Lisø PhD Dissertation Manuscript - NTNU

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northern regions (Nordland, Troms and Finnmark county) the analysis indicates a rather significant increase in frequencies of strong winds in coastal areas, but for these regions the uncertainty in the scenarios is particularly large. Scenarios for climatic extremes are presently being developed in the Norwegian RegClim-project (Regional Climate Development under Global Warming, see www.nilu.no/regclim). The preliminary results (Haugen & Debenard 2002) indicate a rather strong increase in frequencies of heavy rainfall, and a decrease in heavy snowfall episodes in low-elevation areas (see Table 2). Freezing and thawing events were analysed by studying a number of successive 6-hourly values where the temperature passed 0°C, and for most regions a general decrease was found (Haugen & Debenard 2002). 5 IMPACTS OF CLIMATE CHANGE ON BUILDING ENCLOSURE PERFORMANCE 5.1 Introduction Climate change will have different climatic impacts on different types of buildings depending on scale, use, design, construction and location. When designing building enclosures to resist wind actions, extremes are much more important than mean wind velocity values. For certain types of house facings (e.g. rendered (plastered) walls) the duration of rainy periods might be of greater importance than the maximum intensity of precipitation that occurs in the form of driving rain (combined rain and wind). For other types of external walls (e.g. board-clad walls) the intensity of driving rain may be the most important. The total number of freezing and thawing cycles is significant when the whole-life performance of masonry constructions is to be determined. For polymer materials the sum of ultraviolet radiation may determine the lifetime of the products, rather than the yearly averages in temperature. Many parts of buildings’ external enclosures are likely to be subject to faster degradation in parts of the country, for example due to increased frost occurrence or ultraviolet radiation. These few examples are given to illustrate the complex relationship between building materials, structures and climatic impact, and to illustrate the need for more advanced and accurate methods for vulnerability assessment of building performance in relation to climatic impacts. An overall view of building physics challenges concerning the design of roofs and façades is given in the following sections. 5.1.1 Roofs 5.1.1.1 Pitched insulated roofs The traditional type of roof for single-family houses in Norway are ventilated and insulated pitched roofs (illustrated in Fig. 2). The purpose of ventilating such roofs is to avoid damage to the roof structure due to indoor moisture and/or built-in moisture being accumulated in the roof, and to keep the roofing cold so that melting snow, damaging icing and water retention can be avoided. Accumulation of moisture may be caused by condensation problems, rain water and air leakage through minor cracks in the structure, or diffusion. Figure 2. The traditional type of roof for dwellings in Norway are ventilated and insulated pitched roofs (photo: K.R. <strong>Lisø</strong>). Norwegian building tradition places great emphasis on the ventilation of insulated pitched roofs. Still, the need for and design of such ventilation has been a subject of discussion and varying practice for a number of years. This is particularly true for the height of the air gap itself and the size of gap openings (air inlets and outlets) at the eaves, ridges and gables. However, it has proved difficult to provide detailed and general guidelines for the design of air gaps and gap openings. The need for ventilation depends on how much heat is transferred through the roof structure, drying requirements, indoor and outdoor relative humidity, the shape, angle and surface area of roofs, and local climatic conditions such as wind speed, amount of snow on the roof, outdoor temperatures, sun exposure and long wave radiation exchange between the roof surface or the snow cover and the atmosphere. In exposed, cold districts of Norway, driving rain and snow penetration at roof eaves and ridges can be a direct cause of moisture problems occurring in roof structures. The problem is most visible in areas
with large amounts of precipitation and high wind speeds. Normally there is a clear need for air gaps and openings, for reasons discussed here. For reasons of penetration by precipitation and preventing the spreading of fires, air gaps and openings should however be no larger than absolutely necessary. In particularly affected areas, snow penetration problems are often solved by closing all air gap openings, trusting that necessary ventilation will be ensured by air leakiness through small unintentional air gaps in the roof structure. Such proofing can however lead to insufficient ventilation, poor indoor climate and the formation of dampness and mould. The protection of gap openings against snow penetration should be based on local experience from particularly weather-beaten districts in Norway. Figure 3. The present design criteria for air gaps have been found to be the risk of ice formation on roofs caused by melting snow (photo: Y.O. Sæbbe, Nordlys). The amount of building damage on roofs is closely connected with a complicated roof design, insufficient venting (narrow ventilation ducts under the roofing) and other inadequate details. Roofs are not only being exposed to external climatic loads and mechanical stresses. It is also important to understand the impacts of the indoor environment on the design of constructions, especially the air tightness of roofs and outer walls. One of the most common causes of failure in the design of roofs is air leakage from the inside - with subsequent condensation problems. This is especially true when it comes to roofs covering rooms with severe moisture loads. Buildings with large roofs (more than 7 meters from eaves to ridge) and high headroom often experience moisture problems because the same roof design principles are being used as for smaller buildings (e.g. single-family houses). The problems are primarily related to insufficient ventilation of larger roofs, and the consequences are likely to be more severe for these roofs than for smaller roofs. Changes in amounts of precipitation could influence the whole-life performance of roofs. An increase might change the performance requirements of the roofing, the ventilation air gaps and the underlay for roofing. The present design criteria for air gaps have been found to be the risk of ice formation on roofs caused by melting snow (Blom 1990, 2001) (Fig. 3). The design criteria are valid provided the airtight layers (the vapour retarder and the wind barrier) are performing sufficiently. Increasing the thickness of the insulation will decrease the transfer of heat through the roof surface and the likelihood of melting snow. The thermal insulation and continuous wind and vapour barriers in the roof are also deciding factors regarding the prevention of ice formation. Figure 4. A modern pitched insulated roof construction with separate wind barrier and sheathing. The amount of moisture damage in larger pitched roofs calls for a more reliable moisture-proof roof design. Pitched roofs should be insulated continuously from ridges to eaves, and not have partly insulated attics which in practice does not get cold enough to avoid ice formation on the roof. Ideally, roofs should have all thermal insulation placed on the cold side of the roof’s load-bearing structures.
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Building envelope performance asses
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Page 43 and 44: LisÖ et al. on how to cope with an
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Page 75 and 76: cost of repairing process-induced b
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Acknowledgements This paper was wri
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Climate adapted design of masonry s
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2. Masonry defects in Norway 2.1. S
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Fig. 2. Example illustrations of th
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The presented results are in good a
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appropriate in a more severe type o
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screen in a two-stage weatherproofi
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INCREASED SNOW LOADS AND WIND ACTIO
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The hurricane (Beaufort number 12)
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The extensive revisions of the code
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The number of buildings investigate
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As is apparent from the values in t
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climate change is now dominated by
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TABLE 3. Summary of findings Struct
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Lisø, K.R./ Building envelope perf
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temperature of -4°C or less. See F
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Today, frost resistance of brick, c
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degree of saturation, freezing will
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accelerated frost damage or frost d
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A possible objection to the present
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[17] Dührkop H., Saretok V., Sneck
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Decay potential in wood structures
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Wood decaying fungi will only be ac
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to promote decay prevails. Two of t
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Acknowledgements This paper has bee
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(a driving rain gauge), however, is
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Fig. 1. Map of Norwayshowing the lo
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Frequency 25 20 15 10 5 0 highest p
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interesting to compare those result
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Effects of wind exposure on roof sn
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In ISO 4355 ”Bases for design of
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variety of roofs and wind exposures
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Li and Pomeroy [13] evaluated hourl
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Fig. 5. Exposure coefficients for 3
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meteorological stations are expecte
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Many of the meteorological stations
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the ground with return period of 30
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References [1] Standards Norway. De
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