An 81% reduction in carbon emissions from existing and heritage buildings by 2030 is required to achieve climate change mitigation targets of preventing warming above 1.5˚C. A methodology and decision framework is presented for deep energy retrofit analyses that balances trade-offs between conservation and sustainability of a building's components. An historic house in Ottawa, Canada was studied to demonstrate the use of the methodology. The energy retrofit analysis suggests 71% energy savings are achievable through envelope retrofits, upgrading HVAC, sensors, controls, and renewable energy. The simple cost payback period is analysed to estimate feasibility for homeowners to implement deep energy retrofits. The carbon emissions saved over 20 years of operation are estimated. This thesis reveals that heritage conservation and sustainability have intersecting values, demonstrating that conserving and upgrading heritage buildings in a respectful way can play a key role in achieving carbon reductions within the existing building stock.