Knowledge sharing panel on AECB Retrofit


In partnership with AECB, BE-ST, Edinburgh Napier University, NMITE and Passivhaus Trust, TDUK has programmed a series of webinars on climate responsive retrofit to support the 2023 university challenge.

The challenge asks interdisciplinary teams to form to design, engineer and cost a retrofit scheme for an existing timber frame building that needs a second lease of life: the Widemarsh Pavilion in Hereford.

In the third of the webinar series, Andy Simmonds, CEO of the Association for Environment Conscious Building (AECB), and Gervase Mangwana, director at Waxwing Energy, shared their knowledge about the AECB retrofit standard.

The AECB retrofit standard promotes the delivery of net zero carbon retrofits by combining a whole-house fabric-first approach with ambitious energy efficiency measures. It is not just about going as low as you can go in terms of energy: the main aim is for a healthy building that will last another hundred years. The standard therefore also looks at retrofit risks and requires evidence to show mitigation and management.

Explaining the methodology behind the standard, Andy Simmonds introduced the two levels of AECB retrofit:

Level 1 – Heat pump retrofit

This is a lighter retrofit, designed to support a rapid transition to low carbon heating. There is no specific space heat demand target and, overall, less is done to the building – but a heat pump is added. The aim is to be a relatively simple and low capital cost way to get off fossil fuels and reduce carbon emissions. It suits some buildings very well and is unsuitable for others.

Level 2 – Deep fabric retrofit

This is a deeper retrofit. There is a space heat demand, and it accounts for thermal bridging and surface condensation levels. There are many non-energy benefits, such as taking people further from fuel poverty and better air quality.

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In order to meet the criterion, both of the levels require: a long-term energy plan using PHPP software; a flow temperature of lower than 45 degrees; MVHR or (centralised) MEV ventilation; an overheating target of <5% @ 25 degrees.

Whereas a Level 1 retrofit must change the building’s heating system to a heat pump, a Level 2 doesn’t necessarily require a heat pump system, but the design must include future-proofing for low carbon heat. The Level 2 retrofit has a space heat and cooling demand target of 50 kwh/m2 (or 100 kwh/m2 with an exemption), tighter requirements for airtightness (<2 m3/m2.h) and requires a comprehensive retrofit risk strategy covering moisture, radon, flooding, fire, occupancy.

For both levels, Simmonds emphasised, there should be a clear focus on reducing upfront carbon emissions from materials used in the retrofit. Different modelling – including PHPP, DesignPH, PHribbon and AECB Stock Model – can be used to break down the whole life carbon emitted in a proposed project by materials and by building components. This allows designers to compare, at an early stage, their choice of materials.

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Whole life embodied carbon emissions (operational and embodied)

Using an example of a typical semi-detached house, Simmonds demonstrated that the different levels of intervention have different overall impacts in terms of greenhouse gas emissions. He showed that a Level 1 retrofit is a meaningful intervention, dramatically reducing the rate of CO2 emissions through the use of a heat pump – bearing in mind the pump is likely to need replacing every 17 years in order to keep performance going.

“The heat pump is 300% efficient and takes heat from the environment, with a little electricity, so if you have 300% efficiency, you divide the building’s heat demand by three.” Andy Simmonds.

The deeper Level 2 retrofit requires further measures and replacements to achieve the improved U-values. It would see a slight overall increase in CO2 emissions, but Simmonds argues that it is worth investing a little bit of carbon to achieve the many non-energy co-benefits.

Going forward, Simmonds proposed, it is likely there will be several ways to retrofit, each done to different depths. These different levels are likely to be more and less attractive and/or incentivised at different points in time.

Simmonds laid out AECB’s commitments to adaptation in anticipation of temperature increases; to dramatically cut emissions; to change how we think about material resources; and to create a culture of social and ecological respect within design-construction. Retrofitting is a core part of these commitments, as is building (when necessary) efficiently, adaptably and durably, using environmentally benign materials and working to high building standards.

“Building standards will challenge your creativity, but they won’t put it in a box.” Andy Simmonds


Key concepts for retrofit assessment

Using the Widemarsh Pavilion as a case study, Gervase Mangwana discussed some of the key considerations for retrofit.

Form factor (the ratio of: the external walls, ceiling and ground floor in m2; to the internal floor area in m2), he explained, is important for all buildings. Generally, a ratio less than three is considered best practice. For buildings, like the pavilion – that are single storey, with a high ceiling, lots of surface area and little floor area – it is harder to meet space heat demands. The pavilion has a form factor greater than 4. Resource efficiency is therefore important – with the aim of getting as much useable floor space as possible from resources used. For teams considering adding an extension to the Widemarsh Pavilion, Gervase recommended trying to increase the floor area as much as possible whilst minimizing increase in overall surface area.

He went on to attend to airtightness – and the importance of designing this into the envelope plans: considering the detailing of junctions, joints, penetrations and openings – especially for example, when electrical boxes are cut into the airtight layer. This, he explained, is much more challenging in retrofit than new build. Importantly it requires the engagement of all trades particularly the building services who are most likely to penetrate the airtight layer not knowing the importance of this.

Similarly, Gervase flagged the importance in detailing low thermal bridge junctions, warning that, as thermal performance is improved across the overall elements, thermal bridges can easily become the difference between achieving and not achieving a standard.

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In the next and final webinar, we will learn about detailing for timber frame buildings and look more closely at the Widemarsh Pavilion: thinking about the fabric of the building, its airtightness, and the potential use of the landscaping and planting to increase biodiversity and alleviate overheating.

Watch a recording of the AECB webinar on our youtube channel here