Solid Wall Insulation: Is External or Internal Insulation Better? – Can I Just Ask? – Episode 2

Solid Wall Insulation – Is External or Internal Insulation Better?

When it comes to upgrading the insulation of solid-walled homes, one of the biggest decisions is whether to insulate from the outside or the inside. Both external wall insulation (EWI) and internal wall insulation (IWI) offer significant benefits—but each comes with its own set of challenges. Here’s how to weigh up the options and choose the best approach for your property.

If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 2 of Can I Just Ask? below. 👇

External Wall Insulation: Low Risk, High Impact – With a Few Caveats

Let’s start with the external route. EWI involves wrapping your building in a new insulating layer—like putting a big coat over the whole house. Done right, it’s a brilliant way to improve thermal performance and reduce condensation risk. And crucially, it all happens without disturbing life indoors. No moving out, no covering furniture in dust sheets. That’s a major bonus if you’re living in the property during renovation.

EWI can also give tired facades a facelift and protect porous brick or stonework from the elements. It’s especially suited to projects aiming for high insulation values (think 100mm or more of material), because it’s generally easier to fit thick layers externally than internally.

But—and there’s always a but—it’s not as simple as slapping on some insulation and calling it a day. EWI demands proper detailing around windows, eaves, and junctions with neighbouring buildings. These transition points are where things can go wrong if not carefully designed and installed. You also have to factor in roof overhangs, pipes, vents, and boundary lines. In terraced or semi-detached homes, projecting beyond the original footprint can create planning or neighbourly headaches.

Plus, since it’s all exposed to the elements, the timing of installation matters. Cold snaps, rain, or frost can slow down or compromise the work, making EWI more seasonal and potentially pricier than you’d hope.

Internal Wall Insulation: Precision, Comfort, and Control

If EWI isn’t a viable option—or if you’re keen to preserve your property’s external character—then internal wall insulation may be the answer. IWI has the advantage of being more flexible: it can be tailored room by room, and smaller improvements can still yield noticeable comfort gains.

A well-designed IWI system doesn’t just improve thermal performance; it also tightens up air leakage paths. This means you’re likely to feel warmer even with relatively thin layers of insulation, especially in homes where draughts are a major issue.

That said, internal systems do come with their own technical demands. Moisture is the big one. Insulating on the inside changes the dew point (the point at which moisture in the air condenses), so it’s vital to choose materials that manage moisture vapour effectively and regulate flows in and out of your wall construction. The wrong specification can trap moisture and cause mould, rot, or worse.

That’s why internal insulation really benefits from a bespoke approach. You want the system to be tailored to the exact building fabric, taking into account factors like masonry type, existing finishes, location, orientation and ventilation. It’s not something to take on lightly, but with the right design, the rewards can be transformative.

Which Is Better?

As always, it depends. EWI is often lower-risk in terms of moisture management and delivers excellent results—if you’ve got the space and budget. IWI offers flexibility and preserves external appearance, but it demands more careful planning and can be disruptive to install.

In many projects, the best solution isn’t either/or. A hybrid approach—external insulation on some walls, internal on others—can help you get the best of both worlds. It all comes down to the specifics of the building, the constraints of the site, and the goals of the renovation.