Marmox’s Mark Bowman provides an insight into tackling heat loss at the problematic floor-to-wall junction in masonry and hybrid constructions

Ever since the days of the Egan Report and ‘Rethinking Construction’, the building industry has been presented with a bewildering array of system solutions. These often feature off-site technologies and unfamiliar materials, which are not always suited to UK climatic conditions or our workforce’s inherent skillset. The truth is that while panelised or modular designs – especially timber-based systems – have gained market share thanks to Government and public sector housing initiatives, the wider industry remains more comfortable with what are still referred to as ‘traditional’ designs. However, driven by increasingly more onerous energy reduction targets within the Building Regulations and Fabric Energy Efficiency Standards (FEES), masonry construction has metamorphosed from the basic brick and block cavity work which dominated the mid-20th century, to providing specifiers with a selection of well-engineered wall constructions that are able to meet modern performance standards.

Lightweight or aircrete blockwork inner leafs – often combined with full cavity fill insulation and measures such as parge coats – have seen heat loss and air leakage through the external envelope drastically reduced. In fact, hybrid traditional masonry constructions have even proved themselves capable of creating properties designed to the coveted German PassivHaus standard. There remain, though, enduring challenges for consultants and contractors in maintaining continuity of insulation at particularly vulnerable areas of the envelope – namely preventing thermal-bridging at places like the window reveals or the floor-to-wall junctions.

Thermal bridges create cold spots inside the building, which can lead to condensation and mould growth, with consequent health issues for occupants. Approximately 30 per cent of a building’s heat loss can result from these areas, and as properties have become better performing overall, the significance of thermal bridging has grown disproportionally. To counter their impact, regulators have loaded the SAP SEBM calculations to punish poor detailing, with elements such as steel lintels bridging cavities being obvious weak spots. However, it is the floor-to-wall junction, which often accounts for up to two-thirds of thermal bridge losses, that has been singled out for the toughest treatment.

Taking the Transition Heat Transfer Coefficient (HTB) for the materials making the bridge and dividing it by the surface area of the building provides the y-value, but SAP offers three very different approaches to assessing the effect of thermal bridges. The ‘default’ approach means adopting a y-value of 0.15 W/m2K: the equivalent of having a hole the size of a garage door in the wall of a house. Then there are Accredited Details which will reduce the imaginary hole by nearly half with a y-value of 0.08 W/m2K. Best by far though is to use Thermally Modelled Junctions, which are rewarded with much lower y-values. The conundrum for the specifier is to identify an insulation that has a high enough insulation value to extend the heat path around the junction and thereby cut heat loss, but still be able to support substantial structural loadings.

Adopted by countless consultants and loved by bricklayers for its simplicity, the solution developed by Marmox is a product known as Thermoblock. Providing an installed length of 600mm, and available in widths of 100mm, 140mm and 215mm, Thermoblocks transmit the load of the wall above though a series of high strength concrete columns. These are surrounded by XPS insulation, which account for 85 percent of the total cross-sectional area, and thereby minimise heat loss. This produces a combined thermal conductivity of 0.047 W/mK to input into the calculations.

The blocks feature a stepped interlock to which Marmox Multibond Sealant is normally applied. A top and bottom surface of micro-fibre reinforced polymer concrete creates an excellent mortar key, and at 65 mm in height, the horizontal band of Thermoblocks is the same depth as a brick course. The vertical columns of concrete have a compressive strength of 9.0 N/mm², which enables them to be used within building up to three storeys, including non-domestic structures. Under pressure from industry, especially the major housebuilders, Government accepted that further improvements in insulation values had reached a point where diminishing returns could not justify the very large additional costs of achieving them. Zero carbon targets have effectively been put on hold.

What we are presented with today is a far more pragmatic approach to reducing carbon emissions, where the practicalities of modern masonry construction, well suited to the British weather and existing workforce, offer an attractive answer. Marmox Thermoblocks, present an ideal means of addressing the building envelope’s most problematic area for potential energy wastage.

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