The aim of the present work is to improve the thermal performance of lightweight constructions making use of Phase Change Materials (PCMs). Typical lightweight constructions have lack of thermal inertia. The use of PCMs can remediate this problem and improve the building thermal performance. However there are few studies specifying the correct PCM to apply in each situation and the optimal quantity to use. In the present case, it was used a natural PCM – Coconut Oil, placed in plastic containers in the floor. For the study it was used the Test Cells built in the University of Minho. There were carried out several experiments with Coconut Oil, reproducing different thermal storage situations, from the total absence of this material (reference lightweight solution) to the application of all the available PCM. It was concluded that the introduction of PCMs improved substantially the Test Cell thermal inertia, but it would be necessary to also implement some modifications in the South façade (reduction of the glazing area and introduction of more thermal mass) in order to improve the overall thermal performance. Before the introduction of these modifications in the Test Cell, the Program VisualDOE was used in order to forecast the performance of the Test Cell and to optimize the final solution.