I don't know
My gut says around 60 pct or so. Better than fireplaces (when they run ;-) ) but not as good as a poor sealed stove in upper 60s lower 70s.
I presume bholler knows better.
On the other hand, what about a cast iron classic looking more modern stove in your classic home? Saves wood, the environment, and may still look good with the surrounding - even if not authentic.
I have a small Jotul (cast iron) as well as a Woodstock Fireview (soapstone) stove in the main brick part of the house. This is where the living room, bedrooms, and office are. They are both catalytic stoves, and this side of the house is kept heated.
The antique stoves are on the other part the house (made of wood, and it is like a separate house joined to the brick house). I don't really need to update the stoves because:
1) One of them is in my attic... with my drums, etc. There is no heat in that room at all. It is a big iron stove and I would just fire it up occasionally when friends are over. It is sort of special because it is very local, and stoves are not made here anymore. Normally the room is left unheated.
2) There is a tiny stove in the kitchen. They call it a "morning stove". It heats instantly... and is perfect to use when we wake up, or just arrive from out of town, or while we are in there cooking. It is so comfortable and functional... and good in the shoulder seasons too.
3) The third stove is a very pretty one (but maybe the least functional). This fits in the dining room fireplace. I am trying to decide if I will use that stove or use the open fireplace. Either way, it would be burned very occasionally during some dinners.
So as you see, the antique stoves would be burned just occasionally. The rooms there are left cool when not in use.
Information on the stoves is very hard to find. I sent a message to "Efficiency Vermont" program, as they have some nerds that might be able to point me toward some studies.
Honestly it will really depend how often you will be using them. If it's a dozen times a year or so tear them down clean them up and reseal them. So they are safe to use. If it's much more than that I would be looking for something more efficient
I could take apart the one in the attic... but the other two are open as part of the design. It is not that they became unsealed.... they are just primitive stoves.
Although.... the one in the kitchen is probably my favorite stove in the house (functionally).