Arguments for dense, high-rise residential development include shortage of land and the need to house many people in a compact way. This is not strictly true. An equally high overall density, measured in FAR or dwellings per hectare (dph) is achievable with quite low-rise. As shown in the very comprehensive LSE/Eifer study (2014), typical blocks in European cities like Paris, with six to eight floors, have a FAR of over 3.0 and even 4.0. This is not just equal to but considerably higher than the super blocks in Ningbo and similar cities.

Traditional housing in many cultures has been low-dense, from Mediterranean towns to North African medinas or Chinese hutong. These are often favourably situated and formed in ways that exploit local microclimate. Highly

pressured megacities, on the other hand, often have to expand into climatically suboptimal areas such as flood plains or poorly ventilated valleys; or into ecologically valuable agricultural land and green areas. This may lead to residential development in unfavourable areas for both energy efficiency and living quality. Whilst high-rise may be necessary for some city centres, land use is not the real issue. From the perspective of population densities as well as from the ecological perspective of land use, dense and high-rise solutions offer no notable advantage, and significant downsides.

The megacities of the developing world are experiencing increasing urban heat island effects (UHIE), with associated discomfort and mortality. Green spaces have many important functions including microclimatic amelioration, urban ventilation, socialising and recreation (Yu & Nyuk Hien, 2006; Zhang et al., 2014; Cheshmehzangi et al., 2021a). The biodiversity hypothesis also posits that access to nature is a key to health and wellbeing (Hanski et al., 2012; Haahtela et al., 2013). Do high-rise residential typologies, in the typical large urban blocks, favour good living environments?

As regards ecological considerations such as urban microclimate with resultant energy needs for cooling, high-rise blocks do offer more ground level green space, plus that cleaner air and more air movement are available to high buildings. On the other hand, many countries show good microclimate solutions in low-rise traditional typologies, in both hot-and-dry and hot-humid climates. High-rise may cause unfavorable wind pressure zones and eddies. It may also be noted that with low-rise, as in the Ningbo six-storey block, very useful tree shading can extend up to three or four floors, but not higher. Vegetation design can thus provide shade and cooling as well as privacy in low-dense areas but far less so for high-rise apartment buildings.

It can also be noted that the interior landscaped areas in typical high-rise blocks are often little used and are designed primarily from an aesthetic standpoint only; whereas the green (and blue) infrastructures can and should be designed to enhance microclimate, water infiltration, low albedo effect and other specific ecological design objectives. Hence, high-rise is not necessarily “better”; and as illustrated in the block H example the green spaces are limited in size as well as functionality and biodiversity. Heat island effects may be better mitigated by larger (and truly public) urban parks at regular intervals. The potential cooling effect of around 2 - 3 degrees demonstrated in many studies, such as Chen and Wong (2006), offers a major reduction in space cooling needs.

Building services, especially for ventilation, lifts and stairs, tend to take up large amounts of space in high-rise buildings. This has a significant impact on space usage and embodied carbon as well as on construction cost. In low-dense housing solutions, these requirements are significantly reduced, and they can be executed with simpler materials and technologies. This is doubly relevant for low-cost contexts.

Another common argument for large-scale, dense urban development is economies of scale. This is not always valid. There appear to be important cost considerations as regards low-dense compared to compact city infrastructures: “The cost increase from the low to the medium-density scenario is nearly 100%, while the cost increase from the medium to the high-density scenario is just over 50%” (Barter, 2000). These again pertain especially to low-income housing contexts: “High demographic growth, low levels of economic development, high income inequalities, small urban budgets and shortages of environmental infrastructure, shelter and basic services have a critical effect on densification policies and the effectiveness of policy instruments. The merits of densification at a high level of development may disappear at a lower level and be counterproductive without significant improvements to this level” (Burgess, 2000). Whilst the above sources focus on the economic aspect, the costs correlate broadly with higher energy use and emissions.

Renewable energy supplies (RES) integrated into the roofs and facades of buildings in the form of photovoltaic panels (PVs) can cover the entire energy demand when it is as low as in passivhaus type buildings. There is, equally, over a decade of experience with buildings that produce more energy than they need (Figure 5). However, this is only possible in low-dense typologies. A PV roof on a skyscraper will provide only a small fraction of the required power. Similarly, passive solar heating cannot be maximised with high-rise buildings which shade each other from part of the solar radiation. It is also recognized that deep buildings have inherently higher energy demand (Steemers, 2003).

Here again, advantages of low-dense solutions apply not only to housing. In dense cities there are naturally economies of scale and energy efficiency gains to be found in large systems, not least district heating and/or cooling. However, it is a broadly accepted goal that as much as possible should be covered by on-site renewable energy generation. If so, low-dense housing options offer a considerable advantage.

Dense city typologies narrow the choices for ecological design, eliminating quite many recognised solutions. In inner cities, one can seldom choose climatically favourable sites or building orientation. One cannot use courtyards and similar vernacular housing solutions to create an improved microclimate. Local materials will be less applicable; and lightweight materials, preferable in hot-humid climates, are less feasible in high-rise city buildings. Equally, in consideration of the low-income contexts, complex urban residential buildings render self-build or user-led management and maintenance impossible.

Several factors make it challenging, or expensive, to design low energy buildings of high-rise type. Solar protection, one of the absolute keys to low energy design in hot climates, is difficult in high-rise since more of the facades are exposed. Many units have unfavorable East or West orientation. Technologies to reduce solar gain are mostly expensive and seldom applied. Other units in the high-rises in a climate such as Ningbo’s where some winter heating is also needed and sun is welcome, are North-facing only (and fetch correspondingly lower prices).

Further, tall urban buildings are for reasons of economy almost invariably quite deep, requiring large glazed areas, which lead to overheating (Niu, 2004). They tend towards apartments with one-sided ventilation (and poor daylighting), requiring air conditioning, hence increased energy use with mechanical rather than natural ventilation, plus increased energy for lighting. In addition, fire and façade maintenance requirements are far more onerous in high-rise.

The typology of the older low-dense block E, by contrast, permits architectural design using a higher degree of natural ventilation and where all units have sunlight. Low-dense typologies offer more opportunities for regionally adapted, low carbon housing, less dependent on added technology and based on economical, passive design solutions.

Buildings account for some 40% of global energy use and emissions. The largest energy requirement in buildings has normally been operational energy (OE), mainly for space heating or cooling in cold and hot climates respectively, plus for lighting, cooking, hot water and appliances. In hot climate cities, does a compact, high-rise typology offer advantages in this respect? This appears unlikely partly because the typology itself aggravates the heat island effect, thus increasing the need for cooling. Compact building form a good surface-volume ratio is favourable for operational energy efficiency, but as noted above, not in deep buildings where energy needs for lighting as well as for mechanical ventilation are increased. Detailed studies such as LSE Cities/EIFER (2014) and Jabareen (2006) demonstrate that the overall form of dense or high-rise building typologies is in terms of thermal energy performance, no better than low-dense. Between the obviously inefficient extremes of suburban sprawl and excessive high-rise, there is a large range of low to medium-rise solutions that can achieve optimal energy efficiency. Lifetime operational energy efficiency can thus be at least as good in low-dense housing, as in high-rise solutions.

In today’s low energy buildings, such as the passivhaus type, the operational energy needs are reduced to a fraction, often less than a quarter, of conventional buildings. This means that the energy/carbon required to produce the building itself, principally the production of the materials, becomes far more important. The embodied carbon (EC) is an increasing part of the overall life cycle picture; this trend is reviewed comprehensively in Ibn-Mohammed et al. (2013) and Sartori & Hesnes (2007). For example in a recent sustainable office building in Norway, the embodied carbon is very nearly equal to the operational carbon: 69 versus 75 tons CO₂/year respectively (Future Built Program, 2014). In discussions of “net zero” construction and regenerative design, it is also recognised that it is more difficult generally to reduce the embodied impacts than the operational ones (Cole, 2012).

Studies show that the largest carbon items in a building life cycle analysis (LCA) are often cement products and steel. In a Swedish study of an office building, concrete comprised 69.6% and steel 11.4% of the EC (Wallhagen et al., 2011). In an Italian apartment building these two comprised 76% of the EC (Blenghini, 2009). Cement products (reinforced concrete, mortar and blocks) and steel comprise over 70% of the EC in a Chinese high-rise building case study (Zhang & Wang, 2015). In low-rise, simpler materials can be used. These cannot easily be substituted in high-rise buildings. It should be noted, further, that the embodied fraction will increase as operational energy decreases drastically in future low energy buildings.

We need to consider not only the buildings themselves but also the site works associated with different types of housing development. In dense urban projects such as the high-rise Ningbo block, the large green areas between the apartment blocks consist only of a thin green layer on top of extensive engineering works such as underground parking, culverts and other infrastructural services. Embodied carbon will almost inevitably be higher in high-rise structures, as well as their site works, due to the need for carbon-intensive materials such as reinforced concrete (RC) and steel. LCA studies have addressed buildings and seldom the associated site works and infrastructures. The carbon footprint of this kind of inner city infrastructures, including underground parking, may be very large; up to one-third of the total carbon footprint, as a study of the Ningbo block H suggests. This fraction too will increase as buildings themselves become less carbon-intensive.

Two other components of the embodied impacts of buildings are the energy required for transport of materials to site, and on-site energy use. These are fairly minor of the order of a few per cent each: “Embodied energy is dominated by building material manufacturing, representing 90%, and the share of transportation and construction is 4% and 6% respectively…This proportion is very close to the average value of 18 case studies in Sweden and Denmark examined by Nässén et al: 91% for material manufacturing, 3% for transportation and 6% for construction” (Chang et al., 2012). These fractions, whilst minor, are also likely to be higher in the case of large urban buildings, where local materials will seldom be appropriate, and where cranes and other energy-intensive site equipment are needed. Here again it can be noted that they will also become more significant as operational carbon decreases in future.

The preceding point pertains to the initial embodied carbon in construction materials, or a cradle-to-gate LCA perspective. However, the recurrent embodied energy/carbon inputs over a building’s lifetime may for some building components be as much as the initial embodied fraction (Mequignon et al., 2013). Buildings, as well as urban infrastructures, have large requirements for ongoing maintenance, repair and replacement of parts. Complex inner city structures will normally require more onerous recurrent inputs than low-rise areas. Simpler low-rise solutions are less likely to need specialists and are thus more amenable to low-cost housing contexts. Cleaning and repairs may be far more onerous; a particularly demanding example is maintenance of high-rise facades.

The post-use environmental impacts of dismantling and disposing of or recycling buildings have been less studied, although it is often considered to be minor. This phase requires more attention. Recycling aluminum for example saves roughly 85% of the energy that would be needed for virgin aluminum; and recycling steel saves over 50%. But recycling concrete requires 5% more energy than new concrete, “owing to increased energy required to break up the old concrete”; and “recycling plasterboard is 48% more energy intensive than using virgin material” (Gao et al., 2001).

Current LCA methodologies do not account satisfactorily for this phase, where a cradle-to-cradle (C2C) perspective is, at least to some extent, necessary. The post-use phase of urban structures may be far in the future, but here too our choice of urban form will have its consequences. The post-use impacts are almost inevitably higher with dense urban and high-rise housing, compared to low-dense, due to complicated demolition and recycling or disposal of more complex and polluting construction materials and technical components.

Resilience, a keyword in sustainability discussions, has economic and social dimensions but equally technical and environmental ones. This again demands a long term and holistic view of environmental impacts and emissions. Complex, high-rise buildings offer less flexibility or “generality”, hence less resilience to future modification and adaptation, both as regards functional change and technical innovation.

Amongst leading examples of low-dense urban housing in Europe is the Vauban district in Freiburg, Germany (Cheshmehzangi & Butters, 2017; Butters, 2021) (see Figure 6). This urban transformation shows ecological, economic and social resilience. It provides a good example of resilient urban building types. Transformed in the 1990s from a former military area, all the buildings were converted to new uses, both residential and commercial. New low-energy “infill” housing has been added. The buildings, largely housing, are typical of many low-dense European city districts that have shown to be easily transformed to low, even zero energy housing. Many of these typically older buildings are being successfully “greened” whereas more recent, large-scale urban ones are being demolished after less than 50 years. These examples, despite being fairly low-cost (and involving the users), demonstrate types of buildings that due to their low-dense and construction characteristics can be refurbished to very advanced energy efficiency standards, reducing the operational energy not by 20% or 30% but to near zero.

Further, as regards the embodied impacts, both the Vauban and the Swiss examples underline the important point that embodied carbon is in very many cases best addressed by refurbishing existing buildings rather than demolishing them and producing new ones, given the embodied carbon required for new materials. This is confirmed in LCA studies comparing scenarios of refurbishment versus replacement, such as (Gurigard, 2011).

To some extent, the above supports a broad argument in favor of technical simplicity in building solutions. This again applies particularly to housing. Some researchers consider that the trend of extremely energy-efficient passivhaus type housing is too complicated and even risky in relation to everyday operation and occupant behavior (Harrysson, 2015); which is even more likely to be problematic in low-income contexts. Hence here again low-dense typologies offer, certainly not the only but certainly favorable options.

Given that some of the above points may not be relevant in all cases, and that some are of fairly minor import, their cumulative significance nevertheless appears considerable. The 10 points do not pretend to be exhaustive but suggest that low-rise options can offer advantages in terms of environment, energy efficiency and climate emissions.

To summarise briefly, the above 10 points argue in favour of low-rise typologies, highlighting critical attributes to low carbon strategies, better energy efficiency, and more opportunities for sustainable urban design. Moreover, key aspects like urban energy management and low carbon transitions could be benefited significantly if such residential policies could be reconsidered for the ongoing and future urban development projects. To start with, we argue in favour of smaller urban residential blocks, low-rise typologies, and the development of compact urban environments that are integrated with other critical factors of urban mobility, land-use, morphology, spatial planning, public place design, etc. We urge designers and planners not to limit their approaches to these 10 highlighted points. Instead, their role is to advocate potential sustainable paradigms against the current unsustainable trends of urban development, which are often high-rise, large scale, and not energy efficient. The following section provides examples related to housing and energy, urban mobility, and low carbon strategies to discuss other sustainability factors further. More importantly, our findings highlight the scope for reconsidering residential policies to ensure future development approaches align with key directions of carbon neutrality, sustainable cities and communities, and sustainable development agenda or the sustainable development goals (SDGs). China is a suitable example as urban residential typologies have changed drastically before and after the millennium. These changes have not only changed the Chinese cities’ characteristics but also urban morphology/form, spatial layouts, car ownership levels, accessibility, energy efficiency, and other design factors that require much careful attention for future housing reforms and urban design control measures.

In order to maintain a holistic view, a few of the interrelated broader issues are now noted very briefly. Whilst we highlight energy and carbon issues, there are well-known economic and social arguments for low-dense housing environments, including qualities relating to identity, safety and conviviality (Butters et al., 2020). High-rise housing blocks with low surface coverage offer large open spaces but these are often inaccessible to the public, under-used and largely aesthetic only. High-rise environments are also recognized as being problematic for children and security. Many of these well-known factors have particular relevance for low-income housing contexts.

There are in addition, newer “sustainability” discussions relating to qualities of transparency, governance, and participation. Agenda 21 formulated at the 1992 Rio conference states that a prerequisite for sustainable development is user understanding, support and participation. The interrelationship of socio-economic and eco-technical factors must be underlined; the community-enhancing qualities of small-scale, low-dense living are important not only for qualitative reasons but indeed for energy use and climate emissions too. A study of poor communities in Peru found that “social fragmentation, material poverty and marginalization were working against people’s wellbeing and making in difficult for them to live sustainably. The latter was exemplified by increased waste, extensive use of chemical fertilizers and growing deforestation” (Guillen-Royo, 2011). Similarly, a large post-occupancy study on low-energy housing in Europe shows, as do others, that energy efficiency gains are far below what has been expected due largely to cultural and behavioral factors (Sunikka-Blank & Galvin, 2012).

There is a growing awareness that quantitative objectives such as reduced energy use and climate emissions are heavily dependent on non-technical factors. Whilst seeking advances in technical solutions, this interrelationship of technology and sociology should not be forgotten. It appears particularly critical in the field of housing. Low-dense may offer some advantages in this sense too, with positive consequences for water and energy use, wastes and climate emissions.

A note must be added on the issue of urban mobility, although transport energy lies outside the scope of this paper. In developing countries, acquisition of private cars is a seemingly unstoppable ambition or is considered as a matter of status. In hot climates, they add to the heat island effect. Transport is recognized as perhaps the toughest challenge for sustainable cities (Wang & Yuan, 2013). In life cycle analyses (LCA) of buildings, if transport to and from the buildings is included, as in some Norwegian LCA systems, it can be seen to form the major part of the total energy and climate impacts associated with buildings (and principally, their location) (Future Built Program, 2014). This again highlights new perspectives that the sustainability agenda brings to urban planning.

Whatever the urban density, transport is a key to urban energy and GHG reductions. Whilst the “compact city” optimizes the potential for transport hubs and public transit, it often overlooks a key eco-design goal: walkable cities. Where cars are given priority, vast areas of city land are occupied by roads and parking; and congestion (inefficient mobility, the opposite of the goal) is inevitable. In walkable developments, such as the Vauban eco-district, inhabitants possess cars but the key difference do not need to use them much (Nielsen, 2007). The traffic consumes far less energy, occupies much less land, and causes less pollution, noise, and danger. Hence, a walkable city cannot be a car city.

In other words, very high urban density only makes sense if there is low car use. Very low density on the other hand, the “suburban sprawl” paradigm is obviously at the other extreme, necessitating high transport emissions. As noted above, in between the extremes there lie a range of low- to medium-rise options that may represent a good balance and offer excellent energy and carbon performance.

Finally a brief return to the six-storey 1990s housing block E in Ningbo. Discussing advantages of low-dense does not imply nostalgia, nor uncritical approval of such examples of housing. The layout of block E is monotonous and the quality of construction is poor. The area is in disrepair. Car parking and green spaces are not well organized. On the other hand, the green spaces are small but intimate and well used; the one larger communal open space in particular. In addition, there is a pleasant public park nearby which is in constant bustle and activity. The typology is simple, as are the materials, the building envelope is compact and this housing type is easy to construct to a very high level of energy efficiency. The unit cost is low. Given a more imaginative layout and design and up-to-date construction this model could provide many qualities as well as a very low carbon footprint (Figure 7). In consideration of all of the 10 points discussed above, low-dense housing has many advantages and deserves revisiting in updated, sustainable forms.