Try Insulation Lab free in your browser, with nothing to install and no account. Build a box that keeps an ice cube frozen, from real materials on a budget: line the walls, wrap the ice, pack the space, tape the lid. Press Test and watch energy drift in from the warm room while the ice melts on a scale. Change one thing and test again, swap the ice for warm water to see the same box keep heat in, then build it for real.