Is it Possible to Insulate Solid Walls?
Learn about the advantages and disadvantages of solid wall IWI and EWI for your home
Knowledge Article5 min read
Aside from its location at the centre of a 356,000 sq ft production facility, the Vistry Innovation Centre (VIC) looks much like a detached home you might find on any new build estate around the country. Inside, however, the timber frame building houses over 100 innovative technologies, helping it to act as an exemplar for future net zero carbon homes.
The VIC is located within Vistry Works, East Midlands — one of the largest timber frame construction facilities in the UK which, together with Vistry’s sites in Warrington and Leicester, has current capacity to deliver over 7,000 homes via offsite methods. Each room within the VIC explores a different theme including electric heating, water efficiency, sustainable materials, smart tech, and more.
LOCATIONLeicestershire, England
One of the upstairs rooms is dedicated to exploring how Passivhaus can be achieved and includes the use of Kooltherm K118 Insulated Plasterboard on the external walls.
Jack Brayshaw, Head of Technical Innovation at Vistry Group, commented:
“The new Vistry Innovation Centre… is providing us with the opportunity to implement and trial a whole host of innovative technologies as we gear up for the introduction of the Future Homes Standard and the journey to net zero.”
Vistry chose to explore Passivhaus approaches within one of the rooms as a growing number of local authorities are looking to build to this standard and the Scottish Government is looking to introduce its own equivalent of the standard for all new homes.
Vistry chose to demonstrate two different wall specifications within the room, both using Kooltherm K118. The product combines insulation, drylining and vapour control in a single board, providing a fast and simple solution in applications where internal wall insulation is required. Its phenolic insulation core also provides excellent performance with a thermal conductivity of just 0.019 W/mK.
By specifying the boards for installation on the inner face of the external walls within the Passivhaus room, the project team was able to “top up” the insulation performance of the full-fill stud insulation, allowing the U-value targets to be reached, whilst also limiting thermal bridges through the repeated studwork. The product’s excellent thermal performance meant both these objectives could be achieved with a slim board thickness, limiting any loss of internal floorspace within the home.
A 52.5 mm thickness of Kooltherm K118 was used in combination with a VCL and full-fill insulation for one wall – delivering a U-value of 0.15 W/m2K. On the second wall the thickness of K118 was reduced to 37.5 mm, again in combination with a VCL and full-fill insulation, achieving a U-value 0.17 W/m2K.
Hannah Rapson, Group Technical Innovation Co-ordinator at Vistry Group, added:
“The fabric was integral to this room and allowed us to also demonstrate complementary technologies to show an all-round approach to achieving Passivhaus. This included triple glazing, Passivhaus-certified loft hatch, mechanical ventilation heat recovery (MVHR), airtight tapes, and skirting board with rubber strip for airtightness.”
Product technicians from our Product Awareness team provided support with the installation of the K118 boards. To ensure a continuous insulation and vapour control layer on the inner face of the walls, the space was carefully measured and the boards cut to the precise dimensions with a standard hand saw. They could then be easily lifted, butted up to the adjoining board and fixed into place on the inner face of the timber studs with appropriate drywall screws.
Page reviewed 02/03/2026
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