The redevelopment of a popular primary school for Central Bedfordshire Council has included the specification of Marmox Thermoblocks to tackle the critical issue of thermal bridging around the building perimeter, as well as beneath cross-walls to the main structure.

The £7.7 million expansion and refurbishment of Thornhill Primary School in Houghton Regis is being carried out by Ashe Construction to the designs of ECD Architects and Symmetrys Structural Engineers, with MHL Consulting, KP Acoustics, and KLH UK Ltd providing specialist support.  ECD worked with the quantity surveying team from Keegans Group to ensure sustainable building materials were identified so the new facility’s PassivHaus specification would translate into significant benefits in terms of build quality, thermal comfort and lower running costs.

In the first phase of the build carried out this year, 204 of the 600mm x 215mm x 100mm Thermoblocks were supplied by the Hemel Hempstead branch of EH Smith builders’ merchants,  subsequently being installed by the groundworks sub-contractor, Thamesbridge Construction, employing the special Marmox sealant for the overlap joints.

The Senior Architectural Technologist on the project for ECD, John Heaney, explained: “I’ve been aware of Thermoblocks for a number of years through product research and in conjunction with the structural engineers we decided to employ them for the construction of the Thornhill Primary School as we are hoping to have it fully PassivHaus certified, which of course involves successfully addressing cold-bridging around the building envelope.”

“There are other insulation products available on the market, but none with the micro concrete columns which provide high load carrying capacity within the Thermoblocks and are therefore ideal for use in relation to the higher weight of a CLT structure.”

The Technical Director for Ashe Group, Dan Armes, added: “There is a tendency within the construction industry to believe that achieving PassivHaus is primarily about airtightness, when it is only one of the technical challenges, so as the main contractor we therefore embarked on a number of training courses and workshops for key personnel, prior to the start on site.

As a key element to combatting thermal bridging the Thermoblocks have proved straightforward to use, laying them on the strip foundations with a mortar bed to bring them level with the upper, 120mm layer of rigid insulation.  The in-situ concrete slab is then cast across this with an upstand which carries the CLT perimeter walls to the building.  The airtightness layer goes on the outside of the CLT* with vertical insulation installed down across the face of the Thermoblocks to complete the wrapping of the structure.”

Ashe’s Managing Director, Ian Robbins, commented: “We’re thrilled to have been awarded the Thornhill Primary School project. To deliver a PassivHaus scheme of this scale demonstrates our commitment to delivering buildings that leave a long lasting positive legacy for our clients.”

Marmox Thermoblocks are available in widths of 100, 140 or 215mm and are formed from sections of XPS (extruded polystyrene) encapsulating two rows of high strength, epoxy concrete mini-columns.  These are attached at either end to the top and bottom layers of glass-fibre reinforced polymer concrete, to ensure a good bond with the rest of the structure.

As well as combating cold-bridging at the base of blockwork walls, Thermoblocks are also often utilised to support timber frame construction or at vulnerable upper floor junctions including, as in the case of a student accommodation building in Brighton, beneath parapet walls.  Crucially, a variety of details have been thermally modelled by the BRE to provide insulation values for use in SAP or other calculations, avoiding the punishing ‘default’ figure.

* Cross Laminated Timber


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