The article explores how adding internal insulation to buildings can save energy for heating and cooling. The researchers studied four different internal insulation systems in three locations using special software. They found that internal wall insulation can be a good alternative to external insulation for saving energy. However, it may not be cost-effective for low-energy buildings due to long payback times. The type of insulation used can affect the overall cost of the retrofitting project. Ultimately, factors like comfort and temperature control are crucial in deciding whether internal insulation is a good solution for a building.