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Thermoblock Maintain Continuity to “Wrap” for Bespoke Bristol House Build

Already well accepted across the UK construction industry as a quick and effective means of addressing cold-bridging around the perimeter of building oversites, or beneath parapet walls, Bristol based architects Marshall & Kendon have sought technical advice from manufacturer Marmox, regarding the integration of its Thermoblocks into the exterior insulation ‘wrap’ of a bespoke, near-to-zero timber frame construction.


Dovetail Design & Build is building the highly insulated home on an elevated site in Kingsdown Parade once occupied by a 17th century summer house and where the existence of a historically significant cellar required the raft type foundation to permit future access for archaeologists. Marshall & Kendon worked with the timber frame specialist to devise a suitable suspended timber floor design which would create a shallow undercroft. However, maintaining continuity of the insulation and airtightness membrane around the entire envelope posed multiple challenges.

The founding partner of Marshall & Kendon, Jeremy Marshall, confirmed: “It is our habit as a practice to always run insulation across the outside of the structure, wrapping them rather than putting it within the timber frame. In this instance, Thermoblock enables us to make the layer of insulation continuous as it passes through the loadbearing structure, while the airtightness membrane is continued up the outside of the sheathing boards. Our structural engineers were satisfied that Thermoblock offers more than adequate compressive strength and our main contractor, who is very innovative in their approach to timber frame construction, agreed to trial them. Basically the standard TRADA detail for this is 10 years old now and very laborious: while also dependent on all of the insulation being cut very accurately to fit between the framing members. It is far preferable to have a continuous layer of insulation where you can see all of the joints and get at them if necessary should damage occur during the build.”

Marmox stockist, Eco Merchants of Swindon, supplied a total 170 of the 100 mm wide version for the versatile 600 mm long Thermoblocks, with Dovetail’s own operatives utilising an approved adhesive to not only seal the overlap joints, but also to bed the blocks across the OSB base which is covered by a ‘paintable’ membrane. The various lines of Thermoblocks then support the stick-built timber frame exterior and internal walls, with the main isocyanate floor insulation panels infilling the rest of the ground floor area. The Site Manager for Dovetail, Jean Goubert commented: “The fall on the plot and the existence of the old cellar created additional challenges, with a 600mm undercroft being created over the concrete raft using blockwork sleeper walls to support the timber ground floor. While we had to use a diamond disc to cut the Thermoblocks accurately, they have proved really useful in preventing bridging of the insulation, while we are dressing the airtightness membrane up the timber frame and around all the corners. It is a very interesting build.”

With a proven track record going back more than a decade, Thermoblocks are available in widths of 100, 140 and 215mm and feature a unique design where two rows of high strength epoxy concrete mini-columns are encapsulated in sections of XPS (Extruded Polystyrene). They are attached at either end to the top and bottom layers of glass-fibre reinforced polymer concrete to achieve a good bond with the rest of the structure.

How Thermoblocks Help Part L Compliance : Marmox’s Technical Advisory Service is able to assist architects, engineers and other specifiers with incorporating Thermoblock into their designs in order to avoid having to adopt the punishing default values under SAP or similar assessment methods: thereby helping to achieve Part L compliance in a practical and economic manner. The manufacturer’s sales team are further available to advise on project specific applications or logistical considerations – including the use of Marmox’s 360 multipurpose adhesive for sealing the stepped end joints – while Thermoblocks are available from stock through a nationwide network of suppliers.

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