Dr Jesus Lizana (University of Oxford) and Dr Anastasia Mylona (CIBSE)              HRSC0045

Written evidence submitted by Dr Jesus Lizana (University of Oxford) and Dr Anastasia Mylona (CIBSE)











Dr Jesus Lizana

Marie-Curie Research Fellow

Department of Engineering Science

University of Oxford


Dr Anastasia Mylona

Head of Research

Chartered Institution of Building Services Engineers




Introduction to the authors, organisation and reason for submitting evidence:


We are researchers in the UK with expertise in energy, buildings and climate. Dr Jesus Lizana is a senior researcher in the Future of Cooling Programme at the University of Oxford, with almost ten years of expertise working in academic and non-academic building projects, with the last three years focusing specifically on the cooling sector, and Dr Anastasia Mylona is the Head of Research at the Chartered Institution of Building Services Engineers (CIBSE), with more than fifteen years experience in the area of adapting buildings to future changes in climate, especially higher temperatures.

We worked together, with the support of our institutions, to provide written evidence to two questions raised in the call:

We hope you find our evidence valuable and informative.



In this written evidence, we provide recommendations for the steps that should be considered for climate adaptation and sustainable cooling in the built environment (for domestic and non-domestic buildings), highlighting some barriers found explicitly in the context of the UK for their implementation.

The recommended steps to address sustainable cooling and climate adaptation are eight. They involve many robust alternatives available to address cooling, where vapour-compression refrigerant technology (or air- conditioners) is part of the solution (and not the only option to rising temperatures). These eight steps consist of (1) solar protection; (2) heat dissipation; (3) heat modulation; (4) heat prevention; (5) smart management of more comfort variables; (6) occupant-centric technology; (7) focus on the life cycle, and (8) energy consumption based on renewable energy. These solutions, implemented following the right order, can increase the heat resilience of the built environment (steps 1-4), downsize cooling systems and cooling needs (steps 5-6), decrease resource consumption (step 7), and mitigate the environmental impact of cooling (step 8). These steps should be implemented considering current and future weather conditions, heating and cooling as a whole in building design, and socio-cultural factors.

Additionally, some UK barriers to promoting these climate adaptation interventions were highlighted.These barriers are also divided following the eight-step structure and involve criteria related to limitations found in the existing building stock, existing regulations, and occupants’ expectations.


Challenge: Vapour-compression refrigerant technology should be part of the solution rather than being positioned as the sole technological solution to rising temperatures in the UK. If overlooked, air conditioners will be the first option for rising temperatures since they will always be the easiest and fastest response to deal with heat.

Changing climate: There is a limited window to shape the fast-growing trajectory of energy demand for cooling, with rising temperatures driving cooling emissions and hampering progress towards zero-carbon targets. Using mean Cooling Degree Days (CDD) in UK-based urban areas as an indicator to measure heat exposure and cooling needs in the built environment, in a previous study [1], we found that currently, the UK is exposed to a mean value of 43 CDD (mean during 2006-2016). This value is expected to increase to 80 CDD if the global mean temperature rise increases to 1.5ºC, and to 102 CDD if it increases to 2.0ºC. 

Importance: The building design in the UK has been traditionally designed exclusively for the cold season, with a building stock highly unprepared for heat. Buildings in the UK (and northern Europe) are designed to keep heat in by maximising solar gains (e.g. dark roofs, no external windows shutters,…) and minimising ventilation (e.g. windows locked, roofs not ventilated). But in summer, when it is hot, buildings have to do the opposite. So, even with a moderate increase in temperatures compared to other regions, the impact of extra heat will be felt more acutely. Policy, recognising the importance of the risk of overheating, has partly addressed overheating in homes with the introduction of Part O in Building Regulations. Part O introduces the requirement to assess overheating risk in new homes (only) and its mitigation by passive measures such as external shading, increased ventilation, etc. Such policies do not exist for existing housing stock, non-domestic buildings and urban design.

The balance between heating and cooling demands: When designing or adapting buildings, it's essential to consider the overall projected heating and cooling demands in the following decades. For example, maximising ventilation can prevent overheating during summer, but minimising ventilation can help reduce the need for heating during winter. The key is to achieve a suitable balance. Even if there is a slight increase in heating needs after adapting our homes, the investment, maintenance and operating costs of a new energy-intensive air-conditioning system can be avoided or reduced. This approach allows people to stay comfortable during hotter temperatures without exacerbating energy poverty and without compromising the climate further for future generations.

Climate adaptation and sustainable cooling: Even though the easiest and fastest response to address increasing temperatures lies with the installation of air conditioning units, there is a large number of robust low-energy cooling techniques and technologies available to address cooling (Fig 1a), often overlooked. Implemented following the right steps in the right order, they can even eliminate the need for an air conditioner.

In our research [2], we outlined and justified the recommended steps to address climate adaptation and sustainable cooling (Fig. 1b). By following these eight steps in the right order, we can make our built environment more resilient to heat (steps 1–4), reduce the need for large cooling systems (steps 5–6), and lessen the environmental impact of cooling (steps 7–8). These steps are detailed and briefly justified below:


Figure 1. Portfolio of cooling techniques and technologies (a) and the recommended steps to address climate adaptation and sustainable cooling (b). Source: Lizana et al. [2].


The right steps towards sustainable cooling:

1. Solar protection refers to avoiding solar gains in the built environment. Recent research highlighted solar shading as the most effective response to reduce overheating [3–5]. There is an extensive portfolio of available solutions for solar protection (Fig. 2). This can be applied to openings (e..g. windows) and opaque building envelope surfaces (e.g., roofs or walls). The most important actions in windows are low emissivity glazing (shading coefficient of the glass unit > 0.85) and external shading devices. External window shutters can reduce cooling needs by up to 14% [4]. In opaque surfaces, it should be promoted the use of reflective and ventilated surfaces, avoiding dark colours (solar absorptance of 0.70-0.80) and promoting light colours (solar absorptance of 0.30-0.20) [6]. In this step, building design should consider heating and cooling as a whole, maximising solar gains in winter and reducing them in summer.

Figure 2. Alternatives to promote solar protection in buildings.


2.Heat dissipation is based on heat rejection from the building through passive or low-energy techniques, with the support of a suitable environmental heat sink at a lower temperature, such as ambient air, water, ground, and the sky. The most effective alternative is natural/mechanical ventilation [5,7], but we can also implement solutions based on evaporative cooling, radiative cooling, ground cooling, and water cooling. To promote adecuate ventilation, buildings should be designed to promote cross ventilation. If necessary, this can be promoted by incorporating inner courtyards, which can reduce indoor discomfort hours by up to 26% [8]. But additional features, such as ventilation chimneys and roof vents, can be incorporated to further assist airflow (Fig. 3).

Figure 3. Alternatives to promote natural ventilation in building design.


3.Heat modulation consists of using the building's thermal mass to store heat gains and release at optimal times, such as free cooling or natural ventilation at night. This technique depends on the thermal storage capacity of the building materials and the natural heat sink available to receive the excess heat. It is essential to highlight that heat modulation only works in combination with heat dissipation [9]. Otherwise, overheating can increase up to 18% [9].

4.Heat prevention involves mitigating or avoiding external and internal heat gains in the building (sensible and latent heat gains). External heat gains arise from ambient temperature transferring heat through the building envelope, whereas internal heat gains are due to human activities, appliances, and lighting. The most crucial actions here are insulation and airtightness to reduce heat load, but always ensuring that heat dissipation is adequately addressed. Otherwise, again, overheating will increase by more than 5% [5].

These four steps will improve the passive survivability of the building without air conditioning. Only when these solutions are well addressed we should start implementing the best available active cooling technologies. And here, again, we find additional opportunities to eliminate the need for an air-conditioner.

5.Smart management of more comfort variables. The fifth step consists of using all comfort variables to create comfortable environments. Our perception of coolness is not solely determined by temperature. Comfortable environments should be created by controlling temperature, humidity, air velocity and radiant temperature, allowing personal adaptation, and ensuring an adequate supply of fresh air to achieve good indoor air quality. For example, integrating ceiling fans to increase airspeed will enable us to increase the set-point temperature from 24ºC to 27ºC without sacrificing thermal comfort (Fig. 4), reducing energy consumption by more than 20% or even eliminating the need for an air-conditioner [10–13].

Figure 4. (a) Keeping cool only by temperature control; (b) using all thermal comfort variables. Source: Lizana et al. [2]. 


6.Occupant-centric building design. Centralised systems have been proven inefficient since they often end up cooling people down more than necessary or even waste energy by cooling empty rooms. This step combines centralised and personalised environmental control systems (PECS) involving all comfort parameters and allowing adaptability. This requires distinguishing between centralised building configurations and individual adaptive environments for user adaptability according to different cooling needs. It is called an occupant-centric building design. The target is to decrease centralised cooling systems, reducing power and energy demand without sacrificing comfort. A growing market of personal cooling devices, involving personal/desk fans, radiant panels, cooled seats or wearable ac devices are available, which can be applied to non-domestic and domestic buildings [14].

If these steps are well integrated, there is a good chance that an air conditioner will not be needed. However, if additional cooling is still required, the following steps will help to select the most sustainable and efficient cooling technologies:

7. Focus on the life cycle. The seventh step is focused on the life cycle of cooling systems, selecting super-efficient equipment and appliances with high durability, adequate maintenance and ultra-low global warming potential (GWP) refrigerants. Here, product-service business models can be vital to fulfilling all these criteria over time. These business models are based on purchases where the product is joined by more features or service contracts, such as maintenance, repair or replacement, or even scenarios where ownership is not fully transferred (e.g. leasing). Different research has proved how service-centric models and hybrid models that combine product sales and service options are more profitable and environmentally superior to product-centric businesses [15].

8. Energy consumption based on renewable energy. Finally, the last step involves that cooling systems’ operation should also enable higher use of renewable energy sources to reduce fossil fuel dependence. In this case, the consideration of energy storage (electric batteries or cool batteries) for cooling integration can play an essential role in cooling decarbonisation, to maximise the benefits of solar technologies or the use of renewable energy sources in electricity consumption.

It is essential to highlight that the most challenging aspect of sustainable cooling and climate adaptation is time since addressing all these actions requires long-term planning. When we face some heat events in the summer, buying an air conditioner will always be the easiest and faster response. So, it’s vital to start promoting these interventions today, where vapour-compression refrigerant technology should be part of the solution and not be positioned as the sole technological solution to rising temperatures.




Our experience in specific building projects in the UK supported the identification of different barriers that should be addressed to promote climate adaptation and sustainable cooling. We have organised all barriers following the eight-step structure previously defined:

1.Barriers for solar protection:

-Lack of attractive shading options following UK building archetype (e.g. vernacular architecture).

-Some city councils impose restrictions on painting walls white or adding external solar protection devices into building facades, limiting the adaption of these solutions.

-It was found that in many UK buildings, windows open outwards, so it would be impossible to open the windows while the shutters are closed, as is usual in Europe. Alternative options are solar film or internal shutters, which are less effective.

-Most UK roofs are not ventilated, maximising solar gains and overheating when high solar radiation.

-The availability of low-to-moderate cost solutions for external solar protection devices in the market is scarce in the UK.

2.Barriers for appropriate ventilation:

-The understanding that ventilation is just opening a window limits the heat dissipation capacity of homes. The optimal strategy will be to promote cross ventilation in all building design, drastically increasing the air change rate.

3.Barriers to heat modulation:

-Windows are commonly not expected to keep open due to security concerns at night, especially when there is no occupancy. Overcoming this barrier is essential for heat dissipation.

4.Barriers to heat prevention:

-UK policies promote highly insulated buildings and airtightness by assuming that most buildings use heating, ventilation and air-conditioning (HVAC) systems. However, in scenarios without HVAC (or not frequently used HVAC) systems, studies have shown that insulation and airtightness prevent the release of heat when dissipation through ventilation is not adequate, increasing overheating [2,5]. The mechanisms commonly found to dissipate heat from an indoor environment are heat fluxes through the building envelope (managed by thermal mass and insulation), infiltrations (air leakage) and ventilation. If insulation and airtightness are improved without promoting better ventilation, less heat is dissipated and consequently, overheating increases [2].

5. Barriers to using more comfort variables in an indoor environment (e.g. higher air speed):

-Ceiling height is low in most UK building stock, limiting the applicability of ceiling fans with blades.

-The integration of a higher airspeed to create comfortable indoor environments is not fully considered in existing building regulations for cooling. Existing compliance procedures, such as SAP [16] in the UK, calculate and limit cooling consumption and carbon emissions through average climate conditions and a fixed set-point temperature (e.g., 24ºC). For example, integrating fans to increase airspeed with ACs can also increase the set-point temperature from 24 to 27°C without sacrificing thermal comfort, reducing energy consumption by more than 20% [10–13]. However, this integration does not provide a better energy performance certificate (EPC) rating in the UK and abroad.

-To promote a higher air speed in indoor environments, it should be noted that some building guidelines have specific criteria for indoor environmental conditions, but they are not exclusive and should be analysed carefully. For example, ASHRAE 55 allows air speeds up to 0.8 m/s in scenarios where the occupants do not have local air speed control. When occupants have local authority and control, there is no maximum upper limit to the air speed allowed by the standard [17].

6. Barriers to occupant-centric building design:

-Many studies have also highlighted the lack of adaptive opportunities with operable windows, movable shadings, integrated fans or personalised cooling alternatives, partially because of this cultural and building-centric regulatory framework focused on centralised indoor temperature control [2].

-Thermal expectations tend to be strict in air-conditioned buildings, limiting the applicability of lower centralised requirements with individualised environmental control systems. The awareness to change occupants’ behaviour and expectations should be considered.

7. Barriers to focus on the life cycle:

-No adequate labelling of the global warming potential (GWP) of refrigerants in air conditioning systems to empower consumers to make a better sustainable purchase.

8. Barriers to energy consumption based on renewable energy:

-There are no methods that coordinate or promote innovative integrated technologies to maximise renewable use for cooling (and heating). Additional incentives are required that deliver fair payment to consumers to inspire the decision to shift energy demand, potentially integrating thermal energy storage in cooling (and heating) technologies. Some reference examples are the new time-of-use tariffs designed for heat pumps (https://octopus.energy/cosy/), spot price contracts, National Grid’s Demand Flexibility Service (https://www.nationalgrideso.com/industry-information/balancing-services/demand-flexibility-service-dfs) or local flexibility markets.

August 2023


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