Unheated slab and flooring install/home heating question – GreenBuildingAdvisor

Climate zone 6. Monolithic slab on grade with 2” XPS under and on sides. 

 I’ve come to realize that the mass of the slab keeps the house nicely cool in summer, and in the winter I can leave the heat (woodstove) entirely off on the coldest days and the temperature drops around 5 -10 degrees at the most over 24 hours. I’ve left the heat off during winter construction for a week or more at a time and the temperature never approached freezing. The slab’s thermal battery effect, for lack of a better term, does a good job of moderating the temperature.

The only concern I have is that I find that during the fall and spring seasons when day/night temps cycle between low 60’s and upper 30’s that the house “wants” to settle at around 60 degrees. I can light the stove and get the temperature up to 68, but by the next morning it’s right back to 60. It drops about as quickly as it does in the winter. Overall, I know I’m using considerably less fuel than anyone else I know with a similar sized house, but as I approach installing hardwood flooring over the slab I’m wondering if there would be any advantage to putting down a layer of foam insulation over the slab. 

Would that potentially create a layer of isolation against the thermal battery of the slab that would be advantageous, or would it isolate the slab from the heat of the house to my disadvantage? Thanks, Daniel

edit: I should clarify that in my next door cabin I rarely have need to light a fire during the spring/fall seasons, and unless I want to hear the family complain about the 60 degrees temp in the new house I have to have a fire much earlier than I’ve needed to. But again, overall I’m burning a modest amount of firewood.