I feel you. I lived in a converted stable once. The place leaked air like a sieve.

I also discovered the oil fired boiler had 20m+ of unlagged pipes between it and our radiators, running through an unused stable. It took 2 full tanks/winter to just keep it above freezing. It should have been 1/2 a tank to keep it nice and warm.

Modern houses are actually quite a problem for this. A well insulated house also tend to be quite well sealed. I’ve seen my bedroom pass 5000ppm. I suspect a lot of people are working in 1000ppm environments or higher for long periods.

For those interested, IKEA recently released a air quality sensor that does CO2 for a very low price. ALPSTUGA