What temperature are you checking? When I biab, I use a junky glass thermometer and when at temps, put the lid on, wrap it up in an old blanket and walk away for an hour. I usually only lose 1 - 2°.
I've not biab in the winter, the coldest it's been was probably around 45°F outside, so that might factor in.
I'm not familiar with the penrose kettle or your method of heating, but with a 30 qt propane turkey fryer, this works well enough.