Northwich Housing Project

A futureproof extension with 150mm of DriTherm® Cavity Slab 32 

Inox Construction pride themselves on their specialist expertise in wall construction and their most recent project, a large extension to a 1950s semi-detached house, was no exception. Consisting of two stories with a large kitchen, dining room, utility room, bathroom, and two bedrooms, the extension needed to match the brickwork and look of the existing property while meeting thermal performance requirements.  

To achieve this, Inox Construction decided to switch from using rigid PIR boards in a 125mm cavity to using Knauf Insulation’s DriTherm® Cavity Slab 32 in a 150mm cavity. 

Post-war design, modern U-values  

The property sits on a leafy road in the village of Davenham, Northwich, a settlement that is at least 1000 years old and is recorded in the Domesday Book. Although the property itself is only 70 years old, Inox Construction wanted to respect its history while ensuring the extension would be futureproof in terms of thermal performance. 

“The client wanted to extend their post-war 1950s home, but in a way that felt cohesive with the original design. The plan was to achieve the required thermal performance using a 125mm cavity with 100mm of rigid PIR insulation board,” explained Andy Davies, Director of Inox Construction.  

However, Andy’s team was keen to trial a new approach that would give them the flexibility to try different insulants. “It was simpler to tie the courses in using a 150mm cavity and we wanted to use an insulation material that was less time consuming to install and achieved more consistent results,” Andy explained. 

Widening the cavity by just 25mm, from 125mm to 150mm, the team used DriTherm® Cavity Slab 32 to insulate the walls and achieve a U-value of 0.17 W/m2K. 

Buildable, sustainable, non-combustible 

DriTherm® Cavity Slab 32 is a glass mineral wool product designed for use in external full-fill masonry cavity walls. It is made with a water-repellent additive to resist moisture ingress and is non-combustible with the best possible Euroclass A1 reaction to fire classification. Its lambda value is 0.032 W/mK, offering the best thermal performance in the DriTherm® Cavity Slabs range. 

Glass mineral wool is flexible, which means that it adapts to any minor imperfections in the substrate and the fibres will ‘knit’ together at abutting edges to eliminate airgaps. DriTherm® Cavity Slabs can be friction fitted and require no additional taping or fixings. 

Andy said, “It gives our client added peace of mind to know that the insulation is non-combustible. And its lower embodied carbon is good for everyone and helps us reduce our emissions in a practical way.” 

Ease of installation 

“We found that using mineral wool in a 150mm cavity was significantly faster than using boards. The difference in terms of additional brick work is minimal and is paid over multiple times by just how quickly we were able to get a high-standard result,” Andy explained, “You lose so much time installing boards correctly, whereas it was painless to achieve a snug fit with the DriTherm® Cavity Slabs. We achieved a better result for the client.” 

Having found value in this combination of speed and consistent performance, Inox Construction intends to build as many projects as possible using the same approach to cavity walls. Andy’s team were able to build a spacious, tasteful extension that uses sustainable, non-combustible insulation in a 150mm cavity for futureproof thermal performance.