax.) × 50 × 25 mm 3 ) according to [5].  The same indicators had been calculated for submersion test specimens, considering the absorbed and released amounts of water as percentage values, related to the oven-dry specimens' mass. The latter were summarized in Table 5 for all tested materials.

Material
Among the softwoods, Radiata pine SW showed the most rapid and highest change in MC during the absorption step of the submersion test (a 144 = 63.00%), among the hardwoods it was Paulownia (a 144 = 75.42%). Among all materials, the tropical hardwoods and Black locust showed the lowest changes in MC. The other wood species and the treated materials were listed in between (Table 5).
In principle, it became evident that the submersion test resulted in significantly higher water uptakes of the different untreated, modified and preservative-treated materials, compared to the floating test which was in line with the observations made previously by others [20].
The k wa factors based on rm 168 (a), res 312 (b) and MC mean (c) from submersion tests (Figure 3, x-axis) had been compared to those resulting from floating tests (Figure 3, y-axis). The latter showed good correlations between results from floating (50 (ax.) × 50 × 25 mm 3 ) and submersion tests (150 (ax.) × 50 × 25 mm 3 ) with R 2 ranging from 0.7016 (k wa (res 312 )) to 0.8546 (k wa (rm 168 )). Hence, both test set-ups led to similar assessments of the moisture resistance of the individually tested materials like untreated wood, modified wood or preservative-treated wood. In principle, it became evident that the submersion test resulted in significantly higher water uptakes of the different untreated, modified and preservative-treated materials, compared to the floating test which was in line with the observations made previously by others [20].

Outdoor Moisture Performance
After laboratory testing, all tested materials were exposed outside over a period of 10 weeks to determine the moisture resistance of each material under real outdoor conditions. Figure 4 shows exemplarily the MC development in Norway spruce specimens of 50 (ax.) × 50 × 25 mm 3 with increasing MC in response to increasing RH and rainfall.
The mean MC (MCmean) was calculated over the entire exposure period (Equation (10)) and summarized in Table 6 including corresponding kwa factors. Results differed between both, different specimen designs and wood species. Most of the tested materials showed significantly higher MCmean in specimens with free end-grain surfaces (specimens: 150 (ax.) × 50 × 25notched mm 3 ). Considering kwa factors, the narrowest range of data between the different materials was found for small-sized specimens with free end-grain surfaces (100 (ax.) × 10 × 5 mm 3 ) while the widest range was observed for specimens of 50 (ax.) × 50 × 25 mm 3 with sealed sides.

Outdoor Moisture Performance
After laboratory testing, all tested materials were exposed outside over a period of 10 weeks to determine the moisture resistance of each material under real outdoor conditions. Figure 4 shows exemplarily the MC development in Norway spruce specimens of 50 (ax.) × 50 × 25 mm 3 with increasing MC in response to increasing RH and rainfall.   The mean MC (MC mean ) was calculated over the entire exposure period (Equation (10)) and summarized in Table 6 including corresponding k wa factors. Results differed between both, different specimen designs and wood species. Most of the tested materials showed significantly higher MC mean in specimens with free end-grain surfaces (specimens: 150 (ax.) × 50 × 25 notched mm 3 ). Considering k wa factors, the narrowest range of data between the different materials was found for small-sized specimens with free end-grain surfaces (100 (ax.) × 10 × 5 mm 3 ) while the widest range was observed for specimens of 50 (ax.) × 50 × 25 mm 3 with sealed sides. In the following, laboratory indicators from floating and submersion tests according to [1] were considered to predict the outdoor moisture performance of the respective materials. For this, rm 168 and res 312 were calculated from water uptake and release behavior during laboratory testing (lab) and compared to the mean MC (MC mean ) of the same specimen design, which was measured during outside exposure (field). MC mean values of specimens from submersion tests, which were exposed outside with a notch (150 (ax.) × 50 × 25 notched mm 3 ), were compared to respective laboratory indicators (rm 168 , res 312 ) of the submersion test specimens without a notch (Figure 5b,e).
Strictly following the terminology of [1], rm 168 and res 312 were calculated based on the absolute amounts of absorbed water in grams. With R 2 ≤ 0.26, rm 168 and res 312 showed less potential to predict the moisture performance of the same material during outside exposure. The latter became evident independent of the specimen design ( Figure 5). In the following, laboratory indicators from floating and submersion tests according to [1] were considered to predict the outdoor moisture performance of the respective materials. For this, rm168 and res312 were calculated from water uptake and release behavior during laboratory testing (lab) and compared to the mean MC (MCmean) of the same specimen design, which was measured during outside exposure (field). MCmean values of specimens from submersion tests, which were exposed outside with a notch (150 (ax.) × 50 × 25notched mm 3 ), were compared to respective laboratory indicators (rm168, res312) of the submersion test specimens without a notch (Figure 5b,e).
Strictly following the terminology of [1], rm168 and res312 were calculated based on the absolute amounts of absorbed water in grams. With R 2 ≤ 0.26, rm168 and res312 showed less potential to predict the moisture performance of the same material during outside exposure. The latter became evident independent of the specimen design ( Figure 5). The comparison of k wa factors based on MC mean from field exposure with those based on rm 168 and res 312 from laboratory testing (R 2 ≤ 0.34) confirmed that the latter show only very little power to predict the moisture performance during outside exposure (Figures 6 and 7). The comparison of kwa factors based on MCmean from field exposure with those based on rm168 and res312 from laboratory testing (R 2 ≤ 0.34) confirmed that the latter show only very little power to predict the moisture performance during outside exposure (Figures 6 and 7).  Notwithstanding the standard [1], but in line with a study by [19], laboratory indicators rm168 and res312 were calculated considering the absorbed water by the MC as a percentage value related to the oven-dry specimen mass. Compared with MCmean values from field exposure, the correlations between the laboratory indicator rm168 and the moisture performance in field became better, especially for specimens of 50 (ax.) × 50 × 25 mm 3 with an R 2 = 0.57. The residue res312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.14 ( Figure 8).  Figure 6. k wa factors based on MC mean (%) measured during outdoor exposure versus k wa factors of rm 168 (a-c) in grams determined in submersion (a,b) and floating tests (c) according to [5]. The comparison of kwa factors based on MCmean from field exposure with those based on rm168 and res312 from laboratory testing (R 2 ≤ 0.34) confirmed that the latter show only very little power to predict the moisture performance during outside exposure (Figures 6 and 7).  Notwithstanding the standard [1], but in line with a study by [19], laboratory indicators rm168 and res312 were calculated considering the absorbed water by the MC as a percentage value related to the oven-dry specimen mass. Compared with MCmean values from field exposure, the correlations between the laboratory indicator rm168 and the moisture performance in field became better, especially for specimens of 50 (ax.) × 50 × 25 mm 3 with an R 2 = 0.57. The residue res312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.14 ( Figure 8). Notwithstanding the standard [1], but in line with a study by [19], laboratory indicators rm 168 and res 312 were calculated considering the absorbed water by the MC as a percentage value related to the oven-dry specimen mass. Compared with MC mean values from field exposure, the correlations between the laboratory indicator rm 168 and the moisture performance in field became better, especially for specimens of 50 (ax.) × 50 × 25 mm 3 with an R 2 = 0.57. The residue res 312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.14 ( Figure 8). res312 (d-f) calculated as moisture content (%) in submersion (a,b,d,e) and floating tests (c,f) according to [5].
The comparison of kwa factors based on rm168 and res312 from laboratory testing were well correlated with kwa factors based on MCmean from field exposure. The latter was shown for both specimen designs of 150 (ax.) × 50 × 25 mm 3 and 50 (ax.) × 50 × 25 mm 3 , while the residue res312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.08 (Figure 9). The comparison of k wa factors based on rm 168 and res 312 from laboratory testing were well correlated with k wa factors based on MC mean from field exposure. The latter was shown for both specimen designs of 150 (ax.) × 50 × 25 mm 3 and 50 (ax.) × 50 × 25 mm 3 , while the residue res 312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.08 (Figure 9).
The comparison of kwa factors based on rm168 and res312 from laboratory testing were well correlated with kwa factors based on MCmean from field exposure. The latter was shown for both specimen designs of 150 (ax.) × 50 × 25 mm 3 and 50 (ax.) × 50 × 25 mm 3 , while the residue res312 still showed minor power to predict the moisture performance in the field with R 2 ≤ 0.08 (Figure 9). Rather poor (Section 3.3.1) to moderate (Section 3.3.2) correlations were found between the laboratory indicators rm168 and res312 and the moisture performance in the field. Hence, the MCmean was calculated as an alternative moisture laboratory indicator according to Equation (13) based on results from floating and submersion tests. The MCmean considered the MC developments over the entire absorption and desorption process, excluding the MC value detected after 96 h moisture release ( Figure 10). Further on, test specimens from short-term water uptake and release tests (W24-tests) with free end-grain surfaces (100 (ax.) × 100 × 5 mm 3 ) were regarded to predict the moisture performance of respectively tested materials in service. Rather poor (Section 3.3.1) to moderate (Section 3.3.2) correlations were found between the laboratory indicators rm 168 and res 312 and the moisture performance in the field. Hence, the MC mean was calculated as an alternative moisture laboratory indicator according to Equation (13) based on results from floating and submersion tests. The MC mean considered the MC developments over the entire absorption and desorption process, excluding the MC value detected after 96 h moisture release ( Figure 10). Further on, test specimens from short-term water uptake and release tests (W24-tests) with free end-grain surfaces (100 (ax.) × 100 × 5 mm 3 ) were regarded to predict the moisture performance of respectively tested materials in service. with free end-grain surfaces (100 (ax.) × 100 × 5 mm ) were regarded to predict the moisture performance of respectively tested materials in service. MCmean = Mean moisture content over absorption and desorption cycles (%) MCj = Moisture content after j hours of absorption/desorption (%) Laboratory moisture indicators showed better correlations with MC mean values (Equation (10)) resulting from outside exposure, when instead of laboratory indicators rm 168 and res 312, MC mean values (Equation (12)) from laboratory testing were used ( Figure 11). Laboratory moisture indicators showed better correlations with MCmean values (Equation (10)) resulting from outside exposure, when instead of laboratory indicators rm168 and res312, MCmean values (Equation (12)) from laboratory testing were used ( Figure 11).  The k wa factors based on MC mean values tested under laboratory conditions in submersion tests (150 (ax.) × 50 × 25 mm 3 ) showed the best correlation with k wa factors based on MC mean values from field exposure. Hence, the submersion test set-up and the corresponding specimen design showed the best potential to predict the moisture performance of respective materials outside. Surprisingly, the correlation even increased when comparing k wa factors based on MC mean from submersion tests with the moisture performance of the same specimen design having a notch (free end-grain surfaces; Figure 12). Test specimens from short-term water uptake and release tests [3] were used to predict the moisture performance of different materials in the field. Hence, kwa, all factors of 100 (ax.) × 10 × 5 mm 3 specimens had been calculated considering results from 24 h liquid water uptake (W24sub.), water vapor uptake (W24100% RH) and water vapor release (W240%RH) tests according to [3]. The latter showed good correlations (R 2 = 0.7163) with the kwa factors based on MCmean of submersion test specimens during outside exposure ( Figure 13).

MC
The correlation increased further (R 2 = 0.76), considering kwa factors that were solely based on results from 24 h liquid water uptake (W24sub.) tests ( Figure 14). Test specimens from short-term water uptake and release tests [3] were used to predict the moisture performance of different materials in the field. Hence, k wa , all factors of 100 (ax.) × 10 × 5 mm 3 specimens had been calculated considering results from 24 h liquid water uptake (W24 sub. ), water vapor uptake (W24 100% RH ) and water vapor release (W24 0%RH ) tests according to [3]. The latter showed good correlations (R 2 = 0.7163) with the k wa factors based on MC mean of submersion test specimens during outside exposure ( Figure 13). Consequently, over an exposure period of 10 weeks, laboratory moisture indicators from shortterm water uptake and release tests [3] showed good potential to predict the outdoor moisture performance.  Figure 13. k wa factors based on MC mean (%) of test specimens of 150 (ax.) × 50 × 25 mm 3 without notch (a) and with a notch (b) and of 50 (ax.) × 50 × 25 mm 3 (c) determined during outside exposure versus k wa , all factors of 100 (ax.) × 10 × 5 mm 3 specimens determined during 24 h water uptake and release tests according to [3].
The correlation increased further (R 2 = 0.76), considering k wa factors that were solely based on results from 24 h liquid water uptake (W24 sub. ) tests ( Figure 14). Consequently, over an exposure period of 10 weeks, laboratory moisture indicators from shortterm water uptake and release tests [3] showed good potential to predict the outdoor moisture performance.