This text researches how retrofitting to reduce heat loss can improve the energy efficiency of solid-brick-walled Victorian mid-terrace houses. Its intention is to try to understand the technologies of ventilation and conduction heat-loss reduction and their energy performance implications in the context of other concerns such as: those regarding the functionality and aesthetics of the home; the desire to conserve the character of neighbourhoods; the practicalities of construction; and the durability of the building fabric. A literature review outlines the themes, and a review of Victorian terrace retrofit case studies defines what has been shown to be achievable. A design study of a reasonably typical terraced house in Cambridge is undertaken and it is airtightness tested for overall and component leakage rates. Dynamic thermal simulations of various retrofit interventions are carried out. The biggest components of heat loss in the existing house are found to be wall conductance, window/door conductance and ventilation. The house has an ACH50 of 17.5 and it is approximated that 15% of this is via the sash windows with a further 15% via the other windows. Window/door and wall upgrades are found to be most successful. The complete retrofitted house is shown to meet the AECB CarbonLite Silver target for space heating demand (40 kWh/m2/yr). The house's CO2 emissions would be reduced from 8.5 tonnes to 3.3 tonnes/yr by these space heating demand reduction measures alone. It is exposed that at this improved level of airtightness purpose-provided ventilation would be needed to ensure air quality.