Efficient enough (I put my money into "air sealing" and wall and attic insulation). 

However if you own the home free and clear, the cash flow awesome and you are insulated from risks of mortgage rate fluctuations.

Like a barber recommending a haircut, HVAC guy a new unit...energy auditor new insulation.. pest inspector a pest treatment... insurance guy insurance....

@Charlie MacPherson   --- Did you have to add insulation to the rehab as well? 

@Kate Elson - for a two family in Queens, NY we were able to spray insulation in the 1st floor ceiling.

I pay the cost of the permits.To demo the whole house - $3,000 plus I pay for the dumpsters.New Roof - $3,000 Waterproof basement - busted up concrete and installed french drain system  fed into sump pump (kept existing sump pump) and water seal paint the inside (no outside work) $5,000.Frame whole house - $3,000 (basically basement and main level.. upstairs didn't really change)Drywall - delivered, hung, and finished - $32 per sheet he estimates 120 sheets needed ($3,840 total)Insulate all walls and attic - $1500(*) Rough electrical including permits,new panel & wire - $4000  (outlets, switches, cans, fixtures, etc. additional)(*) New duct work - $4,000 ... seems fairly simple with the house demo'd and being such a small house. 

I assumed when we discovered it that they wanted to double insulate it for the garage to not be horrible... but nope... just added3.5" of wall, and no insulation... totally wierd.

. - They almost always need foundation repair, have constant peeling paint, and are not insulated.

I have a 2400 sf home on a 1.5 acre corner lot that has been a gut-renovation -  from new insulation, electric, to all new sheetrock, paint, flooring, all new kitchen and new bathrooms.It is zoned commercially and residential, has an attached 800 sf garage.

Outside of that, however, stop the water, pull the non-salvageable stuff - usually drywall and insulation - spray down the mold with an appropriate chlorine solution, and make repairs.