This problem started happening to me recently. I checked all walls, roofs, no issues there. I have thermostat. Needless to say - it doesn't work. They also have chimney to warm them. Doesn't help really. They're dying regardless of age, during my last gameplay one of the children in my household almost died (I needed to ask Grim to spare him). Everyone in my household have this issue. I keep negotiating with Grim non stop. I tried repairing the game, deleting mods, moving my sims to different houses. Nothing helps… It really negatively affects the gameplay. I wish this get fixed soon…