In this paper, a fast and accurate numerical simulation method on dynamic heat transfer through building envelopes has been developed by using the Krylov subspace and the balanced truncation model order reduction (MOR) algorithms. The computational accuracy and efficiency of the two MOR algorithms are discussed through the numerical simulation on a roof heat transfer in a one-day period, and then the two verified algorithms are applied to simulate the heat transfer through a multilayer wall for a week and the two-dimensional heat transfer through an L-shape thermal bridge. The results show that the relative errors of the two algorithms to the harmonic response method or to the direct solution method are all less than 1%, and the solving time with the two MOR algorithms decreases greatly. In addition, the Krylov subspace MOR algorithm has a faster solving speed and is more suitable for solving the heat transfer through a building envelope than the balanced truncation MOR algorithm.