Preventing heat loss from dwellings as a result of thermal bridges at balconies and other cantilever connections is an important design consideration. However, it is the effect of condensation and mould growth on occupants that is currently very much in the spotlight. To help eliminate this, the use of an effective thermal break module can be critical.
Cantilever balcony connections and other similar construction elements with a high thermal conductivity, and which project through the building envelope breaking the insulation layer in the process, are prime causes of thermal bridges. One of the consequences of this is local heat loss, resulting in more energy being required to maintain the internal temperature of the building. However, while this remains a very important aspect of thermal bridging, there is a more serious issue as far as building occupants are concerned.
(Thermographic showing heat loss from balconies)
Low internal surface temperatures in the area of the thermal bridge can cause if they are below the condensation dew point of the air; and this in turn can lead not only to structural integrity problems with absorbent materials such as insulation products or plasterboard, but the occurrence of mould growth. The effect of this has potentially serious implications in the form of asthma and allergies, particularly for older people and children.
Mould is not a new phenomenon of course, but a combination of circumstances is elevating interest in the problem. Primarily these are better insulated and more airtight buildings, improved energy efficiency requirements – plus much greater environmental awareness. Practically every building, irrespective of its construction, contains mould spores within its fabric which are dormant and completely harmless. However, given the right conditions these spores will germinate. Mould needs very little nutrient and not only grows on walls and ceilings, but spreads into the air.
Research by the Fraunhofer Institute for Building Physics has shown that, with only 80% relative humidity – and contrary to common o pinion – fungal growth can set in even before the condensation point. The temperature zone for optimal growth also lies exactly in the temperature range which human beings find comfortable. Moulds do not contain chlorophyll and as a result do not require light to grow. Also, food substrates are nearly always available, with moulds simply making do with house dust as a substrate in the simplest cases. Paints or wallpaper promote growth and organic coatings, deposits or soiling on the surface of construction elements accelerate growth considerably, the substrate itself then becoming irrelevant for growth of the fungus.
The use of a ‘surface temperature factor – (fRsi)’ allows surveys under any thermal conditions to show areas where there is a risk of condensation and therefore mould growth under different design conditions. 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 – and compares the temperature drop across the building fabric, with the total temperature drop between the inside and outside air.
As the actual surface temperature will depend greatly on the temperatures both inside and out at the time of the survey, crucially, the ‘surface temperature factor – (fRsi)’ has been formulated to work independently of the absolute conditions.
The recommended value for (fRsi) in offices and retail premises is equal to or greater than 0.5; and to ensure higher standards of occupancy comfort, equal to or greater than 0.75 to prevent condensation and mould growth in dwellings. In more extreme conditions of high humidity, such as swimming pools or other wet areas, 0.9 would be anticipated.
In such situations, the incorporation of an effective thermal break module can dramatically reduce thermal energy loss in connective areas – by guaranteeing the homogeneity of the thermal envelope between cantilever structures and the internal slab. This type of module works by sitting directly between the outer slab edge and the balcony, or other cantilever element, blocking the outflow of heat. This is achieved through the use of a high-quality Neopor® insulation body with in which are compression modules and stainless steel bars. These bars allow the transfer of shear and tension forces between the building frame and the balcony in addition to the compression forces transferred through the modules.
(The Type K Isokorb in a balcony application )
These thermal breaks transfer load and maintain full structural integrity, while at the same time maintaining inner surface area temperatures well above those likely to cause mould formation and condensation.
The Fraunhofer Institute of Building Physics (IBP) in Stuttgart deals with research, development, testing, demonstration and consulting in the fields of building physics.
(a single thermal break module)
The Schöck Isokorb® range of thermal break modules controls thermal bridging and maintains minimum surface temperature obligations for structural connections involving balconies and other cantilever construction elements. The range is unique in being able to offer solutions for connections between concrete-to-concrete, concrete-to-steel and steel-to-steel. It is also the only product of its type to provide BBA Certification and LABC registration.
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