A new model for predicting heat transfer in the transitional boundary layer of rough turbine airfoils is presented. The new model makes use of extensive experimental work recently published by the current authors. For the computation of the turbulent boundary layer a discrete element roughness model is combined with a two-layer model of turbulence. The transition region is modeled using an intermittency equation that blends between the laminar and turbulent boundary layer. Several intermittency functions are evaluated in respect of their applicability to rough-wall transition. To predict the onset of transition a new correlation is presented, accounting for the influence of free-stream turbulence and surface roughness. Finally the new model is tested against transitional rough-wall boundary layer flows on high-pressure and low-pressure turbine airfoils.