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Non-insulated balconies and other cantilever construction elements are potentially the most critical thermal bridges in the building envelope. Legislation is tightening and Michael Currier warns that specifiers need to be aware that in some cases claims about product performance levels leave something to be desired.
Preventing thermal bridging (cold bridging), is becoming increasingly critical in the construction process. Predominantly, this is due to legislation and the need for energy conservation and a reduction in CO2 emissions. Other factors too have become the focus of environmental health concern. Low internal surface temperatures in the area of the thermal bridge can cause condensation if they are below the dew point of the air. This leads not only to structural integrity problems with absorbent materials, but the occurrence of mould growth – which can have serious implications for the occupants in the form of respiratory problems and dermatitis.
Thermal bridges occur where the insulation layer is penetrated by a material with a relatively high thermal conductivity and at interfaces between building elements where there is a discontinuity in the insulation. Heat and energy loss in these areas can be as much as 15 to 30 per cent.
So when it comes to an effective solution to thermal bridging, there are a number of criteria to be considered. The module should sit between the outer and inner connection points of a balcony, or other cantilever elements of a building; on the line of the envelope insulation, controlling the outflow of heat through the use of a high-quality insulation material. It should also be designed so that it takes the tension, compression and shear forces between the building frame and the cantilever element. This ensures that load transfer and full structural integrity are maintained, while at the same time providing a measurable and verifiable interruption to heat flow.
Then there is condensation risk. Does the thermal break module comply with this particular stipulation? In the UK, the temperature factor used to indicate condensation risk is (fRSI) – the higher its value the lesser the risk of condensation and mould growth. It is described in BRE IP1/06 – a document cited in Building Regulations Approved Documents Part L1 and L2 and Section 6 in Scotland – where its value must be greater than, or equal, to 0.75 for dwellings, residential buildings and schools. For commercial buildings it must be greater than, or equal to 0.5.
The Government Standard Assessment Procedure, SAP 2005, concerning CO2 emissions from buildings, and respectively heat losses through non-repeating thermal bridges, has to be considered too. Here, the lambda values of a particular module should enable the energy losses through balconies, canopies and other cantilever parts of the building to be significantly reduced.
So, when considering the specification of a structural thermal break, nothing should be taken for granted and key questions need to be asked before accepting or naming a product for incorporation. Not all such products on the market can be considered bona fide elements that will provide reliable performance over the lifetime of the building, or indeed provide compliant performance from time of installation. The specifier should be assured the product has been independently and expertly verified, in combination with all other components which make up the structural thermal connection (particularly in steel connections). For example, an item within the connection which is thermally efficient in isolation may well not perform at all efficiently as an integral part of a composite. Thermal performance must be fully explored and verified before acceptance. |
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