To understand how green roofs affect building energy performance under cold climatic conditions, a proper thermal analysis of the roof and its components is required. To address this, we measured the thermal conductivity of each layer of experimental green roofs, as well as the equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Green roofs were compared to bare roofs (without substrate, vegetation and other green roof layers) to assess the basic functioning and relative performance of the green roof system. Layer analysis at various intensities of frost penetration showed that the thermal conductivity of each layer decreased when penetrated by frost. In particular, thermal conductivity of the substrate and vegetation layers decreased from 0.41 Wm-1 K-1 and 0.34 Wm-1 K-1 prior to freezing, to 0.12 Wm-1 K-1 and 0.10 Wm-1 K-1 after freezing, respectively. This phenomenon is explained by a reduction in bridge-water connectivity during freezing and a volumetric water content that was below the critical threshold value. Overall, a frost depth that extended through the complete green roof yielded the greatest equivalent thermal resistance at a mean value of 2.01 m 2 WK-1. During times of snow cover, snow acted as an insulator and reduced the relative energy saving benefits achieved by green roofs. These results provide information for designing the substrate and vegetation layers of green roofs for optimal insulation.

Silver birch is a common European hardwood species possessing stiff and strong material properties. The mechanical properties of birch timber under compression loads are, however, not well defined as the influence of growth characteristics have largely been disregarded in experimental investigations. To address this, the present study investigated the effect of growth characteristics on the mechanical behaviour of birch sawn timber under compression loading, parallel and perpendicular to the grain direction. The results indicate knot size as the dominant parameter for both loading directions, whereas in specimen without knots, density, distance to pith and growth-ring width are dominant for parallel to grain loading and density and growth-ring orientation are dominant under perpendicular to grain loading. On selected specimens, local deformation and failure behaviour is further investigated by means of digital image correlation technique using a novel contrast pattern. Strain fields on specimens loaded parallel to grain reveal localized strain peaks at the cracks within knots and around the topside of knots where abrupt deviations in the grain orientation occur. Specimens loaded perpendicular to the grain display localized strain peaks near the pith and at growth-ring tangential angles around 45 • .

No abstract

No abstract