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Do you need a ventilated cavity behind internal wall insulation?
It’s a question that keeps coming up, and understandably so. When you start talking about internal wall insulation (IWI), moisture risk is never far behind. Somewhere along the line, the idea has taken hold that introducing a ventilated cavity between the insulation and the masonry wall is a safer option – a kind of insurance policy against condensation.
The short answer? It can be done, and it is recognised in guidance, but in most real buildings it introduces as many problems as it claims to solve. At Back to Earth, we generally recommend fully bonding internal insulation to the wall. Here’s why.
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Two recognised approaches – but not equal
It’s worth saying upfront that both approaches exist in the wild. You’ll find ventilated cavities referenced in some IWI guidance, including parts of the BSI PAS documentation. So this isn’t about one method being “wrong”. It’s about suitability, buildability, and risk.
Our preference is clear: direct bonding of insulation to the existing masonry, particularly when using vapour-open, hygroscopic materials such as wood fibre. That preference comes from experience on site, and from what the hygrothermal modelling actually shows.
The practical challenges of a ventilated cavity
Once you step back and think through what a ventilated cavity really involves, the difficulties start to stack up.
First, you have to make the cavity genuinely ventilated. That means providing reliable airflow paths – and keeping them continuous. Window and door openings immediately interrupt that cavity. Floor zones interrupt it again. Every reveal, lintel and junction becomes a weak point where airflow is either blocked or unpredictable.
Then there’s drainage. If moisture condenses on the cold masonry face (or simply migrates there), how is it supposed to leave the cavity? Designing a cavity that both ventilates and drains internally is far from straightforward.
Add to that the issue of wind washing. Air moving behind the insulation strips heat from its surface, reducing thermal performance and making airtightness much harder to achieve. At that point you’re fighting on two fronts: keeping the building warm and keeping the air where it belongs.
Ventilation doesn’t mean “always dry”
A common assumption is that a ventilated cavity will always be removing moisture. In reality, cavities only ventilate properly when there is sufficient wind-driven pressure difference across the wall. Large parts of the year – particularly in damp, still conditions – that airflow simply isn’t there.
Those are often the same periods when masonry walls are at their wettest. Moisture stored in the wall is released into the cavity, relative humidity climbs, and air movement is minimal. The result can be a persistently damp microclimate sitting directly behind the insulation.
Crucially, this isn’t just about liquid condensation. Dynamic hygrothermal analysis tools such as WUFI consistently show that the bigger risk is time spent below the condensation threshold but above mould growth thresholds. A ventilated cavity can spend long periods in exactly that danger zone.
Throw oxygen into the mix – which ventilation generously provides – and you’ve created very comfortable conditions for mould.
Working with the building fabric, not against it
Directly bonding insulation keeps the insulation layer in hygrothermal contact with the wall. You’re not trying to decouple the new build-up from the old fabric; you’re asking them to work together.
With hygroscopic, vapour-open materials such as wood fibre, this approach allows moisture to be buffered and redistributed rather than trapped in a discrete air space. Peaks in humidity are moderated, and the system behaves in a more forgiving, dynamic way.
Steady-state condensation calculations often overstate risk in these assemblies. When you look at dynamic performance instead, directly bonded systems tend to show lower and more stable moisture loads – precisely because they avoid creating isolated zones with high relative humidity.
Penetrations, timber, and real buildings
There are edge cases where a ventilated cavity can work reasonably well. A single-storey barn conversion, for example, with no embedded floor joists, minimal penetrations, and plenty of uninterrupted wall height, is a very different proposition from a typical two-storey house.
Once you introduce embedded timber – joist ends, beams, wall plates – monitoring and controlling moisture becomes much more complex. Those timbers often sit exactly where airflow is weakest and humidity is highest. Designing a cavity that reliably protects them over decades is extremely challenging.
There’s also the small matter of actually creating the ventilation openings. To achieve meaningful airflow, you often need substantial perforations through external masonry. That’s labour-intensive, visually intrusive, and not particularly kind to historic fabric.
So, do you need a ventilated cavity?
In most cases, no – and often it’s better if you don’t have one.
A well-designed, directly bonded internal insulation system, using vapour-open materials and detailed with care, is generally more robust, more predictable, and easier to execute well on site. It avoids wind washing, reduces mould risk, and works with the building’s natural moisture behaviour rather than trying to manage it with airflow that may or may not exist.
As ever, context matters. But for the majority of traditional masonry buildings, introducing a ventilated cavity behind internal insulation is not the simple fix it’s sometimes made out to be – and it can create exactly the problems it’s meant to avoid.