imagineero
Addicted to ArboristSite
it's all dependant on your climate, and your microclimate within that. Having too much external thermal mass is a sure fire recipe for disaster in a very cold climate. Some external thermal mass can be a good thing in a climate with hot days and cold nights, especially if it faces the sun. With the right thickness, the mass can absorb heat during the day and keep the house cool then warm the house at night with no need for heating. In a true cold environment it makes the most sense to have the best insulation you can get externally (we're talking R100+) and then have your thermal mass internally so as not to lose any of that stored heat. The thermal mass is like a battery bank, and lets you capture some of the heat that would otherwise get lost. It's a hungry beast though, especially at the sizes you've shown. If that mass were external it would soak up so much cold that you'd need a raging fire for days just to get it back to luke warm. In a steady environment of regular fires it can save you fuel and regulate your heat output. In the night crank 'er up, and in the day just barely keep it going. All the heat from the previous nights burn will carry you through till morning easily, and that stone will still have warmth in it 2 or 3 days after the last fire has gone out.