Squaring The Circle

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Introduction
Research and practice into the retrofit of historic buildings has largely focussed on the identification of building physics risks and the development of strategies to mitigate both these and the risks to character and significance, while maximising improved thermal efficiency of heritage assets.
The work of our studio has sought to reduce the energy demand of historic buildings in a way that is both respectful of character and prudent about the risk of moisture build-up and mould growth. Our research published and presented to the 2016 EECHB Conference [1], highlighted the significance of the use of vapour permeable insulations and the critical importance of adequate ventilation in addressing the moisture movement within solid wall constructions. The implementation of building work on the project presented in this paper followed 4 years of careful monitoring and modelling and the conditions within the fabric of the completed project are still being carefully monitored.
This paper responds to the concerns within the heritage sector, and those involved in practical research to date, that uncalibrated responses to a, perceived or political, imperative to retrofit existing, 2 historic buildings will lead to damage to the fabric and historic value. For example, an approach that seeks to simply maximise the thermal efficiency in order to deliver Enerphit or other published standards as a target per se, might lead to the unthinking use of too much insulation and of vapour-tight constructions. Rather than an integrated approach to retrofit, that views thermal and moisture movement together with the use of ventilation systems, there is a risk that adoption of an elemental approach to retrofit risks will cause damage to historic fabric.
Conversely, there are concerns that an overcautious approach to retrofit risks under-delivering on improved performance and the need to re-retrofit projects in the future when it is clear that this is necessary.
In order to provide some context for these, necessarily balanced, judgements about how much improvement is needed to existing buildings to make these sustainable, this paper explores the energy supply side of the equation and, by identifying estimated 'allowances' for the zero-carbon energy use which will be available to existing and new buildings by 2050, paper suggests appropriate standards of performance in use.
Setting energy allowances for completed projects can inform the establishment of design targets that can take account of the key factors, such as the performance gap and comfort taking, that affect the ability of delivered projects to perform as designed, and in order to address the concerns discussed above, the building physics research required to obviate the risks to fabric and historic value while delivering zero-carbon.
This paper aims to Square the Circle by establishing a top-down calibration, and reinforcement, of retrofit standards that have emerged -bottom-up -through project-based research and practice.

The capacity of the UK Grid Energy Supply
For last 3-years, the UK National Grid has published annual plans for the transition to zero-carbon electricity, and other power, supplies in 2050. The latest Future Energy Scenarios (FES) [2] report identifies a number of scenarios for the future of the UK energy supply and for the safe and reliable delivery of low carbon energy to consumers to meet net zero. For each of four scenarios the National Grid have identified energy budgets for residential buildings. The four scenarios are:

Steady Progression.
Representing the slowest credible route to decarbonisation, this scenario is based on minimum levels of behaviour change and adoption of energy efficiency measures. It yields the highest total end-user energy demand, envisaging continued high levels of natural gas usage for domestic heating and industry. This scenario fails to deliver zero-carbon by 2050

System Transformation.
This scenario is based on significant supply-side flexibility to offset a low level of behaviour change and energy efficiency improvement to buildings (predicting over 65% of homes using hydrogen for heating). It envisages a widespread use of hydrogen for home heating, with hydrogen produced in UK through methane reformation and with large requirement for natural gas with Carbon Capture and Storage (CCS).

Consumer Transformation.
In contrast to System Transformation, this scenario is based on high demand-side flexibility and willingness of consumers to change behaviours with significant implementation of energy efficiency measures and the adoption of electrified heating with heat pumps and thermal storage.

Leading the Way.
This scenario is based on elements of the other scenarios and represents the fastest credible route to decarbonisation. It envisages significant lifestyle change with a combination of hydrogen and electricity used in heating. 40% of homes would have heat pumps and thermal storage. Hydrogen being produced in UK with electrolysis from dedicated offshore wind.

Notes on metrics.
This paper has adopted the Passivhaus standard of kWh/m 2 .per annum (kWh/[m 2 a]) as a measurement of energy use and basis for establishing future building allowances. Although this is widely accepted as a simple, measurable figure that is open to very little misinterpretation or misuse, it does embody a basic inequity by effectively favouring larger dwellings (or those not heated or cooled for continuous -annual or diurnal -use) while penalising small, compact and more continuously occupied and serviced buildings. We have considered adoption of other metrics related to occupancy level, compactness factor, or a blend of all of these, to arrive at a measure that is more equitable (to smaller, well-designed or retrofitted projects) but, on the basis of the near-universal acceptance of the kWh/m 2 .a metric, have adopted this as a starting point for, what may be more nuanced and useful, future studies.

Notes on choice of building stock used in this study
This paper uses the UK housing stock as a model for analysis of supply-side study of retrofit energy allowances. The English Housing Survey (2019-20 version) [3] records that 29%, of the housing stock in the UK comprises pre-1919 houses of solid-wall construction. Of these 8.4m houses, more than 7m have heritage features, some have been designated as listed buildings and many lie within, and define the character of, Conservation Areas. By contrast, the particularities of age, use, and character of each of the 400,000 listed buildings in the UK make any modelling of energy use demand reduction too complex for the scope of this exercise.

Note on peak and average energy usage.
The following calculations are based on annual energy supply and demand levels. It is acknowledged that, within these annualised figures, there will be diurnal and seasonal variations in both supply and demand. There is insufficient space here for a more detailed study of these variations or of the effect of these variations on the need for increased peak power supply capacity or power balancing.

Derivation of an energy budget for existing buildings.
For the purposes of this exercise, only the Consumer Transformation and Leading the Way scenarios will be studied. The other two scenarios being discounted on the basis that: • Steady Progression does not deliver zero-carbon by 2050 • System Transformation is heavily dependent on the use of hydrogen to heat homes. The technologies for amenity-scale production of hydrogen and for CCS do not currently exist in a viable form and the cost, programme and disruption of upgrading or replacing the current gas infrastructure, at every level from national distribution to connections within each home, is thought to be unrealistic. It seems more probable that the supply of hydrogen for use in industry, for transport, and in limited use in electricity generation will be more economically deliverable. The application of this formula to the alternate FES scenarios produces the following energy allowances.

Comparison of Energy Allowances with existing metrics
The table below shows a comparison of the energy allowances calculated above, with the performance standards required by the Passivhaus Institute for Enerphit and PHI Low Energy Building Standard [4].

Availability of performance data
The principal constraint on any current assessment of the capacity of retrofit projects -of moderate ambition and householder-funded -to meet the energy allowances under the FES scenarios, is the lack of available performance data for completed projects.
This constraint has affected all efforts to quantify the benefits and rewards of retrofit to all existing, and particularly to heritage properties. In the work lying behind the Sixth Carbon Budget recently published by The Committee on Climate Change [7], the team at University College London (UCL) [8] recognize that their advice was based on very small sample sizes and widespread assumptions. UCL adopted a metric of (£/TCO2 saved) to compare the carbon saved by retrofit with that saved, for example, by a switch to hydrogen as a fuel for housing. The assumptions used for proposed fabric performance improvements were also adjusted to allow for broad assumptions about the Performance Gap (between 7-50% (28% assumed) and behavioral factors like Comfort Taking (<33%) or other Behavioural Factors (+10%). Due to the large factors of uncertainty and the metric used this data has not been convertible or useful for this paper.
Similarly, the current work on Decarbonising the Public Sector [9], that has been commissioned by the Department of Business, Energy and Industrial Strategy, has adopted a metric of (£/TCO2 saved over the 'lifetime' of each measure or intervention). The added complexity of assessing the lifetime of fabric and systems measures makes this dataset very difficult to convert to meaningful use in the current paper.
In the absence of published data, this study has relied on the post-occupancy monitoring undertaken by built-environment professionals. This has proved immensely difficult to obtain during the current pandemic. Of the 70 case-studies originally offered, only 6 have been forthcoming. Whilst this sample is statistically insignificant, it does suggest the outcomes that might be expected of a wider study and the capacity of normal levels of intervention, that are able to gain Listed Building or Conservation Area consent, to deliver improvements in performance that meet predicted energy allowances.

Summary of data available
The data summarised here is set out in more detail in Appendix A -with details of original condition of property and the retrofit measures involved (constructions and systems).

Discussion
A number of issues arise from the above findings The assumption that all new houses in the UK will be built to Passivhaus Standards relies on a degree of political will and acceptance by the UK construction industry and housing market. Failure to deliver new housing to this standard will not only reduce the energy available to existing housing but increase the number of houses (by then existing) that will need to be retrofitted to these tighter allowances. Table 3 demonstrates that the allowances derived the adjusted grid capacity for the two FES scenarios -60 and 64 kWh/[m 2 a] -lie between the standards required by Enerphit and PHI Low Energy Building Standard at c.60 and 75kWh/[m 2 a] respectively. Perhaps confirming the relevance of these standards.
It is worth noting that the performance data in Table 4 is limited in timescale -often for no more than one year's energy usage -so should carry a caveat with regard to the influence of variation in the length and severity of heating seasons.
The delivered performance of the completed projects falls within a range of between 59-90 with 3 of the 6 projects falling within the target allowances of between 60 and 64 kWh/[m2a]. The project that falls significantly outside this target performance being the retrofit of a late C19th house in which the existing gas boiler has been retained -until the end of its lifetime -at which point this will be replaced with an ASHP).

Conclusions
The outcomes above suggest that the range of retrofit measures commonly undertaken to existing, and specifically historic, houses are capable of delivering the reduced levels of energy demand that align with the anticipated grid capacity under the FES Scenarios considered while adopting a careful approach to building physics and moisture risk.
The levels of energy demand reduction recorded have all been delivered in projects that have incorporated ventilation systems with heat recovery as well as fabric improvements to insulation and airtightness within an integrated package of measures to deliver both energy and carbon saving targets and fabric and occupancy comfort. Any adoption of targets such as the energy allowances derived above, will necessitate that the retrofit of existing buildings be considered in this holistic and careful manner and on a case-by-case basis.
The close relationship of the performance of these completed projects to the extant Passivhaus metrics for retrofit suggests that these metrics are relevant and that it would be appropriate to adopt the Enerphit and PHI Low Energy Building Standards as a yardstick not only for all retrofit projects but as a tool for policymakers in assessing the alternative routes to zero-carbon, future Carbon Budgets, Future Energy Scenarios, and initiatives to Decarbonise the Public Sector.
The tiny size of this sample -even allowing for the constraints imposed by the pandemic -and the broader lack of available data on the performance of retrofit, suggests that extensive post-occupancy monitoring of energy usage in retrofit project -grant-aided if necessary -is required to sensibly inform future policy and funding initiatives.

European Energy Scenarios
This study has outlined the future energy context in which existing and historic buildings in the UK will be heated/cooled and occupied in 2050. This would not have been possible without the publication of the FES by the National Grid.