Planning and Design

Mixing external and internal wall insulation on the same building is sometimes unavoidable, especially where planning or architectural constraints limit what can be done on certain elevations. This episode covers how to manage the junctions between EWI and IWI — keeping the thermal envelope continuous, minimising transitions, and avoiding the cold spots and moisture risks that come from getting the detailing wrong.

Not all moisture analysis methods are created equal. The Glaser method is widely used and fine for simpler constructions, but it was never designed for solid masonry walls or vapour-open materials, and it misses too many real-world variables to be reliable. For internal wall insulation projects, dynamic hygrothermal simulation using WUFI gives a far more accurate picture of how moisture moves through a wall over time, helping you avoid the kind of slow moisture build-up that causes serious long-term damage.

More insulation isn't always better when it comes to internal wall insulation. The safe limits depend on wall type, exposure to weather, and what's embedded in the masonry. Solid brick walls typically allow 60 to 80mm before moisture risk becomes a real concern, while stone walls need an even more cautious approach. The bigger threat isn't always condensation — it's the slow build-up of humidity that can cause timber decay long before any visible damage appears.

U-values are a useful starting point for retrofit projects, but the right target depends heavily on how you're insulating and what you're working with. Building Regulations point to 0.3 W/m2K for thermal elements, though 0.7 W/m2K is the compliance threshold in certain circumstances. For external insulation, 0.3 is achievable and relatively low risk. For internal insulation, a more conservative 0.5 to 0.7 is often wiser — thinner insulation layers carry less moisture risk and still deliver real comfort improvements.

For most natural insulation work, a formal certification isn't essential. What matters far more is attention to detail — a carefully installed 20mm of insulation will outperform a poorly fitted 100mm every time. Natural materials use familiar trade skills, and resources like Fibres Academy mean good guidance is readily available for anyone who wants it. That said, some specific systems like NatureWall do have a recommended training course, not as a box-ticking exercise, but because consistent detailing is what makes the system perform.

Partial insulation is possible but comes with real risks. Insulating only part of a building creates a mix of warm and cold surfaces, and the colder uninsulated areas end up carrying a greater moisture burden, increasing the risk of condensation and mould. Where insulation stops, thermal bridging becomes a problem too. If a staged approach is necessary, keep the gap between phases short and prioritise a thin continuous layer across the whole envelope over a thick layer in just one area — heat will always find the easiest route out.

Airtightness, breathability, and vapour control are often treated as conflicting priorities but they work together when detailed correctly. Airtightness controls bulk air movement through gaps and cracks, while breathability refers to vapour diffusing through materials molecule by molecule — the two are not the same thing. A lime plaster or intelligent membrane can be airtight and vapour-open at the same time. The key is choosing the right vapour control layer for the build-up, and making sure the wall can dry in the right direction when conditions change.

External insulation wraps the building like a coat, keeping disruption out of the home and making it easier to achieve thicker insulation values with lower moisture risk. The downsides are planning constraints, tricky detailing around windows and junctions, and weather-dependent installation. Internal insulation is more flexible and preserves the external appearance, but it shifts the dew point inward, making moisture management critical. The right choice depends on the building, the site, and the budget — and in many cases a combination of both is the best answer.