t has been a long-held view by some that renovation of old housing stock can make a positive contribution to the longer-term housing shortfall in the UK. As long as the schemes are cost-effective and create energy-efficient buildings that perform to the required standards of new builds. These days, you can add the renovation and retrofitting of former office premises into the mix as well. There were few of this type of development before 2013, but now that the planning laws have been permanently relaxed, applications and starts for office-to-residential conversions across the UK have risen dramatically.
There are, of course, many challenges in undertaking both housing stock renovation and office-to-residential conversion. But the replacement of thermally-inefficient cantilever balconies, or the retrofitting of new cantilever balconies, has its own demands – particularly the avoidance of thermal bridging, which is an important issue.
Consequences of thermal bridging
Cantilever balcony connections with a high thermal conductivity and which break the insulation layer when projecting through the building envelope, are prime causes of thermal bridges and should be given special attention. One of the consequences is local heat loss, resulting in more energy being required to maintain the internal temperature of the building. This is certainly a very important aspect of thermal bridging, but there are other issues too. Low internal surface temperatures in the area of the thermal bridge can cause condensation if they are below the 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. Once mould has formed, it has potentially serious health implications, particularly for older people and children, in the form of allergies and respiratory diseases.
At the very least the solution must meet UK Building Regulations
Meeting current UK Building Regulations with the chosen balcony solution is the very minimum standard and, ideally, the approach taken should demonstrate compliance with other industry certification and registration standards as well. There are three main dedicated methods for renovation and retrofitting that enables cantilever balcony connectivity to meet regulatory thermal performance standards.
The first technique is the incorporation of an IPE beam into the existing ceiling, where concrete is then poured in to prevent any slippage of the construction towards the balcony. A structural thermal break is already fixed to the top plate, before it is positioned in the ceiling, ready to be connected to the cantilevered steel balcony. This implementation option has proved itself many times and is economically justifiable, but is only workable if it is possible to access the ceiling interior. This solution is, therefore, not ideal. A more traditional alternative is ‘posted’ balconies with four supports. However, with inevitable security concerns and an ever-increasing need for ground floor vehicle access, especially in busy urban areas, this is no longer such a favoured option.
By far the most attractive proposition is to attach a load-bearing structural thermal break directly into the concrete slab face of the building, to which a cantilevered steel balcony can then be attached. This solution requires drill hole positions to be marked with a template on the face of the building. Holes are then drilled into the concrete slab, adhesive injected and the load-bearing reinforcing rods of the thermal break slipped into position and anchored by means of the pre-injected adhesive. Structural screed is poured into a special pocket between the concrete slab and the thermal break to ensure a perfect contact between the two structural elements. Once both the mortar and the screed are cured, the structure is ready for the new balcony to be connected. One of the most effective structural thermal breaks on the market for this purpose is the Isokorb, which is a ready-to-install component.
It has a 120mm insulation thickness and standard heights of 160 to 220mm, which offer different design options for integrated, energy-efficient connectivity. It is ideal for office-to-residential retrofitting, or housing stock renovation. And, in the case of existing housing stock renovation, because the installation is entirely external, any residents present can remain in the building; and there is no risk of damage to internal finishes and decoration either. An additional feature is that this particular product has been fully certified as an ‘Energy-saving component’ by the Passivhaus Institute in Darmstadt, Germany. When this type of thermal break is incorporated, the required domestic dwelling fRsi value (the temperature factor used to indicate condensation risk that must be equal to or greater than 0.75) is always comfortably met.
An experienced design team is essential
Unlike new builds, product selection for renovation is subject to certain restrictions. The solution will depend on the specific project requirements, the existing building and its structure, as well as the development of customised design proposals. It is essential that an experienced design team is on hand to provide extensive technical support on all these issues throughout the project. This should include product selection advice for the different options; building physics detail; framework conditions; methods for undertaking a building inventory; dimensioning examples and a comprehensive checklist.
Atrium Point – one of the largest conversion projects in London
Atrium Point at Sudbury Hill in North West London, previously the London headquarters of the international contractor M.W Kellogg, is a good example of an office-to-residential scheme. The existing office buildings are being sustainably refurbished, with solar panels, air source pumps and heat recovery units, all contributing to meet CO2 reductions in the redevelopment. It is currently one of the largest schemes of its type in London and will realise a mix of 290 studios, one- and two-bedroom apartments, with many of the units benefitting from a retrofitted cantilevered steel balcony.