Permarock systems are regularly used on new-build projects across the UK and, recently, those have included a growing number of residential schemes built to the Passivhaus standard.
Permarock Mineral Fibre External Wall Insulation systems are currently being used to enhance the thermal performance of two new Passivhaus housing projects in Yorkshire, and these follow new-build and retrofit schemes in Sunderland, Derbyshire, Hereford, Norfolk, Essex and London.
The latest Passivhaus scheme to feature Permarock products is another residential new-build project, which is now being delivered in Nailsea in North Somerset. The project, led by Stonewood Homes, began in 2023 and will ultimately see the creation of 52 sustainable homes; a mix of two, three, and four-bedroom layouts. It will be marketed as ‘Elm Grove’, and it is due for completion in 2025.
The development features timber-framed designs with substantial insulation, energy-efficient triple glazing, solar photovoltaic arrays and mechanical ventilation heat recovery units. Importantly, each home will also have an almost completely airtight membrane, which will help to control water vapour permeability and minimise the transmission of heat.
Part of the specification calls for Permarock Exterior Render to be applied onto Euroform Versarend carrier board, followed by SiliconeUltra K-Finish in a number of colours. This will form part of a back-ventilated render rainscreen system for many of the houses. The work is being carried out by Harbour Façade Systems, a Permarock Registered Installer.
PermaRend is an impact-resistant, polymer-modified render that resists cracking and is suitable for use on brickwork, blockwork and concrete substrates. Easily applied in multiple coats, it is light but robust, and very flexible.
SiliconeUltra K-Finish is lightweight and water repellent, and it exhibits excellent water vapour permeability. It also sheds dirt very effectively and incorporates a preservative that deters the growth of algae and fungus. As a result, it retains a clean, bright appearance for much longer than conventional render systems.
A notable feature of the architect’s design is that it requires special detailing to highlight the windows. This is being achieved by applying the SiliconeUltra K-Finish in different grain sizes: 2mm in bands around the windows, and 1.5mm elsewhere.
The scheme has been the subject of numerous media articles. (Example: Builders Merchant News.) It is set on land owned by North Somerset Council, which recently stated that the project is part of its “ambitions to tackle the climate emergency and reach net zero carbon emissions by 2030.”
Lee Zeffiretti, Stonewood Homes’ Contracts Manager, added: “Collaborating with Permarock systems on our Elm Grove development in Nailsea has been a remarkable experience. Their professionalism and top-quality materials have played a crucial role in the project’s success, helping us achieve the highest standards and deliver exceptional results.”
Timescale & Rationale
Many of us ask why has it taken this long and why was it needed in the first place?
I’ve been working in the sector for 30 years and can take you to projects where verge trims installed all that time ago are still proving effective. So why has the verge trim got such a bad reputation? What has changed?
Properly designed and correctly fitted, a verge trim is a very effective method of capping an EWI system and should remain watertight for the lifetime of the building. It should go without saying that any product or system not correctly fitted is not going to perform effectively and is likely to fail. It is not difficult to correctly fit a verge trim.
So why the need for additional controls? First, because the old verge trims were often installed with little or no consideration given to cold bridging. And second, in the early days of funding for EWI, the emphasis was very much on insulation, cost and payback from savings on energy bills. The focus was on insulating as many properties as possible as cheaply as possible. A basic principle of construction was forgotten. No element of a building operates in isolation. If one element of a construction is failing, it will impact negatively on others.
Before controls were put in place, and before the introduction of the PAS standards, external wall insulation was sometimes being fitted on properties that were already failing with failing roofs, leaking guttering and windows that needed replacing. Most EWI systems will be able to cope with a limited amount of water ingress, but a leaking roof or guttering may compromise even the best fitted EWI systems.
Notwithstanding the above, we also must agree that the interfaces between products and systems are as important as the products and systems themselves. Well-designed interfaces are critical if a building is to resist water ingress and prevent thermal bridging. To this end, RCS design in the new guidance was based on the following principles:
• Water ingress resistance – weather tightness
• Exposure – suitable for all exposure categories
• Redundancy – multiple layers of weathering protection
• Continuity of insulation – elimination of thermal bridges
Routes to Compliance
There are two routes to compliance.
Firstly, an initial survey is carried out to identify whether the existing roofline can use one of the thirteen standard details in the ‘Weathering and Thermal bridge guide (June 2024 edition)’. If such a detail is identified, work can proceed using this detail.
The second route is taken when the survey identifies an RCS need that is not covered in the guide; in this case, the contractor provides the RCS survey to the system designer who, with the Retrofit Designer and RCS fabricator, develops a bespoke solution following the RCS design principles.
In either case, the initial RCS survey should identify any existing issues with the roof / wall interface and detail the remedial works required. (For example: broken slates, missing or ripped felt, cracked or missing cement fillets and the position and thickness of existing roof insulation). Any identified remedial works then need to be completed prior to the EWI system and RCS being applied.
Principles & Considerations
It should be remembered that an EWI system on a retrofit property is not the primary weather protection for the building. The building should be sound and free from moisture ingress prior to its application.
Also, all building systems have the potential to fail, but normally there are warning signs. It is critical that if cracks are observed, render is delaminating or prolonged damp patches are present, they are investigated and the cause rectified. If you saw a damp patch on your bedroom ceiling, you wouldn’t wait until water was dripping on your head to investigate.
And here lies another issue. Many people who have bought their own council property won’t necessarily be able to afford to maintain it to an adequate level. I remember when I was first involved in the application of EWI, the private properties on the estate were mostly the best maintained. Unfortunately, now when I visit sites, where EWI has been applied, it is mainly the private properties that are in disrepair.
It must be realised that external wall insulation is critical to improving the thermal performance of a large proportion of the UK’s housing stock, but this needs to go hand in hand with investment in the maintenance of that stock. Funding policies need to consider this and allow the flexibility to include private properties within schemes.
Decarbonisation of the housing stock is a critical element of achieving net zero. EWI is vital to improving the thermal efficiency of our older solid wall and non-traditionally built properties. But EWI is so much more than just insulation; EWI that is designed and installed correctly will increase the value of a property, extend its life, and improve its aesthetics. Delivered as part of a community scheme, it can also transform a run-down estate to look like a virtually brand-new development. It’s well documented that the environment we live in has a profound effect on our wellbeing. External wall insulation has the potential to improve the lives of individuals and communities as a whole.
It is important that we get it right and continue to develop our products and systems so that they are more effective, easier to apply and generally foolproof. The work done by the industry to develop Roofline Closure Systems is part of a process of continual improvement.
Mitchell Gee is a former chair of the Insulated Render and Cladding Association (INCA) and a former managing director of a leading external wall insulation system provider. He is currently a board member of both INCA and SWIGA. He has 30 years’ experience of working in the sector.
For details of Permarock’s own roofline closure systems or for associated technical support, please call us on 01509 262924 or email permarock@permarock.com.
