Continuous external insulation

Why insulation between the studs is not enough

Because the section is where the insulation is not. Steel conducts heat orders of magnitude better, so on the line of the sections the wall is substantially worse than in the bays between them.

The result: the actual heat transmittance of the wall is much worse than a naive calculation from the layer thicknesses would give.

The solution

A continuous insulation layer running outside the sections. With it:

• the section stays on the warm side,
• there is no through heat path,
• the internal surface does not cool down on the line of the sections,
• the condensation risk drops considerably.

Where it can go

Outside

The most common and the most effective: insulation running on the outer face of the frame, with the façade layer over it.

Inside

Less often, and with care: supplementary insulation on the inside puts the section on the cold side, which is risky in vapour terms. Only with the calculation to back it.

The logic of the build-up

The arrangement that works in practice is a doubled wall with cement board on the outside: the outermost layer is then a hard, impact-resistant board rather than the insulation itself. That is a difference you can feel on the finished house.

The common counter-example — OSB outside, polystyrene on it, mineral wool between the studs — does exactly the opposite: if we already have a board that provides hardness, why should it not be the outermost layer?

Filling the insulation

Where the regulations allow it, spray foam completes the wall cavities and the outside of the house in one operation. The foam expands everywhere, so quality does not hang on how carefully the gaps are stuffed — and for its thickness it gives the best thermal value.

In fire terms it is more constrained: where the regulations do not allow it, mineral wool gives just as good a result, but there the careful filling of the gaps is critical. Fire questions around insulation materials.

What the fixings mean

Fixings passing through the external layer are small thermal bridges themselves. So the material, diameter and spacing of the fixing count — and their effect appears in the calculation.

The point

Continuous external insulation is not an extra but a condition of a good light gauge steel wall. Without it the thermal figure is good on paper and not in reality.