Using existing slab
I’m putting a one-room addition (about 11×14′) on the back of our walkout basement on our 1960 ranch in northern Utah (dry and sometimes snowy winters; dry and hot summers).
The site is difficult to access and everything needs to be hand carried to the back and debris hand carried to the front, including hand digging the foundation trench and hand-mixing bags of concrete.
My contractor has the idea of leaving the existing concrete patio slab, which doesn’t have a crack in it after 60 years, in place and only sawing out 2-3′ wide strips of the patio where necessary to dig out and put in the foundation wall. The old slab would be joined to the new foundational wall in some way.
This would obviously save time and money but also the embedded energy and debris waste in retaining the existing patio slab and not using new materials. For reference we used about 500 therms of natural gas in 2018 for heating 1450 ft2 above grade and 1200 ft2 walkout basement with our 80% efficient furnace – about $500/yr locally.
We wouldn’t be able to put rigid insulation or a vapor barrier under that part of the slab. Our existing house obviously doesn’t have these either. I assume it wouldn’t meet energy codes but if the local permitting authority would approve it…
Structurally is this a bad idea?
What are your thoughts on the energy and/or carbon balance of reusing the concrete slab vs. saving some energy by properly insulating a new slab?
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