1. Introduction
Energy efficiency is one of the key measures needed to achieve the net zero targets announced by the UK and many other governments worldwide [
1]. Domestic emissions account for two-thirds of the total emissions and the UK’s housing stock is one of the most inefficient in Europe [
2].
The UK’s Standard Assessment Procedure (SAP) rates the energy and environmental performance of houses from bands A to G (with A and G being the most and least efficient, respectively) [
3]. The creation of energy efficiency strategy is a devolved power, but the UK government publishes general guidance for all regions [
2,
4,
5]. The UK government sets energy efficiency targets focusing on improving the efficiency, and consequently averages the SAP ratings of the housing stocks across all regions. In 2017, the Clean Growth Strategy stipulated that all UK homes reach an SAP rating of C by 2035, and all fuel-poor and rented households reach an average SAP rating of C by 2030 [
2].
Presently, the UK is targeting net zero greenhouse emissions by 2050, with current building regulations reflecting fabric and heating system standards [
6]. Thus, advancing the energy efficiency of UK buildings has been listed as a national infrastructure priority, with the highest possible standards implemented for proposed energy efficiency upgrades of existing buildings and new-build constructions [
4,
7].
Existing studies on the impact of retrofit on existing housing show that retrofitting can yield considerable reductions in domestic demand and carbon emissions, can provide socioeconomic benefits and can provide insights for existing building regulations updates. In 2010, approximately 4 million jobs were saved and created due to the construction of 125,000 new energy-efficient homes in the United States through a scheme facilitated by the American Recovery and Reinvestment Tax Act [
1]. There were significant savings in energy bills, which low-income/fuel-poor households benefitted from. Various energy efficiency schemes across Europe, such as the scheme funded by the KredEx Revolving Fund in Estonia and the INTENSE project in South Moravia, have shown that energy efficiency can improve the economic and environmental value of entire neighbourhoods [
8].
Rosenow et al.’s 2018 study considers insulation (loft, wall and floor), glazing, boiler upgrades, heat networks, heat pumps and household appliances [
9]. They find that the demand of the existing UK housing stock can be cost-effectively and technically reduced by one quarter and one half of the current demand by 2035, respectively [
9].
Colclough et al. investigated the post-occupancy performance of social houses renovated to the Nearly Zero Energy Building (nZEB) standard in south-east Ireland [
10]. These renovations featured increased fabric insulation, ventilation and heating system upgrades and the addition of solar panels. The occupants reported increased levels of comfort, which reinforces the benefits of highly energy-efficient fabrics and heating systems. The Energiesprong project develops net zero energy homes and their retrofit projects include the transformation of houses in Nottingham and Essex, among others [
11]. Their projects demonstrate that fabric retrofit, including increased insulation, high-performance double-glazed windows, can facilitate improved thermal comfort [
11].
Furthermore, high fabric energy efficiency has the potential to reduce future heat demand if superior standards of energy efficiency are enforced in new homes [
4]. New builds would need first-rate levels of insulation, airtightness and ventilation, triple glazed windows and external shading, low carbon heating, water management and cooling, and flood resilience [
4]. Consequently, passive house standards may be incorporated into new-build regulations. The Committee on Climate Change (CCC) Sixth Carbon Budget advice states that new homes would need passive shading measures and mechanical heat recovery and ventilation to reduce future overheating [
12]. It provides a more detailed analysis on the distribution of low-carbon heating systems among new homes across all UK regions.
Nevertheless, the impact of the proposed energy efficiency measures for new homes on future domestic demand and emissions remains unclear for future UK homes. Investigations into the impact of fabric energy efficiency on existing and new domestic buildings should be carried out with consideration of local context [
4]. Nonetheless, most UK-based studies on energy efficiency upgrades are largely based on the English housing stock, and do not consider the individual residential characteristics of the other regions.
Among all UK regions, Northern Ireland (NI) has distinct domestic energy efficiency needs. NI building thermal standards are the most outdated in UK as they were last updated in 2014, while those of Wales, England and Scotland were updated in 2016, 2018 and 2020, respectively [
6,
13,
14,
15]. Glazing standards were updated in 2002 for the rest of the UK, while these standards were not updated until 2006 in NI [
16]. The average SAP rating of NI housing stock, rating D (66.32), is the highest in the UK; this is due to the large share of social houses in the region. However, NI’s average SAP rating is only slightly higher than that of English housing stock (rating D: 63.21) which is in second place [
17,
18,
19,
20,
21]. Houses in NI have larger floor areas than houses in other regions; the mean domestic floor areas for Northern Irish, English, Scottish and Welsh housing stocks are 107 m
2, 90 m
2, 104 m
2 and 79 m
2, respectively [
19,
21,
22]. Consequently, the costs of domestic retrofit schemes in NI would be potentially higher than in the rest of the UK.
Additionally, the level of fuel poverty in NI is the second-highest in the UK as 18% of all NI households are fuel poor [
12,
23,
24]. Each region is responsible for developing its own fuel poverty strategy [
24]. While England aims to improve all of its fuel-poor homes to a SAP rating of C or higher by 2030, the Scottish government states that reaching EPC C will not be sufficient to lift all houses out of fuel poverty [
24,
25]. Therefore, the Scottish government proposes that the fuel-poor households in its stock would reach an SAP rating of C by 2030 and an SAP rating of B by 2040 where technically and economically feasible [
25]. The Welsh fuel poverty strategy states that the energy efficiency of Welsh fuel-poor houses will be improved by 2035; however, it is unclear which target SAP rating has been set [
26]. In Northern Ireland, the most recent fuel poverty strategy was published in 2011 [
27]. The strategy states that improved energy efficiency is one of the tools needed to eradicate fuel poverty. However, no clear targets towards fuel poverty mitigation are indicated, and the NI fuel-poor strategy has not been updated since 2011.
Nevertheless, existing energy efficiency programmes in NI are mainly targeted at fuel poverty; examples include the Warm Homes Scheme, the Affordable Warmth Scheme and the Northern Ireland Sustainable Energy Programme (NISEP). Through recent programmes, 9000 cavity walls were fully insulated and 66,000 lofts were topped up with insulation between 2016 and 2018 [
23]. Energy efficiency schemes are run by local authorities and government departments such as the Northern Ireland Housing Executive (NIHE) and the Department of Communities [
17]. Still, more efforts are needed towards creating energy efficiency programmes for NI.
In 2020, the Zero-In on NI Heat networking project initiated conversations on barriers and opportunities for heat decarbonisation in NI among market experts and consumers [
28]. The project revealed that consumers are willing to facilitate heat-sector decarbonisation, and government leadership is important in this transition. Currently, all energy efficiency schemes in NI implement retrofit measures for about 16,500 buildings per year [
29]. Still, existing studies indicate that more houses would need to be retrofitted annually to achieve significant domestic energy efficiency improvements that align with national UK targets [
12,
29,
30]. As such, there should be more emphasis on developing more energy efficiency schemes and producing a strategy that can drive improvements in the energy efficiency of existing and new domestic fabrics.
Energy Efficiency Potential across NI
The Department for the Economy (DfE) released the latest energy strategy for NI in December 2021 [
30]. The strategy states that up to GBP 2.4 billion will be spent on reducing energy use via the introduction of low carbon heating and investing in energy efficiency measures in the domestic sector between 2021 and 2030. During this period, 50,000 buildings (approximately thrice the current number of buildings retrofitted annually in NI) will need to be retrofitted annually. Additionally, a large pilot domestic retrofit scheme will be launched in 2022; this scheme will consider the requirements for heat pumps and be aligned with current and other pilot retrofit programmes. While this new strategy gives some general direction for energy efficiency improvements in NI, it does not provide a much-needed comprehensive framework on domestic energy efficiency implementation.
In 2014, the Department for Communities (DfC) opened a consultation for a new fuel poverty strategy for NI [
31]. The new strategy would address the Warm Home Scheme delivery model, qualification criteria and available energy efficiency measures. This strategy is yet to be published. National Energy Action (NEA) and Energy Action Scotland (EAS) have recently published a report with some recommendations for fuel poverty mitigation in NI [
32]. The report states that all NI fuel-poor homes can be upgraded to SAP rating C between 2025 and 2030 at a cost of GBP 440 million. However, there are no details on how this can be achieved.
The DfE’s new energy strategy aims to tackle fuel poverty by implementing measures that will cause carbon emission and energy bill reductions [
30]. Still, there is insufficient detail on what measures would be applied, what percentage of the fuel-poor housing stock would receive these measures, a target SAP rating and a retrofit timeline.
The CCC’s Sixth Carbon Budget (6CB) advice suggests an 11% reduction in total domestic demand and 4% removal of emissions (based on 2016 levels) in NI by 2050. This can be achieved if fabric retrofit measures are implemented in 410,000 existing NI dwellings (including fuel-poor households) by 2050 [
12,
33]. However, the 6CB dataset does not provide a comprehensive breakdown of the number of NI houses (in terms of age or type) to receive the recommended retrofit measures. New builds will be zero-carbon; nevertheless, there is insufficient detail on what measures would be applied to the building envelopes of new buildings.
The NIHE’s 2019 report on the cost of improving the SAP rating of NI dwellings indicates that 23,200 F- and G-rated occupied and vacant houses can be improved to rating E via fabric retrofit at a total cost of approximately GBP 87 million, with an average cost of GBP 3700 per house [
34]. Although this study presents the costs of improving the efficiency of NI houses with the lowest SAP ratings, it does not investigate the impact of energy efficiency programmes for houses of EPC bands D and E. Furthermore, this study does not consider the impact of its recommended improvements on the SAP rating of the entire NI housing stock and there is no analysis for future housing stock.
Another 2019 NIHE study outlines eight scenarios specifying fabric and heating system upgrades [
35]. A total of 632,000 houses received at least one retrofit measure, at a total cost of GBP 2.4 million with an average cost of GBP 3133 per house. The average SAP rating for the domestic stock increased from 65.8 to 73.8 (from band D to band C). However, this study did not specify the impacts of these scenarios on heat demand or carbon emissions. Additionally, there is no consideration of future domestic heat demand, carbon emissions and new builds.
ARUP’s most high-reaching scenario for NI aligns with UK net zero targets. It suggests a 55% reduction in energy demand by 2050 and states that NI policy must promote the retrofitting of a maximum of 50,000 buildings per year [
29]. Although ARUP presents an analysis that considers the effect of potential NI energy efficiency policy outlines on domestic retrofitting, the study does not give a detailed breakdown of how retrofitting can be achieved. The number of houses to be retrofitted and changes in demand per measure are not clearly represented, and this limits the feasibility of the study. The 2021 NIHE report on the cost of carbon savings in Northern Ireland’s housing stock states that 390,000 dwellings can be improved to band C at a total cost of GBP 2.4 billion, with an average cost of GBP 6200 per dwelling [
36]. Moreover, 3.2 tonnes/year of CO
2 emissions can be removed with improvement to band C. Additionally, 586,000 houses can be improved to band B at a cost of GBP 9.2 billion and at an average cost of GBP 15,600 per house. However, this study is not sufficiently detailed to suggest a defined pathway for NI domestic energy efficiency. Moreover, there is no breakdown of the archetypes recommended for retrofit or impacts of the suggested scenarios on domestic heat demand.
The novelty of this study is the initiation of a comprehensive framework for achieving fabric energy efficiency in NI through the implementation of measures and prescribing specific numbers of existing and new households. It outlines three varying levels of retrofit with corresponding changes in demand and carbon emissions that consider government guidance for NI housing stock (including fuel-poor households). Additionally, this study provides a retrofit cost breakdown per scenario and presents the socioeconomic impacts.
3. Results and Discussion
3.1. Change in Demand and Emissions across All Scenarios
There is a 10% reduction in the heat demand of the existing housing stock by 2050 (see
Figure 5 and
Figure 6). This reduction is slightly lower than the 6CB’s estimated 11% reduction in NI’s existing domestic demand by 2050 from 2018 levels due to retrofitting only [
66]. The highest changes in demand come from improved airtightness, because almost all of the existing housing stock is recommended for airtightness retrofitting. Other significant changes in demand come from cavity wall and solid floor insulation, which are recommended for approximately all houses needing these measures.
The highest percentage reduction in demand comes from solid wall insulation (see
Table 2). However, houses with solid walls are the least efficient in the stock; they are also often heritage houses [
69,
70]. This makes them difficult and expensive to insulate. Therefore, a low uptake of solid wall insulation is recommended, and this limits the potential of this measure. Around 6% of domestic carbon emissions are removed by 2050. However, the 6CB advice suggests that 4% of domestic emissions are removed by 2050 (see
Figure 7). The CCC’s percentage of emissions removed might be less due to dissimilarities between the breakdown of heating types in houses recommended for retrofit by this study and the CCC’s study. The highest removals of carbon emissions come from improving airtightness, cavity wall insulation and loft insulation top-up.
In the High-Level Scenario, there is an 18% reduction in the heat demand of the existing housing stock by 2050 (see
Figure 5 and
Figure 6), which is significantly above the 6CB’s estimated 11% reduction in NI’s existing domestic demand by 2050 [
66]. The highest changes in demand come from houses retrofitted with solid floor insulation. Solid floor insulation incurs the third-highest reductions in demand and is applied to about half of the total number of houses requiring retrofits. Other significant changes in demand come from pre-2006 double glazing to triple glazing improvements and improved airtightness. Around 12% of domestic emissions can be removed in this scenario; the highest removals of carbon emissions come from improving pre-2006 double glazing to triple glazing, solid floor insulation and improving airtightness (see
Figure 7).
The Passive Scenario results in a 42% reduction in the heat demand of the existing housing stock by 2050 (see
Figure 5 and
Figure 6). In the Passive Scenario, the highest changes in demand come from houses retrofitted with solid floor insulation. Other significant changes in demand come from improved airtightness and cavity wall insulation. Furthermore, this scenario estimates that 27% of the current emissions can be removed by 2050; the highest removals in carbon emissions come from solid floor insulation, improved airtightness uptake and cavity wall insulation in this scenario (see
Figure 7).
3.2. General Cost Impacts and Potential Improvements in SAP Rating
The Central Scenario costs approximately GBP 2 billion (see
Table 5). Overall, cavity wall insulation, upgrading pre-2006 to post-2006 double glazing and solid floor insulation are the costliest measures to implement. Furthermore, the bungalow and the flat/apartment are the most and least expensive house types to retrofit, respectively. Although the detached house has the largest floor area, the bungalow has the highest building cost per square metre of all house types because it occupies a high value of land for its floor area [
71]. The average cost of retrofit across all house types is GBP 3700, with bungalows and flats showing the highest and lowest average costs of GBP 5500 and GBP 1900, respectively. These costs are broadly in line with other studies and assumptions mentioned earlier.
However, the 6CB Balanced Pathway Scenario shows that the average household will need a GBP 9000 investment in retrofit [
66]. Houses are recommended for a range of retrofit packages costing between GBP 1000 and 10,000, depending on their retrofit needs [
66]. The CCC considers additional energy efficiency improvements such as heating system upgrades, which accounts for a higher average cost of retrofit per house than those in this study. This study’s Central Scenario only proposes fabric retrofit measures with average costs of GBP 50–7400 per house.
The implemented measures and accompanying SAP points in the Central Scenario are outlined by building regulations [
21]. In the Central Scenario, 23,000 existing houses receive at least one retrofit measure annually by 2050. Thus, the average SAP rating for existing and new NI housing stock increases to 73 (band C) by 2050 (the average SAP points for a new build is estimated as 86 (band B)) [
22]. If Northern Ireland aims at improving domestic energy efficiency in line with other regions, the Central Scenario must be delivered through accelerated schemes implementing the retrofit of 75,000 houses annually with at least one measure by 2030.
The NIHE’s cost of improving 23,200 occupied and vacant F- and G-rated dwellings to band E through envelope and heating system retrofitting is GBP 87 million, with an average cost of approximately GBP 3700 per dwelling [
34]. This study estimates a similar average cost of retrofit (GBP 3700) with approximately 670,000 houses being recommended for retrofitting with measures including costly and potent measures such as solid wall and solid floor insulation.
Additionally, the NIHE suggests the average SAP rating for the domestic stock will increase from band D to band C, with oil and gas heating installations and fabric (easy-to-treat cavity wall insulation, loft insulation and double glazing) upgrades of 632,000 homes at total and average costs of GBP 2.4 million and GBP 3133, respectively [
35]. However, this present study recommends that the NI stock can achieve an average SAP rating of C by 2050 due to fabric retrofitting only, at an average cost of GBP 3700.
The High-Level Scenario costs GBP 5.9 billion; the measures that are most expensive to implement between 2022 and 2050 are upgrading pre-2006 to triple glazing, solid floor insulation and cavity wall insulation (see
Table 6). The individual costs of applying all measures to all house types rise due to the increased costs of improving glazing specifications. Consequently, the average cost of retrofit across all house types rises to approximately GBP 4500, with bungalows and flats showing the highest and lowest average costs of GBP 7000 and GBP 2000, respectively.
The Passive Scenario costs GBP 10.7 billion, and the measures that are most expensive to implement between 2022 and 2050 are upgrading pre-2006 double glazing to triple glazing, solid floor insulation and external solid wall insulation (see
Table 7). Wall insulation is the most expensive retrofit measure to implement per house (with an average cost of approximately GBP 38,000). This high cost is due to the extremely airtight construction, minimal thermal bridging and significant thickness of the insulation required (the thickness of a passive wall would range between 300 and 500 mm) [
73]. The detached house and the terraced house are the most and least expensive house types to retrofit, respectively. The detached house has the highest number of windows and largest floor area among other house types [
74]. Therefore, the cost of installing triple glazing and loft installation to passive house standard in the detached house is impacted by its size, making it more expensive than other house types. The terraced house is the cheapest house type to retrofit to passive house standards of all five house types, as it has the lowest average wall area of all house types (see
Table 8). The average cost of retrofit across all house types rises to approximately GBP 11,000, with detached houses and flats showing the highest and lowest respective average costs of GBP 15,000 and GBP 6000.
Across all scenarios, the cheapest overall costs are attributed to improved airtightness and loft insulation installations, while the cheapest measures per house are loft insulation top-up and improved airtightness (see
Table 5,
Table 6 and
Table 7). Cheaper measures such as improved airtightness and loft insulation top-ups can be installed by homeowners themselves; the airtightness of a flat/apartment can be improved at costs as low as GBP 12 (see
Table 8). Additionally, external solid wall insulation remains the most expensive measure to implement per house.
3.3. Cost Impacts and Improvement in SAP Rating for Fuel-Poor Households
In the Central Scenario, the total cost of retrofitting all fuel-poor households by 2030 is GBP 930 million, which is 45% of the scenario’s overall cost (see
Table 5). In the High-Level Scenario, the cost of retrofitting all fuel-poor houses by 2030 rises to GBP 1.1 billion due to pre-2006 double glazing to triple glazing retrofitting (see
Table 6). This cost accounts for 18% of the total cost of the scenario. In the Central and High-Level Scenarios, cavity wall insulation, solid floor insulation and pre-2006 double glazing to post-2006 double glazing retrofitting maintain the highest gross retrofit costs among all measures applied to the fuel-poor housing stock.
In the Passive Scenario, the cost of retrofitting all fuel-poor houses rises to approximately GBP 1.8 billion due to the scenario’s ambitious retrofit standard (see
Table 7). This accounts for 17% of the total cost of the Passive Scenario. In this scenario, the overall costs of implementing pre-2006 double glazing to triple glazing, solid floor and external solid wall insulation retrofit in the fuel-poor housing stock are the highest among all of the costs of the passive retrofit measures. In the High-Level and Passive Scenarios, the percentage costs of retrofitting all fuel-poor houses based on the total scenario cost decreases due to the higher numbers of non-fuel-poor houses retrofitted by 2050. The average retrofit costs per house and measure for the general housing stock remain the same for the fuel-poor housing stock.
The average SAP rating for the NI fuel housing stock can improve from band D to band C (an average SAP rating of 72) by 2030. To achieve this, at least 12,300 fuel-poor houses would need to receive at least one retrofit measure between 2022 and 2030. This number is slightly less than the average number of houses currently retrofitted annually in NI, indicating that NI currently has the potential to eradicate fuel poverty over the next 10 years.
3.4. Impacts of Scenario Outputs
3.4.1. Economic Impacts
The retrofit costs across all scenarios were analysed according to the costs of labour and materials used in the construction industry [
74]. In the Central and High-Level Scenarios, loft insulation costs may be broken down into 47% and 53% for materials and labour/transport, respectively, as one square metre of 100 mm insulation thickness costs GBP 1.
In the Passive Scenario, 93% of the loft insulation retrofit cost is claimed by labour. The labour costs for loft insulation are significantly higher than in the other two scenarios, due to the specialist level of skill required to install this measure to the passive house standard [
75]. For insulated doors, it is estimated that 81% of the retrofit costs are allocated to materials, as the addition of insulation increases the unit price of the door [
33].
Across all scenarios, 31% and 69% of the glazing retrofit costs are allocated to labour and materials, respectively, according to Building Cost Information Service (BCIS) index specifications. The airtightness improvement costs are only associated with materials [
74]. Approximately half of solid wall insulation costs are claimed by labour costs, while the remainder is allocated to materials (53%, 48% and 49% in the Central, High-Level and Passive Scenarios, respectively). For internal wall insulation, 70% of the cost per house is spent on labour; for external wall insulation, the BCIS index specifies that 38% is spent on labour [
76].
For cavity wall insulation, most of the retrofit cost is spent on labour (97% in the Central and High-Level scenarios, and 98% in the Passive Scenario), as insulation materials cost GBP 2/m
2. Similarly, most of the solid floor insulation costs are allocated to labour (97% in the Central and High-Level Scenarios, and 99% in the Passive Scenario). Labour costs for solid floor insulation are considerable due to the rigorous installation process. Typically, solid floors are either overlaid with expensive hi-tech insulation materials, or the concrete must be broken up and reinstated with new rigid insulation below the screed [
77].
Overall, in the Central Scenario, circa 70% and 30% is attributed to labour and materials, respectively. In the High-Level Scenario, the retrofit costs are split roughly halfway between labour (54%) and materials (46%). In the Passive Scenario, approximately 60% and 40% of the retrofit costs are assigned to labour and materials, respectively. A cumulative 5%, 3% and 2% saving due to economies of scale may be applied to the Central, High-Level and Passive Scenarios. Savings due to economies of scale are attributed to 5–10% savings from the labour and transport required for installing wall insulation in over 10 houses [
74].
Fabric retrofit solutions facilitate job creation and employment opportunities [
2]. The potential number of construction-related jobs that could be generated annually between 2022 and 2050 is 72,000, 170,000 and 348,000 from the Central, High-Level and Passive Scenarios, respectively. This general estimation is based on the mean weekly earnings and the number of weekly and annual working hours in the NI construction industry. In the NI construction sector, the mean weekly earnings and working hours are GBP 590 and 43 h (including overtime), respectively; the average number of hours worked in a year is estimated as 1367 [
78,
79,
80].
In NI, the rate of unemployment for people over 16 has increased and the proportion of people aged 16–64 who were not working and not looking for or available to work decreased in the last year [
81]. The Central, High-Level and Passive Scenarios can contribute total labour costs of GBP 1.4 billion, GBP 3.2 billion and GBP 6.6 billion, respectively, to NI’s local economy. Therefore, there is substantial potential for job creation through these massive retrofit programmes in NI, implying economic benefits worth exploring. Furthermore, the demand for retrofit materials can lead to further employment in the manufacturing sector. Materials may be sourced internationally or locally, but labour costs will be of economic benefit to the local NI context.
Table 5 shows that across all scenarios, the cost of implementing all retrofit measures to each house type in each scenario is generally cheaper than its corresponding new build cost. However, the cost of retrofitting a terraced house to passive house standards is more expensive than the cost of rebuilding it. In this case, it would be better to rebuild a highly energy-inefficient terraced house than retrofit it to passive house standards.
3.4.2. Socio-Environmental Impacts
Retrofitting will reduce the heat demand and energy bills per household, making heat more affordable for all income classes, especially those in fuel poverty. This study shows that if all measures are applied across all house types, the demand required to heat a bungalow, terraced house, semi-detached house, detached house and flat can be reduced by up to 11%, 11%, 9%, 8% and 8%, respectively, in the Central Scenario (see
Table 9). The more ambitious scenarios can yield greater reductions in demand per house type. The highest percentage of house types requiring retrofits is attributed to the terraced houses. The most ambitious Passive Scenario can result in up to a 48% reduction in the heat demand of a terraced house, if all passive measures are applied.
Fuel-poor people live in cold, damp homes during winter and this can be fatal [
82]. If more homes are well heated, the rate of deaths from extremely cold indoor conditions will reduce. With better levels of retrofit, domestic building envelopes would be better insulated, and thermal comfort levels would improve. Higher levels of thermal comfort and, hence, better quality of indoor air can reduce rates of cardiovascular disease, asthma and cancer [
83].
Retrofitting the building envelope eases the transition to a low-carbon heating system. Building envelope retrofitting may help initiate positive consumer attitudes towards adopting low-carbon technologies [
84]. Currently, NI consumers do not have adequate education and support to invest in energy efficiency measures [
28]. This study provides information on energy savings and the cost of retrofit, which consumers will find useful.
Some relatively affordable measures such as glazing and airtightness improvements, which have some appreciable impacts on heating demand, can be included in existing and future government retrofit schemes. This would encourage NI homeowners to invest in improving the energy efficiency of their dwellings. Improved domestic energy efficiency would lead to reduced carbon emissions, contributing towards climate change mitigation in NI. Although the 6CB advice specifies that 4% of NI’s residential emissions can be removed from 2018 levels by 2050, this study shows that higher amounts can be removed (6–27%) by improving the efficiency of the building fabric. Additionally, with well-insulated houses in NI, a lessened reliance on high-carbon heating fuels, especially oil, is feasible.
3.5. Conclusions and Recommendations
This study investigated the socioeconomic and environmental impacts of different levels of domestic fabric retrofit in the context of Northern Ireland. The study makes the following recommendations, which are applicable in other contexts and were illustrated by this Northern Irish case study.
Firstly, different archetypes will have techno-economic variations, which should be considered in the application of fabric retrofit when retrofit programmes are developed and executed. In the NI context, flats and terraced houses are the cheapest to retrofit to passive house standards among the different archetypes. However, it is cheaper to rebuild a highly energy-inefficient terraced house than to retrofit it to passive house standards. Terraced houses and flats/apartments have the highest and lowest numbers, respectively, requiring most retrofit measures such as wall insulation, loft insulation top-ups, solid floor insulation, glazing, and airtightness improvements. These two house types have the lowest costs of retrofit across all scenarios, and this can inform the creation of current and future energy efficiency grants.
Secondly, the installation costs of retrofit measures should be an important consideration for the creation of energy efficiency schemes, especially those targeted at fuel poverty. In NI, cavity wall insulation, solid floor insulation and upgrading pre-2006 double glazing to post-2006 double glazing/triple glazing are the costlier measures. Policy can encourage the creation of grants/schemes to offset some of the cost of these measures for owners of house types (bungalows, detached houses and semi-detached houses) with high retrofit costs. Current and future energy efficiency programmes for low-income households can make provision for solid floor insulation and glazing improvements. Similar considerations can be made for households in fuel poverty. The following categories of fuel-poor households show the highest numbers needing the assigned measure:
1965–1980 bungalows—external solid wall insulation.
Pre-1919 semi-detached houses—internal solid wall and cavity wall insulation.
1980 bungalows—loft insulation top-up.
1965–1980 terraced house—loft insulation installation.
Pre-1919-post 1980 terraced houses—solid floor insulation, glazing and airtightness improvements.
Approximately 85% of all fuel-poor houses recommended for retrofitting are off the gas grid.
Similarly, some measures that have greater environmental impacts than others can be easily implemented by homeowners due to cheap material and installation costs and ease of installation. Local policy can encourage the implementation of such measures first and at a higher rate than others. Airtightness improvements, cavity wall insulation, loft insulation top-up, solid floor insulation and glazing improvements yield greater potential for reductions in demand and emissions than other measures in the NI context. A total of 23,000 houses can be retrofitted annually from 2022 to 2050 with a range of measures with average costs of GBP 60 to GBP 8000.
Airtightness improvement and loft insulation top-up have the cheapest costs per house among these potent measures and can be easily implemented without professional help [
72,
74]. Loft insulation installation is required by an evident percentage of vacant houses, which can be retrofitted to provide high-quality living spaces. Policy should facilitate incentives encouraging homeowners to invest in airtightness upgrades and loft insulation top-ups. An airtightness test should be carried out on every house built pre-2006 before airtightness upgrades are carried out.
Finally, local authorities can create and accomplish high-reaching energy efficiency programmes and targets, while using national guidance as a springboard. The development of energy efficiency frameworks can inform building regulations for existing and new buildings. This would encourage the uptake of optimally energy-efficient domestic fabrics, which can improve quality of life and aid an easy transition to low-carbon heating. This study’s NI scenarios demonstrate that the heat demand and resulting emissions of the existing housing stock can be reduced further than UK government advice shows by 2050. However, if NI must achieve this, 23,000 houses will have to be retrofitted with at least one measure from now till 2050. Consequently, more efforts towards the creation of energy efficiency schemes in NI are needed. NI currently has the potential to abate fuel poverty between 2022 and 2030. The average SAP rating of NI’s fuel-poor housing stock can improve from band D to band C by 2030, if 12,300 fuel-poor houses are retrofitted with at least one measure between 2022 and 2030.
Around 74% of the houses recommended for retrofitting are off the gas grid, indicating that retrofitting is a critical first step in preparing the NI housing stock for the uptake of low-carbon heating systems. The improvement of the energy efficiency of domestic envelopes should be prioritised as the first step towards achieving net zero targets in NI. The scenarios’ cost estimations suggest a guide to the amount of public funds that can be assigned to energy efficiency programmes for all households. Finally, this study’s findings can inform revisions of NI building regulations towards local and UK-wide decarbonisation targets.