Building a custom masonry heater

[thunderSSSS2]

I want to build a masonry heater to replace the inefficient open fireplaces that are so common where I'm from. I am planning a house renovation and think this would be a much better way to stay warm. I have watched plenty of videos but I am not sure if that is enough information to actually start the project. I know the foundation has to be very strong to support the weight of the bricks. But I am still confused about what kind of mortar mix works best for the high heat. And I keep seeing people use something called basalt cardboard between the inner furnace and the outer walls. I want to understand what that material does and how to layer it correctly

 

[ChicagoDogz]

that sounds awesome. for the mortar, steer clear of standard bag mix. you need Refractory Mortar. unlike regular mortar, it won't crumble at 2000°F
as for that basalt cardboard, it’s a life saver for the structure. It acts as an expansion joint between the inner firecore and the outer stone/brick facing
without that buffer layer, the expanding core would literally crack your outer walls apart like an eggshell

 

[wildcherries]

I want to build a masonry heater to replace the inefficient open fireplaces that are so common where I'm from. I am planning a house renovation and think this would be a much better way to stay warm. I have watched plenty of videos but I am not sure if that is enough information to actually start the project. I know the foundation has to be very strong to support the weight of the bricks. But I am still confused about what kind of mortar mix works best for the high heat. And I keep seeing people use something called basalt cardboard between the inner furnace and the outer walls. I want to understand what that material does and how to layer it correctly

A masonry heater needs a solid base because it is very heavy and itt should use high heat refractory mortar since regular mortar can crack from repeated fires. The basalt cardboard works as insulation between the hot inner core andthe cooler outer shell so the outside stays safe to touch. It also gives the inner firebox a little room to expand without pushing on the outer walls.