4.8 Introduction to 3 CLR Modelled House Types

1. Background to the CarbonLite Retrofit energy targets

This lesson starts by considering the kind of space heating targets that might be appropriate for different retrofits.

Modelling of 3 common UK house types has been carried out by the CLR team, and the results are explained in Lessons 4.8, 4.9 and 4.10.

Before we look at the results from the 3 modelled house types (before and after retrofit) in detail, here are some key points arising from this work:

The space heat demand reductions that CLR outlines are currently only recommended space heating targets, but the AECB is currently working on developing these into a moisture-robust standard for domestic retrofit projects along with plans to incorporate the proposed standard into a self-certification system.

DOWNLOAD A SUMMARY OF THE CLR SPACE HEAT DEMAND TARGETS BELOW:

CLR SSHD-headlines V4d (updated 15.12.20 and now includes EPC bands)

Note: CLR modelled space heat demand for the ‘unimproved’ dwellings used as a baseline for each form factor category has been cross checked with other national modelling results as well as official government measured consumption data. The data has been sense checked against the authors’ own measurements and those of other AECB experts. This provides consistency and a degree of confidence that the category baselines are realistic and meaningful.

Other information:

• In Module 8, the financial performance of a fairly large number of whole house retrofit scenarios (for the same 3 house types) is considered.
• CLR accepts that retrofitters will be balancing the cost against performance on each individual project – the energy targets set are called ‘CO₂ prioritised’ in the course material. A second set of less ambitious energy targets – notionally called ‘Cost Prioritised’ are used to illustrate solutions for each house type, at around the minimum 50% reduction mark.

2. The 3 modelled house types and their key features

In the table below, some key dimensions of each of the 3 modelled house types are listed.

3. Tools used in the Scenario Modelling

AECB used PHPP to carry out its scenario modelling. PHPP is a proven and reliable modelling tool, because it allows the user to specify factors ignored or set as default in simpler models. Some of these factors (e.g. the exact performance of windows) can take a major toll on energy performance, leading to a big performance gap.

For those interested to explore these differences in more detail this is a useful starting point: ‘Projecting Energy Use and CO₂ Emissions from Low Energy Buildings’ http://www.aecb.net/publications/projecting-energy-use-and-co2-emissions-from-low-energy-buildings/

By using PHPP analysis, THERM (thermal bridge calculation software), and by allowing for a realistic level of ‘comfort take’ on the part of the occupants after retrofit, the modelling goes a great deal further than the familiar SAP and RdSAP approximations for retrofit, and as such we believe gives a much more realistic estimate of the kind of real life savings that could be anticipated, providing more confidence to retrofitters and customers alike.

The energy modelling is based on a number of assumptions, including:

• The average whole house temperature during the heating season (pre-retrofit) is 17°C .
• The average whole house temperature during the heating season (post-retrofit) is 20°C (i.e. the occupants are enjoying increased comfort at the expense of some energy savings).

What if I use SAP rather than PHPP?

• The space heat demand figures have been produced using both PHPP and (full) SAP in order to give users of both software packages a familiarity with the discrepancies arising between the two (see section 4 below).
• A report by Alan Pither on this SAP – PHPP comparison exercise is available here: S1 Final_Methodology used for the SAP assessment _UploadReady
• Tthe AECB maintains a background level of error checking and further development of this work: as and when required any significant results will be fed back into the CLR course material.

4. The range of energy targets proposed (for the 3 house types)

While the features of the 3 house types modelled are covered in more detail in the next 2 lessons, the headline results for specific space heating demand (SSHD) before and after retrofit are summarised house by house and in the table below.

Remember, all results assume a pre-retrofit house temperature of 17°C and a post-retrofit temperature of 20°C.

4.1 The bungalow

• The original house before retrofit has a PHPP modelled SSHD of 230 kWh/m².a
• So the minimum or “backstop” energy target would be 50% of this: 115 kWh/m².a
• The model calculated “CO₂ prioritised” retrofit using IWI as 75 kWh/m².a (a 70% reduction)
• The model calculated “CO₂ prioritised” retrofit using EWI as 51 kWh/m².a (a 78% reduction)

For comparison, using SAP instead:

• The original house before retrofit has a SAP modelled SSHD of 162 kWh/m².a
• A minimum or “backstop” energy target would therefore be 50% of this: 81 kWh/m².a
• The model calculated “CO₂ prioritised” retrofit using IWI to give a 75% reduction
• The model calculated “CO₂ prioritised” retrofit using EWI to give an 82% reduction

4.2 The town house

• The original house before retrofit has a PHPP modelled SSHD of 117 kWh/m².a
• So the minimum or “backstop” energy target would be 59 kWh/m².a (a 50% reduction)
• The model calculated “CO₂ prioritised” retrofit using IWI as 40 kWh/m².a (a 70% reduction)
• The model calculated “CO₂ prioritised” retrofit using EWI as 18 kWh/m².a (an 86% reduction)

For comparison, using SAP instead:

• The original house before retrofit has a SAP modelled SSHD of 112 kWh/m².a
• A minimum or “backstop” energy target would therefore be 56 kWh/m².a (a 50% reduction)
• The model calculated “CO₂ prioritised” retrofit using IWI to give a 65% reduction
• The model calculated “CO₂ prioritised” retrofit using EWI to give an 80% reduction

4.3 The semi-detached house

• The original house before retrofit has a PHPP modelled SSHD of 179 kWh/m².a
• So the minimum or “backstop” energy target would be 90 kWh/m².a (a 50% reduction)
• The model calculated “CO₂ prioritised” retrofit using IWI as 62 kWh/m².a (a 71% reduction)
• The model calculated “CO₂ prioritised” retrofit using EWI as 26 kWh/m².a (an 86% reduction)

For comparison, using SAP instead:

• The original house before retrofit has a SAP modelled SSHD of 140 kWh/m².a
• A minimum or “backstop” energy target would therefore be 70 kWh/m².a (a 50% reduction)
• The model calculated “CO₂ prioritised” retrofit using IWI to give a 70% reduction
• The model calculated “CO₂ prioritised” retrofit using EWI to give an 85% reduction

4.4 Comparison table

The key figures listed above are brought together in the table below so that the 3 house types can be compared pre-retrofit, and with IWI or EWI retrofits. Some SAP figures are given to compare with the PHPP figures. Please refer to the latest (2020) key figures, these now include EPC bands.

• The numbers in red (above and in the table below) are the CLR calculated typical specific space heat demand figures for the three typical house types in the un-retrofitted state i.e. the benchmarks for each category.
• The black figures under the red arrows figures are exactly half these, i.e. they represent the minimum recommended CLR post-retrofit specific space heat demand reduction of 50%.
• The numbers in the green boxes are the targets, notionally termed ‘CO₂ prioritised’ specific space heat demand figures that CLR energy and cost-benefit calculations have suggested as a financially attractive way for the UK to meet climate change obligations, reduce fuel poverty and NHS costs, whilst building owners and the State benefit from a range of valuable co-benefits (more on this in Section 8).
• The grey numbers (above and in the table below) are taken from SAP.

The challenge for retrofitters is:

• to use robust investment appraisal methods that aim for suitable targets, and
• to choose the most effective strategies possible within the limitations presented in each project

For projects that are not identical with one of the three representative house types but fall between these in form factors, a chart of SSHD targets by form factor has been drawn up to refer to (see Section 4). To check the design heat demand of their retrofitted project, retrofitters can use PHPP or SAP, together with a set of the thermal bridge calculations that the AECB is making available through this programme.

This information may be used to help guide the development of a whole house retrofit plan, though it is important to remember that they are no more than ‘example scenarios’ and it is up to individuals to assess appropriate use for informing specific projects’ retrofit measures.

We think the recommendations represent realistic levels of ambition that remain challenging and ambitious, yet recognise the very real challenges of retrofitting buildings of different shapes and construction types. Following any future feedback these figures may be refined or altered.

How does CarbonLite fit with other energy standards?

The CarbonLite Programme takes a holistic approach that embraces energy saving, health and comfort standards, and moisture safety. It is also compatible with a number of other energy standards, some of which may aim for especially ambitious levels of energy saving. A range of available standards is shown in the table below.

Combining the figures from the CLR table above and the Energy Standards table directly above, we can see that there is some overlap:

• For the town house with deep IWI retrofit (40 kWh/m².a), an AECB Silver target (of 40 kWh/m².a) is achievable, based on the modelled assumptions used.
• For the bungalow with deep EWI retrofit (51 kWh/m².a), the AECB Silver target may also be achievable or nearly so.
• For the semi with deep EWI retrofit (26 kWh/m².a), the EnerPHit target (of 25 kWh/m².a) is achievable.
• For the town house with deep EWI retrofit (18 kWh/m².a), the Passivhaus target (of 15 kWh/m².a) may be achievable.