Lisø PhD Dissertation Manuscript - NTNU

appropriate in a more severe type of climate. The presented lessons learned are highly
relevant also for geographical areas other than Norway, e.g. the most exposed parts of the
UK, and is a first approach towards improved design guidelines for masonry structures in
harsh climates.
3.2. The two-stage tightening principle
The results clearly illustrate that the employment of the two-stage tightening principle (i.e.
separate wind and rain barrier) is crucial in climates with severe driving rain exposure.
This is in agreement with the recommendations of Stirling [8]. However, there is a wellestablished
tradition in several European countries with fully filled cavity. Hens et al. [9]
suggests that such walls, even with open head joints, can maintain a satisfactory rain
protection, dependent on the severity of climatic impact and the combination of materials
used. Yet, there exist no distinct guidelines for when and where to use fully filled cavity
walls. The development of such guidelines may be based on the methodologies given in
BS 5628-3:2001 [10], which do not advise full cavity fill of unprotected masonry on the
west coast of Scotland, parts of North West England, much of Wales, and South West
England, due to driving rain exposure (according to assessment methods given in BS
8104:1992 [11]).
The investigation indicates that one weephole per meter is sufficient to drain water
from cavity trays at the lower edge of cavity and veneer walls in climates with sheltered or
moderate driving rain exposure. There should be two weepholes per meter where severe
driving rain exposure is expected.
3.3. Position of window
The correct positioning of windows in cavity and veneer walls is important to avoid
defects. Windows should be installed parallel to, or flush with, the thermal insulation layer
to avoid thermal bridges and subsequent heat loss. As a main rule the turn-up at the rear
edge of the weatherboard flashing should rest directly against the cold side of thermal
insulation layer. This positioning of the window ensures a low heat loss, a robust rain
protection (especially important in climates with high driving rain impact) and a low risk
of moisture defects.
The window can be positioned further to the warm side of the wall in cold inland
areas with a low driving rain impact, in order to reduce the risk of internal condensation
problems. But, in these cases the tightening details around the window must be carefully
considered.
3.4. Frost resistance
The frost resistance of masonry depends on a complex set of material properties and on the
climatic impact on the material, e.g. [12]. In part due to this complexity frost resistance of
brick and rendering mortar is still tested according to rather simple methods given in
different international and national standards. There are designated test methods for
different countries, but the results presented in this investigation shows that the
performance of masonry depends on climatic exposure at the very local level. Annex B of
NS-EN 771-1:2003 [13] includes exposure examples for masonry or masonry elements
dependent on the design of the construction, i.e. the risk of frost decay in a given climate.
However, the climatic conditions are not specified in the standard. Simple climate adapted
design recommendations for the use of bricks in the United Kingdom is provided by
Hanson Brick [14]. However, a more pronounced relationship between frost resistance and
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