Calculating the U-value of a steel frame

Why the simple formula is not enough

The traditional layer formula assumes homogeneous layers stacked on one another. In a framed wall that does not hold: there are steel sections in the plane of the insulation, conducting heat an order of magnitude better.

Ignore that and the calculated U-value comes out substantially better than the real one — the wall complies on paper and not in fact.

What has to be taken into account

The proportion and spacing of the sections

The closer the frame, the greater the thermal bridge effect. The difference between 600 mm and 400 mm centres is noticeable.

The geometry of the section

The flange width determines the area in contact with the lining, and the web depth the length of the heat path.

The continuous layers

Insulation outside the frame improves the result dramatically — because it does not reduce the size of the thermal bridge, it breaks the heat path.

The fixings

Metal fixings passing through the external layer are point thermal bridges.

How it is calculated

The building physics standards give a procedure for determining the transmittance of structures with inhomogeneous layers, and pay separate attention to metal-framed structures, because the usual simplifications are not accurate enough there.

For more complex connections a numerical (two- or three-dimensional) calculation is made — at the installation plane of a window, say, or at wall junctions.

Junction thermal bridges are a separate item

Alongside the U-value of the wall surface, linear thermal bridges — corners, the floor junction, window installation, the foundation connection — contribute separately to the loss. In a well-designed structure they are small; in a badly designed one they are comparable to the surface figure.

Where it connects

The calculated value is compared against the energy requirements. Energy requirements in Hungary.

The same calculation shows where the surface will be cold, and therefore where condensation threatens. Condensation risk check.

The point

In a framed structure the U-value is not the sum of the layers. Calculate it with the simple formula and the result will be prettier than reality — and the thermal camera will show it.