At EWI Pro, we specify many projects throughout the UK. They vary in size, scope, and system application. From large-scale commercial projects to small-scale residential, we cover every type of project. As we’ve discussed recently, we provide a comprehensive range of testing processes. These include wind-load tests, moisture testing, and pull-out tests. Our Technical Team also provides expert U-value calculations to ensure the system is suitable. In our new series of blogs, we will shine a light on specific projects and the build-ups they utilise. This project focuses on our brand new Insulating Cork Render and Lime Plaster and how it can be incorporated for heritage retrofits.

Lime render is split into hydraulic and non-hydraulic lime; EWI Pro’s Lime Render is hydraulic, which means it sets through hydrolysis. EWI-747 Lime Render and EWI-748 Lime Basecoat are a dry powder mixed with water on building sites. The NHL tag distinguishes hydraulic lime renders; EWI-747 is NHL 3.5, which makes it regular setting and ideal as an external finish. EWI-748 is NHL 5, a stronger and faster setting; this makes it suitable for the adherence of insulation boards, particularly mineral wool. The combination of lime render and mineral wool produces a fully breathable system. Lime renders and mortars must be set and cured, a process that can take several days. However, due to its natural composition, lime is considered self-healing as minor cracks can fill. This accommodates natural building movement and temperature changes.

Project details

• Where: Yorkshire 
• Property: Old stone farmhouse 
• Room: Bathroom extension 
• Walls: Stone facade with internal blockwork and 20mm plaster
• U-Value change: 2.77 to 0.94
• Process: Stripped 20mm plaster that was on the block work and applied 30mm of EWI-745 insulating cork render and a 4mm coloured lime plaster on top
• New wall: Stone facade with internal blockwork, 30mm insulating cork render, and 4mm lime plaster

The project – Insulating Cork Render

The old stone farmhouse in Yorkshire displays the versatility of the EWI Pro Heritage Lime Range. In this instance, the retrofitting is completed internally, which is crucial for maintaining the external appearance. Being an old farmhouse, the stonework is naturally porous, allowing moisture to pass through it. Internally, the blockwork was covered with plaster that needed to be removed. The U-value of the original stonework construction was 2.77 W/m²K, which is extremely poor thermally. The poor performance of stonework results in a colder interior environment and much higher heating bills.

To insulate the affected area (bathroom extension), 20mm of old plaster was stripped away from the blockwork walls. Blockwork presents a flat, uniform, and dimensionally stable substrate to install onto; therefore, this project did not require a parge coat of EWI-748. Instead, EWI-745 Insulating Cork Render was installed directly onto the internal walls. Due to the effectiveness of the thermal insulation, only 30mm of EWI-745 needed to be installed. With this thickness, the resultant U-value is down to 0.94 W/m²K. Crucially, the bathroom fittings, like the sink, could be left in their original condition and place. Thicker insulation would result in the need for a complete bathroom extension rebuild. However, thanks to EWI-745 Insulating Cork Render, the thermal comfort is significantly improved.

EWI-745 also maintains the overall breathability and porosity of the stonework and blockwork. This is particularly crucial as bathrooms are subjected to high humidity and moisture levels. The breathability allows this humidity and moisture to pass through the cork-insulating layer and evaporate through the wall.

Lime Plaster finish

The finish to the insulation system is provided by EWI-749 Lime Plaster, which provides an incredibly even and smooth finish whilst maintaining the breathability that the whole system hinges upon. The Lime Plaster is also coloured to a muted finish that harmoniously blends with the rest of the property and heritage features. Lime renders and plasters will naturally provide a heritage patina; the ability to use either render or fine finish plasters provides exceptional versatility. The façade, internal or external, can be finished smooth or textured.