Everything thing Wesley stated with acknowledgement given to the HVAC system, from equipment selection, proper operation and/or installation, properly sized and installed supply air ducts/return air. A mech. ventilation system has proven to contribute significantly to efficiency, comfort, and health benefits worth taking note of.
closed cell or if un-affordable open-cell polyurethane spray foam for all exterior walls, roof-deck, and/or separating conditioned space from building envelope. the slab is another area to look at improvements on. The polyurethane spray foam will leave you with a excellent thermal infiltration ratio/thermal barrier. the home will be tight tight “like a yeti cooler” A controlled and i recommend the forced air application of a fresh air intake system using an ERV (Energy Recovery Ventilator) or HRV (Heat Recovery Ventilator) will balance everything and make a very comfortable and efficient home. upgrading to a SV95 furnace or any 90%- 95% AFUE (condensing furnace) if gas is used for heating will be needed for adequate combustion and/or consumer safety