As buildings are very long-lived and a large proportion of the total building stock existing today will still exist in 2050 in developed countries, retrofitting the existing stock is key to a low-emission building sector. Numerous case studies of individual retrofit projects (in which measures, savings, and costs are documented) are reviewed in Harvey (2013), but a few broad generalizations are:
(1) For detached singlefamily homes, the most comprehensive retrofit packages have achieved reductions in total energy use by 50–75%;
(2) in multi-family housing (such as apartment blocks), a number of projects have achieved reductions in space heating requirements by 80–90%, approaching, in many cases, the Passive House standard for new buildings;
(3) relatively modest envelope upgrades to multi-family housing in developing countries such as China have achieved reductions in cooling energy use by about one-third to one-half, and reductions in heating energy use by two-thirds;
(4) in commercial buildings, savings in total HVAC energy use achieved through upgrades to equipment and control systems, but without changing the building envelope, are typically on the order of 25–50%;
(5) eventual re-cladding of building façades—especially when the existing façade is largely glass with a high solar heat gain coefficient, no external shading, and no provision for passive ventilation, and cooling—offers an opportunity for yet further significant savings in HVAC energy use; and
(6) lighting retrofits of commercial buildings in the early 2000s typically achieved a 30–60% energy savings.