How Much Internal Insulation Can You Add? – Can I Just Ask? | Ep.15

Internal insulation is often a go-to strategy for upgrading thermal performance in older buildings—but how much is too much? The answer isn’t straightforward. It depends on wall type, location, materials, and crucially, how the insulation interacts with the building fabric. Here, we explore the practical and safe limits of internal wall insulation, with a focus on brick and stone construction.

If you’d prefer to watch the video, just click play below to watch Chris and Andy from our technical team talk through this question in Can I Just Ask? – Ep.15 👇

How Much Internal Insulation Can You Add? Understanding the Limits and Risks

Internal wall insulation can be a powerful tool for improving the energy performance of older buildings, but there are limits to what’s feasible, and more importantly, what’s safe. While some projects push for as much insulation as physically possible, the reality is that more isn’t always better. The constraints are largely determined by the construction type, the building’s exposure to weather, and the presence of moisture-sensitive materials like embedded timber.

Brick Walls: A Practical Starting Point

Let’s start with brick. For single-skin 100mm brick walls, the capacity for internal insulation is very limited, typically no more than 20–40mm. Anything more than that becomes risky, particularly in wetter climates or where the wall has poor drying potential.

More commonly, we’re dealing with 225mm (9-inch) solid brick walls. In these cases, assuming a reasonably well-maintained exterior with no significant render or cladding, you can usually install between 60 and 80mm of internal insulation. In some cases, particularly in the drier, more sheltered climates of southern England, you may get away with pushing this to 100mm, but only with careful detailing and good moisture management.

As the wall thickness increases, such as with 330mm bricks, there’s more leeway, but again, local climate and wall condition remain key factors.

Stone Walls: A Complex Challenge

Stonework, especially random rubble or rubble-filled cavity walls, introduces far more complexity. Not only is stone highly variable in its porosity and thermal behaviour, but it’s also particularly prone to wind-driven rain, which can drive moisture deep into the structure. Add internal insulation to the mix and you increase the risk of trapping that moisture, especially if vapour movement is restricted or drying potential is compromised.

This is where materials like wood fibre come into their own. They’re hygroscopic, vapour-permeable, and can help buffer moisture fluctuations. But even wood fibre has its limits, it doesn’t eliminate the risk, it just helps manage it. In the absence of protective measures like external render or rainscreen cladding, the insulation build-up must be relatively conservative to avoid overloading the wall with moisture stress.

Why More Insulation Isn’t Always Better

It’s tempting to approach internal insulation with a “how much can I squeeze in?” mentality. But as post-occupancy evaluations consistently show, performance isn’t just about thickness, it’s about execution. Insulation installed with excellent continuity, minimal thermal bridging, and robust airtightness detailing often outperforms thicker, sloppier installations.

There’s also the issue of moisture thresholds, especially with embedded timbers. While many worry about reaching the dew point and causing interstitial condensation, long-term damage often occurs below that level. For example, at around 87% relative humidity, masonry can store enough moisture to enable fungal decay in adjacent timber elements – even in the absence of visible condensation.

This is why joist ends, lintels, and bonding timbers embedded in external walls pose a significant risk. If insulation prevents drying and raises local humidity, timber decay becomes a serious concern, regardless of how well the insulation performs thermally.

The Real Limiting Factor: Moisture Risk

In practice, moisture is the defining constraint. Technically, you can insulate to much higher levels than we often recommend. But doing so without a full understanding of the wall’s behaviour, and without accounting for rain exposure, thermal gradients, and timber vulnerability, is a recipe for long-term structural issues.

This is why, at Back to Earth, we advocate for a cautious, fabric-first approach. That means:

• Assessing wall thickness, exposure, and condition
• Selecting vapour-permeable, moisture-buffering materials
• Prioritising build quality over insulation thickness
• Avoiding high-risk details like buried joist ends without appropriate mitigation

Final Thoughts

Internal insulation is as much about managing moisture as it is about reducing heat loss. The right thickness isn’t just a number, it’s a carefully balanced decision based on wall type, location, and the existing building fabric. Done well, even a modest layer of insulation can yield significant comfort and efficiency gains. Done poorly, it can quietly undermine the structure it’s meant to protect.

If in doubt, less is often more, especially when it’s well detailed and intelligently integrated into the building’s overall performance strategy.