The thermal performance of a modular lightweight steel framed wall was measured and calculated with three-dimensional finite element method model. The focus of this article is on the effect of flanking thermal losses. The calculated heat flux values varied from –22% (external surface) to +50% (internal surface) when flanking loss was set to 0 as a reference case, thermal transmittance equal to 0.30 W/(m²·K). Other critical parameters were the existence of fixing ‘L’-shaped steel elements and the perimeter thermal insulation (10 cm XPS).