Facade-layout synthesis occurs when renovating buildings to improve their thermal insulation and reduce the impact of heating on the environment. This interesting problem involves to cover a facade with a set of disjoint and configurable insulating panels. Therefore, it can be seen as a constrained rectangle packing problem, but for which the number of rectangles to be used and their size are not known a priori. This paper proposes an efficient way of solving this problem using constraint programming. The model is based on an open variant of the DiffN global constraint in order to deal with an unfixed number of rectangles, as well as a simple but efficient search procedure to solve this problem. An empirical evaluation shows the practical impact of every choice in the design of our model. A prototype implemented in the general purpose solver Choco is intended to assist architect decision-making in the context of building thermal retrofit.