Staying clean: heat and damp
Breaking the mould
26 September 2018
Structural breaks can reduce thermal bridging and the risk of mould growth, explain Jon Denyer and Chris Willett
Heat loss from thermal bridges at balconies and other cantilevered connections is a critical design consideration for dwellings, and the risk of condensation and resultant mould growth is just as important a matter.
Concern about these coincides with efforts to minimise energy use in buildings and improve the thermal performance of the envelope, which has become increasingly important in the drive for sustainability and energy efficiency. We have seen the adoption of more demanding requirements for the thermal performance of the building envelope through voluntary certification schemes such as BREEAM and Passivhaus. There are also the stringent criteria of Part L1A of the Building Regulations, which state that:'
'The building fabric should be constructed so ... there are no reasonably avoidable thermal bridges in the insulation layer caused by gaps within the various elements, at the joints between elements, and the edges of the elements, such as those around window and door openings.'
To compensate for heat loss at any thermal bridging points by adding more insulation elsewhere is therefore not an effective way of reducing the risk of surface condensation at thermal bridging points with low surface temperatures. Where structural components such as balconies, parapets or canopies penetrate the insulation layer, though, thermal separation of the exterior structure from the interior structure can significantly reduce heat loss.
Structural thermal breaks
Separation can be achieved effectively by incorporating a structural thermal break at the various critical connections. However, it is important that the break incorporated is tested to ensure that it meets the required performance standards. Typically, highly conductive materials at the point of connection such as reinforced concrete – which has a thermal conductivity, or λ, of 2.2W/(m K) – or structural steel, with a λ of 50W/(m K), are replaced with expanded polystyrene (EPS), with a declared conductivity, λD, of 0.031W/(m K). Used at a minimum thickness of 80mm, this ensures effective thermal separation.
EPS is non-structural and constitutes the main body and surface area of the thermal break. To conserve the structural integrity between exterior elements such as balconies, parapets or canopies and the interior structure such as the floor slab, reinforcement bars are therefore used to connect both sides and transfer tension and shear loads. These traverse the thermal break’s insulation body, and ideally should be made of high-strength stainless steel, λ = 17W/(m K), rather than carbon steel, = 50W/(m K). This not only reduces thermal conductivity but also guarantees longevity through its inherent resistance to corrosion.
To transfer the compression loads, the thermal break uses special compression modules made of high-strength concrete, λ = 0.95W/(m K), as these offer better thermal performance when compared with compression bars made of carbon steel or even stainless steel. Thermal bridges can lead to greater energy use for both heating and cooling, potential non-compliance with Building Regulation guidance, and problems with the structural integrity of absorbent materials such as insulation products or plasterboard. However, such bridging may occur wherever you have a structural connection. With non-insulated cantilevered elements such as balconies, the cantilever itself has a cooling fin effect, and interacts as a geometric thermal bridge with the material one where the reinforced concrete or steel penetrates the insulation layer, leading to significant heat loss and reduced internal surface temperatures.
Cantilevered balconies and exposed slab edges are thus considered to be the most critical thermal bridges in a building envelope. These are obviously crucial aspects of thermal bridging, but for the building occupants there are further, potentially serious, health implications.
Low internal surface temperatures in the area of the thermal bridge are likely to result in condensation, which can lead to mould growth. This in turn can create potentially significant health problems, particularly for the elderly and the very young, who may be exposed to health risks ranging from simple skin irritation, dizziness and flu-like symptoms to serious illnesses of the respiratory tract such as allergic asthma.
Mould is not a new phenomenon of course, but a combination of circumstances is leading to increased concern. Although buildings are better insulated and more airtight as a result of improved energy efficiency requirements, the fabric of almost all buildings, irrespective of their construction, contains mould spores. At construction these are dormant and entirely harmless, but given the right conditions they will germinate.
Fungal growth can set in even before the condensation point, and the temperature zone for optimal growth also lies in the range that human beings find comfortable. Moulds do not contain chlorophyll and as a result do not require light to grow. Substrates for mould growth are nearly always available as well, and they even make do with house dust in the simplest cases. Paints or wallpaper promote growth in some instances, and organic coatings, deposits or soiling on the surface of construction elements accelerate it considerably, the substrate itself then becoming irrelevant.
Areas of risk
The use of a surface temperature factor, fRsi, allows surveys under any thermal and design conditions to show areas where there is a risk of condensation and therefore mould growth.
This is a ratio that compares the temperature drop between the outside air and the internal surface of the building with the total temperature drop between the inside and outside air. It is described in BRE IP1/06, where standardised boundary conditions are assumed, and this paper is cited in Building Regulations Approved Documents Part L1 and L2 and section 6 in Scotland.
The actual surface temperature will depend greatly on both the internal and external temperatures at the time of the survey. So, crucially, the fRsi is formulated to work independently of the absolute conditions.
The recommended value for fRsi in offices and retail premises is equal to or greater than 0.50. To ensure higher standards of comfort for occupants in residential properties and to prevent condensation and mould growth, it should be equal to or greater than 0.75. In conditions of high humidity, such as swimming pools or other wet areas, an fRsi of 0.90 would be anticipated.
In such situations, the incorporation of structural thermal breaks can significantly reduce energy loss in connective areas. This is achieved by minimising the extent of thermal bridging penetrating the thermal envelope between cantilevered structures and the internal slab.
As an example, Schöck produces a range that allows connections to be made between concrete and concrete, concrete and steel, and steel and steel. Verification of performance values means these products comply fully with relevant Building Regulation guidance, and with the Standard Assessment Procedure for carbon dioxide emissions and heat loss. It has also been approved by the NHBC, has registered details with the LABC, and been certified by the British Board of Agrément (BBA).
BBA assessment of the thermal break connectors required explicit identification of the dimensions and thermal conductivity of the product components. When reviewing, the BBA checked for components that were thermally insulating or non-standard, and third-party initial type-testing data. Components were also assessed as part of an ongoing factory quality-control process to confirm the validity of the declared or designed thermal conductivities.
It was discovered that the psi value for heat loss and the minimum temperature factor of a junction depend significantly on its overall construction and the insulating properties of the wall and floor materials around the thermal break connector. Assembly and junction section drawings were therefore reviewed to identify typical UK junction details, to estimate best and worst cases in terms of overall junction minimum temperature factors and additional heat-loss psi value.
These sections were then numerically modelled by BBA experts, using finite difference software complying with EN 10211 and the UK modelling conventions document BR 497. The outputs were compared with benchmark values in the guidance documents supporting the Building Regulations across the UK. The acceptable constructions were illustrated in an Agrément Certificate with positive statements of regulatory compliance, provided that other aspects of performance such as structural adequacy are also satisfactory.
When it comes to dealing with problematic details at the beginning of the building design process, professional certification of products and systems is a must for everyone involved – from architects and specifiers to manufacturers and contractors.