Effect of steel framing in attic/ceiling assemblies on overall thermal resistance

Experiments have been performed to assess the impact of cold-formed-steel framing on the thermal performance of attic/ceiling assemblies. Test configurations duplicated features of full-sized, truss-based and conventional joist-and-rafter assemblies away from the edges of the ceiling. Steady-state tests were done at winter conditions in a climate simulator. In truss systems, strong thermal bridges due to framing members that penetrated through the insulation to the bottom chords persisted as the insulation level increased. Without penetrations, the effect of steel framing eventually disappeared as insulation level was increased. For negligible effect of the framing, framing spaced 41 cm oc required greater insulation depth than did framing spaced 61 cm oc. Without penetrations but with enough insulation to cover framing with depths of 8.9 cm, 20.3 cm and 30.5 cm, greater framing depth yielded slightly poorer thermal performance. In some tests, a continuous layer of extruded polystyrene foam insulation was placed between the C-shaped bottom chords of trusses and the gypsum board ceiling. System R-values improved slightly more than the R-value of the foam insulation. A three-dimensional model of the thermal behavior of the assemblies was used to extend the test results to the entire range of steel-framed attic/ceiling configurations. Equations generated from this and related work can be the basis for changes in codes and standards that reflect the effect of steel framing on the thermal performance of attic/ceiling assemblies and discourage allowing steel framing to extend beyond insulation in the assemblies.