ThermoShell IWI

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ThermoShell IWI LogoThermoShell Internal Wall Insulation (IWI) System

Polystud SystemThe ThermoShell IWI System is an internal wall insulation system designed for upgrading existing solid external walls. It consists of thermally engineered insulated studs and water repellent glass mineral wool insulation slabs, which can be combined to provide greater thicknesses of thermal insulation than achievable by using a single thickness.

EcoStuds are a composite of high performance extruded polystyrene (XPS) insulation and Oriented Strand Board (OSB). The OSB is manufactured from timber supplied from Forest Stewardship Council (FSC) certified forests. The IWI System has been designed to simplify the process of upgrading solid masonry walls while delivering high levels of thermal performance. The system can be installed by any competent builder.

Please note - only ThermoShell IWI Approved Installers can purchase our IWI products.

Find out more about becoming an ThermoShell IWI Approved Installer here

The Products 


EcoStud - extruded polystyrene bonded to Oriented Strand Board (OSB)


Earthwool EcoBatt  - water repellent glasswool

Thermal performance

A typical 225mm (nine inch) thick un-insulated masonry external wall with dense plaster internally will achieve a U-value of approximately 2.10W/m2K.

The same wall insulated with the IWI System, using 80mm thick studs, will achieve a              U-value of at least 0.35W/m2K, an improvement in thermal performance of over 80%.

If a typical three bedroom semi-detached house were to be upgraded in this manner, it would reduce the carbon emissions associated with the house by approximately 2.40 tonnes per year.



Typical U-values (W/m2K) for 225mm solid masonry walls*

EcoStud Thickness




65 0.44
80 0.35
95 0.30
130* 0.25
160* 0.19
 * Combination of two studs required to achieve this thickness 

Find out more about the Internal Wall Insulation (IWI) System:

BBA Certificate


Case Studies


PDF IconInternal Insulation of Solid Walls - IWI Design Guide PDF


PDF IconA Simple Solution to a Difficult Problem Brochure PDF


PDF IconThermoShell IWI Homeowner Guide