In my northern Medical hideout inside temps drop to 2 to 3c. There isn't much I can do except dress for it. Throughout my long life I have learned there is nothing as relaxing as falling into a warm bed. So I have a heated mattress pad. An extremely soft cap for my head really makes a difference.
You are fervently against unvented catalyst heating so this is the price you must pay. Picture framing a catalyst heater in CO2 detectors is my idea of soothing sensitivities about CO2 poisoning.
When disqualifying alternatives it is very easy to paint one's self into a corner.
Insulated underwear help as do slippers.