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BBSR-Online-Publikation 01/09, Eds.: BMVBS/BBSR, January 2009
Editing:
Technische Universität München (contractor)
Univ.-Prof. Dr.-Ing. Gerd Hauser (project leader), Michaela Hoppe
Federal Institute for Research on Building, Urban Affairs and Spatial Development, Bonn
Andrea Vilz
andrea.vilz@bbr.bund.de
When structural changes are made in existing buildings, these changes have to meet certain standards that are defined by the German Energy Saving Regulation (Energieeinsparverordnung, EnEV 2007). These standards pertain to either the energy performance of the building as a whole or to the thermal transmittance values (U-value) of single structural elements. All requirements of the Energy Saving Regulation are subject to economic, structural and historical factors. The energy cost reduction achieved by the applied refurbishment measures should amortize within the buildings remaining economic life, and the applied refurbishment measures must not lead to any damage of the building structure. In addition, when structural changes are made to historical buildings the original appearance must be preserved.
For these reasons, the Energy Saving Regulation allows for the possibility to apply for an exemption on the basis of a Hardship Case Clause (Härtefallklausel). However, if this exception is claimed the building's energy saving potential can not be tapped to its full extent. In order to find suitable solutions for this problem, the following building situations were chosen for further investigation as they represent typical cases that often lead to an application for exemption:
The problems underlying these conditions are compiled in individual chapters along with energy refurbishment recommendations that take into account economic and preservation aspects of both, building substance and design.
Constructive measures to improve the energy performance of buildings with specialist facade designs, e.g. Wilhelminian buildings with stucco decorated facades, are limited due to the cultural necessity for preserving their original appearance, which also lessens these buildings' energy saving potential. By applying insulating render, only modest U-value improvements can be achieved. Internally applied insulation, however, results in a reduction of the usable space. Consequently, the construction thickness is limited to a certain maximum, which in turn limits the energy saving potential of this measure. Furthermore, internally applied insulation moves the dew point position slightly towards the core of the building element. Inaccurate detailing and execution of the work can, therefore, lead to interstitial condensation, that can cause condensation damage. Further problems are thermal bridging resulting from adjoining building elements such as floor slabs or internal walls. A table showing achievable U-values, which relate to humidity protection, serves as decision-making instrument for matching refurbishment measure and existing building structure. An example project illustrates the impact of these limitations on the overall energy efficiency of the building as well as design scopes for the combination of different measures.
Timber-framed buildings are very sensitive structures in terms of building physics. Their refurbishment, therefore, requires extraordinary detailing accuracy regarding heat and humidity protection, especially when the timber framing is exposed. In this case the wish to preserve the original appearance implies internally applied insulation. The risk of interstitial condensation has to be taken into account when choosing fitting construction materials and compositions. Furthermore, it has to be assured that any moisture, penetrating the construction due to wind driven rain, will dry of before penetrating the construction. As a consequence of the humidity problems mentioned above, the possibilities for energy refurbishment measures are limited in this case. Reasonable constructions in respect of building physics result in relatively poor U-values. Therefore, appropriate refurbishment measures are outlined, along with their influence on the building's energy performance. Further information on this topic can be found in the data sheets on timber framed building renovation, published by the International Association for Science and Technology of Building Maintenance and Monument Preservation (Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege e.V.).
Until the 1980s balconies and canopies were usually constructed by cantilevering floor slabs. These severe thermal bridgings often lead to increased mould formation. This problem will be further exacerbated by the use of externally applied insulation that is interrupted at the cantilevering balconies. Approaches to improve this situation vary from the above mentioned possibility of interrupting the thermal insulation at the cantilever to encasing the cantilever with insulation material. Demolishing the balcony and reconstructing it as a thermally independent structure causes increased construction costs, but allows a structurally sound execution of the insulation works, along with the possibility to adjust the balcony size to present standards. Transforming the balcony into a winter garden, by closing it with a glass construction, is a further way to reduce heat losses that also offers a better usability of the balcony, e.g. in cold periods. The refurbishment measures mentioned above vary in terms of their energy saving potential, construction costs, thermal bridging and usability. A profound analysis of these factors provides assistance in choosing an appropriate measure.
Adding an additional thermal insulation layer on top of an existing roof deck is likely to cause problems at structural connections, e.g. thresholds, due to the increase of the overall floor thickness. Parapets or railings might need to be raised in order to provide the obligatory fall protection. Replacing the whole roof build-up is one alternative which results in an only modest increase in thickness of the overall roof build-up, but also in higher construction costs. The use of vacuum insulation panels can be reasonable in this case, as it combines a low roof build-up with an excellent insulation value. The higher material costs amortize as building costs, caused by the necessity to raise the parapet and railings, as well as problems at structural connections, no longer arise. Applying thermal insulation from below can also be an appropriate solution for rooms with high ceilings but need to be thoroughly detailed to avoid any future humidity problems. Attention should also be paid to thermal bridge problems along the parapet. A comparison of different roof build-ups and achievable U-values assists in finding the appropriate measure.
In buildings without a basement, an additional thermal insulation can only be applied to the ground floor slab from above. In doing so, the floor build-up increases and, thus, reduces the clear floor to ceiling height and causes difficult connections at thresholds. An alternative is the replacement of the whole floor build-up which results in a modest increase of the floor build-up. Using vacuum insulation panels, instead of conventional insulation materials, results in an overall low floor build-up along with excellent insulation value, however, also increased costs. A comparison of different floor build-ups and achievable U-values assists in finding the appropriate measure.
If only one single house within a row of staggered terraced houses is refurbished by using externally applied insulation the construction inevitably protrudes beyond the property line, due to the necessary material thickness. Accordingly, the owners of the adjacent properties have to agree, otherwise this externally applied insulation can either not be realised or can be realised only with insufficient material thickness. Possible solutions include not to insulate the affected walls (not recommended) or to reduce the material thickness. A further alternative is to change the construction system and effect an internally applied insulation on the concerned walls. Combining internally and externally applied insulation usually leads to thermal bridging at the structural connections that is likely to cause mould growth. Therefore, it is necessary to develop solutions that minimise this problem. All possible solutions affect the energy performance of the building. These consequences can be quantified, according to the depth of the stag between the houses.
The abstract is part of the German publication "Wärmeschutz für Sonderfälle", BBSR-Online-Publikation 01/09, Hrsg.: BMVBS/BBSR, January 2009, Bonn
ISSN 1868-0097, urn:nbn:de:0093-ON01209NR224
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