Finishing A Detached Garage Conversion

[Greg Kitzmiller]

I recently bought a house with a partially-converted detached garage. It has a mini-split for heating/cooling. The issue I've ran up against is the mini-split does nothing against the concrete slab which seems to absorb and slow-release whatever outside temperature it's been in the past 12 or so hours.

I'm a complete noob. Doing some research it looks like I can do a moisture barrier and sub-floor - not sure if that'll help improve things. Running the mini split 12 hours a day gives me a large bump in my electricity bill. My only other thought is just that maybe the mini split is old and not particularly good.

Any help/advice would be appreciated.