I don't think there's much concern there for heat loss... I mean, the door's wide open! Must be a seriously hot stove in there!
No, it is just that one get's accustomed to the cold living in that all winter long. Besides, one needs some fresh air in there occasionally.
I've been very happy with this: https://billboardtarps.com Same price as blue tarp at the hardware store even when you factor in the shipping.
That is a great resource! I've had the NOT so pleasurable experience of having to tear a billowing in the wind billboard down....that sucker was heavy! It filled up the bed of my then employer's Tacoma.... Had I foreseen marrying my wonderful bride of 3 years today, AND choosing to heat with wood, I'd have saved that thing for top covering....