Effect of wood density and cracks on the moisture content of coated Norway spruce (Picea abies (L.) Karst)

Abstract

A protective coating is often used on the cladding of wooden facades to limit the absorption of moisture. Low wood moisture content (MC) is essential to obtain satisfactory durability performance. Wood density is known to influence the water sorption and crack formation of uncoated wood. However, the effect of density on the aforementioned behaviors of coated spruce is not yet fully understood. Six-years of data on the crack formation and the MC variation of outdoor exposed panels are analyzed in this article. The outdoor test was complemented by a subsequent laboratory experiment, wherein the MC variation was monitored at different depths on the board during artificial water spraying. The aim of this research was to increase the knowledge about how wood density and aging affect the water sorption of coated spruce through the crack formation. The results indicated that wood density had an impact on the overall sorption behavior of coated spruce. Low-density spruce contributed to faster water absorption and desorption processes than coated samples with higher density. However, the observed correlation to density was limited to a condition with an intact coating. High-density characteristics contributed to more crack formation, and the density-sorption relationship reversed with a cracked coating. A cracked coating caused a strong local increase in the MC of the wood at the location of the cracks. Weather-exposed replicates without cracks had a higher MC in the core of the board compared with the value beneath the coating. The higher MC is probably due to the water sorption of the uncoated backside of the panel. Such an occurrence raised awareness for future studies to account for multidimensional sorption behavior from all sides of the panel. The local difference in MC also raises awareness for future studies to investigate local MC variations (as opposed to the global average of the panel) in research on the durability of coated wood.

Effects of Fertilization on Wood Formation in Naturally Regenerated Juvenile Silver Birch in a Norway Spruce Stand in South Sweden

“…The cell structure is mainly influenced by cambium age and ring width [26], and the within tree radial variations can be useful for distinguishing between age and growth effects [27]. In some conifers, wood density is directly affected by the tree growth rate, as shown for Pinus cooperi [28] and for Norway spruce (Picea abies Karst), where density is negatively correlated with ring width [29,30]. Generally, a wood fiber's function is to support the tree, thus their ultrastructure is of great importance to define the mechanical properties of wood [24].…”

Artificial Weathering of Contact-Charred Wood—The Effect of Modification Duration, Wood Species and Material Density

“…Continuous shrinking/swelling stresses create microcracks that create paths for moisture to reach the more hydrophilic parts of the wood. For example, Sjökvist et al [ 9 ] noted that high-density spruce samples cracked more in natural weathering, and as a result, absorbed more moisture than their lighter counterparts. The length, width and depth of cracks on the coating as well as the wood surface had a clear effect on the increase in moisture content.…”

“…Wood density affects weathering by decreasing the erosion rate [ 7 , 8 ]. Sjökvist et al [ 9 ] noted, however, that in spruce claddings, this effect was limited to an intact coating. The formation of cracks reduced the impact of density and leveled moisture content differences between high- and low-density spruce.…”

Evaluation of Untreated and Surface-Treated Wooden Facades of Buildings in Sweden

“…When a wooden facade is painted, moisture absorption is reduced by slowing down diffusion and preventing the wood from absorbing water by capillary action [17]. The moisture ratio differs depending on the paint used [22]. A wooden facade that is covered with paint can be expected to have less rot and, thus, an increased durability [23].…”