Cantilever balcony connections and other similar construction elements with a high thermal conductivity, which project through the building envelope and break the insulation layer in the process, are prime causes of thermal bridges. There are two major consequences. One is local heat loss, resulting in more energy being required to maintain the internal temperature of the building. The other, condensation, is arguably more serious for building occupants. Low internal surface temperatures, in the area of the thermal bridge, can cause condensation if they are below the dew point. This is likely to result in structural integrity problems with absorbent materials, such as insulation products or plasterboard or worse the development of mould growth. This can be a concern in any location, but in a residential situation, there are serious implications in the form of asthma and allergies, particularly for older people and children.
To avoid these issues, calculating the condensation risk and then taking appropriate preventative action is critical. A formula that allows surveys under any thermal circumstances to show areas where there is a risk of condensation and therefore mould growth under different design conditions is fRsi, or the surface temperature factor (1).
It is a ratio described in BRE IP1/06, a document cited in Building Regulations Approved Documents Part L1 and L2 and Section 6 in Scotland. It compares the temperature drop across the building fabric, with the total temperature drop between the inside and outside air. The recommended value for fRsi in offices and retail premises is equal to or greater than 0.50; and to ensure higher standards of occupancy comfort, equal to or greater than 0.75 for residential buildings.
Once the surface temperature factor is established, it is imperative that thermally efficient elements are installed which enable inner surface area temperatures to remain well in excess of those likely to cause mould formation and condensation.
A highly effective solution to thermal bridging issues is provided by the market leading range of Isokorb structural thermal break modules from Schöck. The Isokorb blocks the outflow of heat using a high-quality, polystyrene insulation foam, that sits inside a compression module. Stainless steel bars pass through it, which take the tension and shear forces between the building frame and the balcony. In addition to its exceptional thermal performance ratings, the Schöck Isokorb range is unique in providing connectivity solutions for concrete-to-concrete, concrete-to-steel and steel-to-steel applications. It also provides BBA Certification and LABC Registration and complies with the Government Standard Assessment Procedure, SAP 2009, concerning CO2 emissions from buildings, and respectively heat losses through non-repeating thermal bridges. Here, the lambda values of the Isokorb enables energy loss through balconies, canopies and other cantilever parts of buildings to be reduced by as much as 84% to 91%.
Chester Balmore and meeting Passivhaus standard
The current Chester Balmore project, near Highgate in North London, part of Camden Councils community investment programme, was once a 1970s estate. The blocks on Chester Road and Balmore Street were in poor condition and refurbishment simply was not a cost-effective option. Now, the project developed by Rick Mather Architects, is designed to achieve Code for Sustainable Homes Level 4 and is mixed-use for around 53 new homes. There are three separate blocks with additional commercial space, making it one of the largest ever residential schemes to meet Passivhaus standard.
The Schöck solution specified for this scheme is the latest generation Isokorb for concrete-to-concrete applications the type KXT. It is a product that has been awarded the "low thermal bridge construction" certificate by the Passivhaus Institute in Darmstadt, confirming its suitability for Passivhaus construction.
A major reason for the superior performance of the type KXT is the thickness of the insulation body increased from the standard 80mm thick to 120mm providing an even more efficient solution. It not only improves thermal insulation performance by up to 30% in comparison to the standard range, it also improves impact sound insulation by around 50% as well.
Tim Paul from Rick Mather comments: "Balconies offer practical and aesthetic advantages for residents, but they have not always been a popular choice in the context of high efficiency Passivhaus design. However, the advanced technology and superior performance of the Schöck Isokorb XT range offers such a high level of insulation, that we had no problem in incorporating balcony design into the Chester Balmore project