Lisø PhD Dissertation Manuscript - NTNU

precipitation during summer is the largest contributor. Øygarden at Skjåk (Oppland
County, located less than 150 km in overhead line from Brekke) has the lowest annual
normal precipitation with 278 mm. This is lower than the normal monthly precipitation for
the 6 wettest months of Brekke. However, the lowest recorded precipitation for one year is
only 118 mm, measured at Saltdal (Nordland County) in 1996.
The development of design tools for the assessment of frost decay risk is important
because freezing and thawing of porous, mineral materials in combination with large
amounts of precipitation represent a significant challenge in the design and construction of
building enclosures in Norway. In this paper we present a simple climate exposure index
for the assessment of geographically dependent frost decay risk based on multi-year
records of air temperatures and rainfall in different parts of Norway. Ways of further
development of the index are also discussed.
2. Background
The climate in Norway puts great demands on the design and geographical localization of
buildings and the correct choice of materials and constructions. Since 1953 the Norwegian
Building Research Institute (NBI) has undertaken analyses of building defects, both on
behalf of the construction industry and in comprehensive field investigations. Results from
an ongoing investigation of process induced building defects investigated in the 10-year
period 1993-2002 (2378 building defect cases registered and described in 2045 assignment
reports) reveals that defects related to the building enclosure constitute about two thirds of
the cases. Moisture as the main source causing the defect replies for as much as 76% of all
investigated cases in the 10-year period [1]. The lifetime, or decay rate, of building
materials is of course strongly dependent on the local climatic conditions at the building
site (i.e. temperature conditions and precipitation in the form of driving rain). Frost
resistance may be a problem for porous, mineral materials, if exposed to repeated freezing
and thawing when the moisture content exceeds certain critical values.
The development of design guidelines for buildings in severe climates through
different climate index approaches has often proven a successful line of action in the
pursuit of high-performance building enclosures. Scheffer [2] developed an index of the
relative climate (using temperature and rainfall) to estimate the potential for decay of
above ground wood structures. Cornick and Dalgliesh [3] have developed a moisture index
for building enclosure design proposed for mapping North American climate regions
according to moisture loading and the potential for drying. Djebbar et al. [4] introduces a
freeze-thaw index as one of three hygrothermal indicators to characterise the moisture
durability performance of masonry wall assemblies in high-rise buildings. Hoppestad’s
driving rain map for Norway [5], developed as early as 1955, is widely recognized. A new
method for assessing driving rain exposure based on multi-year records of synoptic
observations of present weather, wind speed and direction coupled with average annual
rainfall totals has been developed by Rydock et al. [6].
In Norway, there are many weather-beaten areas where it is particularly important
to take into account climatic challenges at the local level. The basis for calculating
characteristic wind and snow loads on buildings is regulated by Norwegian and
international standards. At present there are no corresponding, easily accessible design
guidelines for the quantifying and sizing of e.g. moisture loads and frost damage risk.
Furthermore, and even more disturbing, knowledge of sound building traditions and
practice adapted to local climatic conditions seems to vanish in our quest for standardised
cost-effective solutions. A navigable way of ensuring high-performance building
enclosures is in our opinion to develop climate classifications or climate exposure indices
for different building materials and building enclosures.
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