I find myself very curious about the construction of your home. Thanks for putting up with it. The walls do appear to be a single solid timber. If it’s 15cm thick that would be an R value of 5.6, where I live in the United States, Wisconsin (cold but probably warmer than you) the minimum code required r value for an exterior wall is 30. Was this a permitted residential structure? The gap that you show in your photo is an eve vent and is unfortunately necessary to keep the roof cold and prevent ice damming. Usually it’s fine though because the living space is sealed and insulated from the cold attic above. You might have a design KW required to heat your home as constructed somewhere in the building/permitting paperwork. If not you could find a home heat loss calculator online (builditsolar.com has one I like) and enter everything you know about your home construction in to see what kind of power you need to heat it. You can modify it with planned insulation projects to see where you get the best return. Blown in in the ceiling like you have planned will be a total game changer if there is currently nothing there, but if you already have some ceiling insulation it might not be a magic bullet.
A smallish stove like that should kick out 5 kw easily, and do 10 kw when you’re around to push it. A big stove could maybe do double that if you are willing to tear through a lot of wood. Long term you will be better off modifying the house to fit the stove you have, even if it means covering the inside or outside of those beautiful Timbers with house wrap (has to be plastic) and some additional insulation (could potentially be some kind of wool) Honestly I’m really surprised it doesn’t do a better job for a small place, but the heat loss numbers will tell you where you stand after the major build issues are fixed.
The sill plate where the wood wall meets the foundation might be another spot to check for gaps. This one you can fill with whatever you can find.
Good luck! Fantastic project and dedication.
