A self-build project in Herefordshire gave STEICO the opportunity to demonstrate how integrating STEICOjoist (I-joists) and STEICOzell (air-injected wood fibre insulation) into Oakwrights’ new Passive wall panel, helped achieve airtightness and energy efficiency. With tested fire performance, proven offsite compatibility and practical benefits for both production teams and end users, STEICO solutions played a key role in helping to deliver a high-performance home ready for future building standards.
Having an airtight and weathertight building envelope is critical when it comes to constructing dwellings. So, when Oakwrights needed their closed timber-frame panels (Passive timber frame system) to perform to the highest level at a self-build in Herefordshire, it made sense that STEICOzell breathable wood fibre insulation and STEICOjoist I-joists were used.
Over the past 25 years, Oakwrights has established itself as a go-to name in timber frame building, creating bespoke homes nationwide for self-builders and developers. The Passive Wall panel first put to the test on a live 333m² self-build in rural Herefordshire. Designed and constructed by Oakwrights, the three-bedroom home combines traditional oak framing with a contemporary panelised system. It features a cantilevered flat-roof porch, Rationel windows and doors, and a mix of vertical timber cladding and brickwork.
How did STEICO products help make the Passive Wall panel perform on this project?
For this self-build in Herefordshire, Oakwrights used their newly developed Passive Wall panel system in combination with STEICO products. The goal was to achieve thermal performance, airtightness and ease of installation on a real, live site.
Each prefabricated panel was constructed using STEICOjoists — an engineered timber solution chosen for its strength, stability and minimal thermal bridging — and filled with air-injected STEICOzell wood fibre insulation. Together, these components created a breathable and performant wall and roof build-up.
“The advantage of STEICOjoists lies in the availability of high-capacity sizes,” says Alex Lesenechal, Head of Production at Oakwrights. “This provides greater flexibility, either to enhance U-value performance with deeper joists, or to achieve longer structural spans with the range of lengths and widths available.”
For this build, the use of STEICOjoists and STEICOzell played a key role in helping the wall panels achieve a U-value of 0.13W/(m²K), with the roof panels reaching 0.11W/(m²K). Airtightness tests delivered strong results, with air changes per hour (ACH) as low as three.
Oakwrights is a member of the Structural Timber Association (STA). A condition of membership is a third-party quality assessment of their manufactured panel systems including STEICO products. Oakwrights achieved the highest membership accreditation level of STA Assure Gold[1].
Fire resistance testing for loadbearing elements was carried out on the panels to the method within BS EN 1365-1:2012[2] using the general requirements laid out with BS EN 1363-1: 2020[3].
Notably, a thinner version of the panel used here had already passed a 52-minute REI fire test. As the minimum period of fire resistance for dwellings (where the height of the top floor is up to 5m above ground) is 30 minutes[4], the result offers reassurance on safety margins as fire regulations continue to tighten.
What benefits did the use of STEICO products bring to the build?
Alex Lesenechal, Head of Production at Oakwrights, says their incorporation of STEICO wood fibre insulation products into their Passive wall system helps to underpin the energy performance of the panels.
Like the panels, trees are the raw material for STEICOzell insulation where they make up at least 80% of its composition[5]. That means that they have bound within them biogenic carbon from the atmosphere. Although this can eventually be released at the end of life, new insulation and wood products will bind more carbon from the atmosphere. In this way they do not add to long term carbon emissions. Each cubic metre of STEICOzell binds 63kg CO2eq[6] and can thus contribute to climate protection.
“Wood fibre STEICOzell complements our timber frame approach,” Alex says, “allowing us to offer clients what we believe to be a more sustainable product solution.”
For this project, STEICOzell was injected at a minimum of 45kg/m³ into the joist cavities. The insulation conformed closely to the structure, resisting settlement during transport and installation. That helped maintain long-term airtightness, a key performance goal on this self-build.
The specific heat capacity of STEICOzell allows the insulation to act like a buffer in summer. The heat of the day can be stored in the insulated building envelope for hours. A large proportion of the heat does not reach the interior at all and can be dissipated outside again during the colder nights. This may help to mitigate against the effects of overheating and could support a reduction of internal temperature fluctuations for a more comfortable internal climate.
“Even though the STEICO system had a higher material cost,” said Alex, “the savings in process efficiency and quality made it a smarter investment for Oakwrights.”
How did STEICOjoists support structural performance and flexibility?
On this build, the STEICOjoists formed the core structural element within the wall and roof panels. Made from laminated veneer lumber (LVL) flanges and Natural Fibre Boards, the joists offered excellent strength while keeping the system lightweight and easy to work with onsite.
The I-joist shape also helped avoid thermal bridging, and its open profile made it easier to route services without compromising the structure. When paired with STEICOzell, the full cavity could be insulated without gaps, contributing to the project’s airtightness results.
Multiple depths and web sizes gave Oakwrights the flexibility to match structural needs with thermal goals on this specific project, supporting both the architectural layout and energy targets for the home.
Why Oakwrights values STEICO support
More than just a supplier, STEICO played a proactive role in helping Oakwrights bring the Passive timber frame panel to life. From supporting structural and fire testing to advising on the technical details of panel design, they’ve been embedded in the journey.
Besides the product performance that facilitates such builds, Oakwrights also values the back-up and wider service STEICO provides. “What stands out most is their exceptional technical support team and the continuous innovation they bring to their products, backed by detailed technical information,” says Alex. “The STEICO team has been an excellent partner, consistently supporting us in developing bespoke technical solutions and testing. Additionally, they've been active partners in marketing collaboration. We would recommend working with them.”
If you would like more information about the STEICO range of wood fibre insulation, please contact our technical team where one of our experts will be happy to answer any questions you may have about STEICO products and systems. If you would like to know where your nearest local supplier of STEICO products is, we have a list of our UK supply partners here.
Editor’s note: Whilst this project is well underway, it is not due for completion until sometime in early 2026. Once the build is complete this case study will be updated to reflect any new information.
[1]www.structuraltimber.co.uk/wp-content/uploads/2024/06/6191-STA-Membership-Quality-Standards-v4.0-JUNE-2024-v3-240624.pdf
[2]knowledge.bsigroup.com/products/fire-resistance-tests-for-loadbearing-elements-walls-1
[3]knowledge.bsigroup.com/products/fire-resistance-tests-general-requirements-2
[4] From Table B2 on page 136 of Approved Document B : assets.publishing.service.gov.uk/media/67d2bb074702aacd2251cb94/Approved_Document_B_volume_1_Dwellings_2019_edition_incorporating_2020_2022_and_2025_amendments_collated_with_2026_and_2029_amendments.pdf
[5]www.steico.com/fileadmin/user_upload/importer/downloads/umwelt-produktdeklaration_epd/STEICO_IBU-environmental-product-declaration_zell_EUR_en_i.pdf
[6] Calculation according to EN 16449, life cycle stage A1 according to EN 15804+A2