Best insulation for masonry cavity walls

What’s the best insulation for a masonry cavity wall? 

As regulations tighten, the thermal, fire safety and carbon reduction requirements for new homes are becoming more demanding. Masonry cavity wall insulation needs to tick a lot of boxes, so what are the specification factors you need to consider? 

Energy efficiency 

The latest updates to Approved Document L (Energy Efficiency) brought tighter limiting U-values for new homes and introduced the Building Regulations England Part L report, requiring photo evidence of thermal continuity.¹ It’s clear that today’s homes need to translate designed performance into real world performance but there is still a recognised ‘performance gap’ between the two. To achieve real world thermal performance, masonry cavity wall insulation must be correctly installed with no air gaps. However, this can be challenging with certain types of material. For example, rigid board insulation won’t sit flush against a cavity unless it is perfectly uniform and flat (which is rare). Boards also need to be cut and taped with precision, introducing more opportunity for human error. The best cavity insulation is one which is easier to install correctly, under real world site condition.

DriTherm^®Cavity Slab 32 glass mineral wool insulation is flexible, allowing it to adapt to minor imperfections in the substrate and fully fill the cavity. Where two slabs or rolls meet, the strands also ‘knit’ together, minimising gaps and maximising thermal performance. A 150mm cavity, filled with DriTherm^® Cavity Slab 32 is easier to install correctly, making it more likely to achieve its designed thermal performance. 

Fire safety  

There is a growing awareness around the impact of construction products on fire safety. In 2022, Approved Document B (Fire Safety) tightened the guidance on using combustible materials² and London Plan guidance now states that major development proposals must be submitted with a Fire Statement confirming that no combustible materials will be incorporated in the development’s external walls.³ There’s a clear direction of travel towards using fewer combustible materials and the best cavity insulation will always be one that prioritises occupant safety. Specifying 150mm of DriTherm^® Cavity Slab 32 allows you to get ahead of tightening regulations because the insulation is non-combustible with the best possible Euroclass A1 reaction to fire classification. 

Embodied carbon 

Current regulations aim to reduce the operational carbon emissions from new homes. But as operational carbon reduces, embodied carbon is likely to gain more attention. That’s the total greenhouse gas emissions released in producing the building,⁴ from things like raw materials and transportation. Whilst embodied carbon isn’t regulated yet, industry bodies are supporting a proposed ‘Part Z’ Building Regulation, to measure whole-life carbon emissions.⁵ A 150mm masonry cavity wall filled with DriTherm^® Cavity Slab 32 is a more sustainable choice because glass mineral wool has the lowest levels of embodied carbon of any mainstream insulation material.   

By specifying a wider masonry cavity and DriTherm^® Cavity Slab 32, you can achieve the thermal performance you need, alongside fire safety and lower embodied carbon. Glass mineral wool insulation like this helps you to meet the requirements of today and prepare for tomorrow. 

Learn more about DriTherm^® Cavity Slab 32. 

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