The relative accuracy of current thermal simulation algorithms is evaluated against an intricate multiphysics hygrothermal computational fluid dynamics (CFD) building envelope analysis that incorporates, in addition to real-life conditions, a meticulous representation of cracking in building envelopes. The study found that even the most advanced current algorithms have up to 96.13% relative error versus CFD analysis. A new model for combined heat and air infiltration simulation is developed and presented. The model resulted in up to 91.6% improvement in relative accuracy over current algorithms. It reduces error versus CFD analysis to less than 4.5% while requiring less than 1% of the time required for a complex hygrothermal analysis. The model used is demonstrated to be easy to integrate into other simulation engines as a standalone method for evaluating infiltration heat loads. This will vastly increase the accuracy of such simulation engines while maintaining their speed and ease of use characteristics that make them very widely used in building energy design. (C) 2014 Elsevier B.V. All rights reserved.