One of the most common and inexpensive ways to perform thermal insulation is through blankets (mineral or glass), which can be applied to both walls and ceiling.
Other options are glass wool, expanded polystyrene (styrofoam), polyurethane foam, vertical gardens, vermiculite (applied to ceramic blocks), among other materials.
It is worth mentioning that vertical gardens, in addition to improving the humidity of the environment and helping to regulate the temperature, are sustainable and bring a beautiful aesthetic appeal.
