The ins and outs of the EnerPHit retrofit standard

Climate response

In the second #TDChallenge23 webinar, three professionals working at the forefront of retrofit in the UK provided a closer look at the EnerPHit standard.

EnerPhit is a passive house standard specifically intended for existing building projects. To learn more about what it is and how to do it, we heard from Sarah Lewis, Rachel Michell and Chris Morgan. With practical insights and detailed information, the three speakers discussed EnerPHit strategies using the challenge building – the Widemarsh Cricket Pavilion – as a case study.

Sarah Lewis, research and policy director at Passivhaus Trust, described the scope of the retrofit challenge the UK faces. With a large amount of some of the oldest and least efficient building stock in the world, and with the urgent need to decarbonize our building sector working practices, Lewis underlined the necessity for making deep energy improvements to our existing stock.

To achieve this, there are two retrofit standards: EnerPHit and AECB.

Although there are some variations in terms of fabric performance, limit on energy demand and emissions, comfort and outcomes, and closure of the energy performance gap, both have the same 5 principal targets:

  1. Continuous thermal envelope
  2. Solar gain
  3. Thermal bridge free
  4. Draught free
  5. Fresh air

How retrofit differs to new build standards

As it may not be possible to reach the same standards with an existing building as a new build, the EnerPHit standard takes these limitations into account and slightly relaxes the requirements. Lewis explained that EnerPHit offers a little bit of leeway on airtightness as well as on space heating compared to Passivhaus. The other criteria requirements are mostly the same.

EnerPHit is a worldwide standard, meaning there can be EnerPHit buildings anywhere, and it is quite flexible – with two means of compliance:

  1. The heat demand approach – based on the requirements for heating demand
  2. The component approach – based on the requirements for individual building components

Certification can be applied for and achieved through either approach. EnerPHit can also be done step-by-step – extended over years or decades.

Both methods use the Passive House Planning Package (PHPP) software – essentially a series of clever, interconnected spreadsheets that allow users to effectively model the overall energy demand, heat loss and overheating risks of their building. Lewis emphasized that PHPP is a very useful asset and in-demand skill for practitioners today – and for which all students participating in the challenge will get access and receive training.

“Adaptive reuse of buildings is clearly the future when we have so many buildings that need to be upgraded before 2045. For this reason, it’s really positive to see students so engaged with retrofit.” Sarah Lewis.

Component method

The ‘component’ aspect of the approach refers to the component parts that make up a building: the roof, windows, doors, floor etc. With this method, limits can be set for the thermal performance of building elements alongside the same targets for airtightness, ventilation and surface temperatures.  This means the comfort and health part of the standard is the same, but it is more flexible in terms of approach to individual elements.

The component method may be a preferable option, Lewis explained, if a building has been designed in such a way that will not meet the heat demand requirements. For example, if a building has poor orientation, the form is inefficient, or the existing glazing is not good for getting solar gain.

Enerphit slide

Retrofitting a small community-use building

The comparison of heat demand vs component methods of compliance was continued by Rachel Mitchell, a passivhaus designer from Greenbox Associates, who considered both specifically in relation to the Widemarsh Cricket Pavilion.

The 2023 university challenge asks interdisciplinary groups of students to propose a retrofit scheme for an old pavilion that sits on the edge of what was a cricket pitch. The pavilion is a timber frame, community-use building, and comes with certain constraints.

Important aspects identified by Mitchell were:

  • the building’s timber frame
  • the requirement to keep the front façade
  • its use as a non-domestic building
  • its small size – i.e., its ‘form factor’

A crucial concern for retrofit, ‘form factor’ refers to the relationship between external heat loss surfaces (roof, walls, floors) and the internal floor area. The more internal floor area and the smaller external surfaces the lower the form factor and the easier to meet the EnerPHit standard.

Ideally, Mitchell explained, form factor should be kept below 3 – if possible, closer to 2 or 1. She acknowledged that this is likely to be difficult for the pavilion and suggested using the opportunity for extension of the building to improve its form factor.

Enerphit slide

How could EnerPHit be achieved in this case?

Mitchell compared some of the steps that would likely be necessary for the pavilion to meet the EnerPHit standard through both methods. Outlining the variations in requirements, she demonstrated that both would need triple glazing, space for a MVHR unit, and the same levels of airtightness. However, as the component method gives more flexibility, allowing for higher U-values in some places to achieve overall targets, less insulation would be required in the floor and walls than for the heat demand method.

“Its really about the flexibility that you want. The space heating demand approach is definitely more challenging but its more energy efficient.” Rachel Mitchell


Timber-frame: the biggest challenge for retrofit

The final speaker, Chris Morgan, an experienced passive house designer and director at John Gilbert Architects, focused on the challenges of retrofitting timber frame buildings.

Considering the Widemarsh Pavilion, he laid out a series of questions he would ask of the overall building:

  • Form factor? Not ideal: if you have a good one it makes everything easier and cheaper.
  • Exposure? Doesn’t look too bad.
  • Function? A lot of hot water, meaning lots of heating needed and lots of vapour.
  • Moisture risk? Several rooms with high temporary moisture load.
  • Solar gain? Little help from the sun.


Enerphit slide


This was followed by a series of questions specific to the walls:

  • Enough insulation? No.
  • Thermal bridging? Yes, because the timber frame is not being insulated.
  • Air leakage*? Guaranteed. *Air going straight through walls.
  • Thermal bypass**? Yes. **Air going in but not necessarily out – causing mischief within the walls.
  • Vapour permeable? i.e., Is it safe for moisture? Probably – but only because it’s so draughty.
  • Robust? Yes, again only because it’s so draughty. It would last for a long time but with poor energy efficiency.
  • Insulating timber frames

Morgan advised that it is best not to use rigid insulation in a timber frame because you want something squishy (like rock wool) to fit closely around the timber frame. He detailed a variety of insulation approaches: using different depths and placements, inside and/or outside and with or without a vapour control layer, exploring the outcomes of each of these.

“We can save buildings and make them better than a new build. It’s simply not true that we need to knock down and start again. We are doing retrofits that are more energy efficient than new builds now: that’s satisfying because it’s a quarter of the price – and being able to tell clients that is a good thing.” Chris Morgan.

The practical and strategic advice of the three expert practitioners helped to deepen understanding of the possibilities and challenges of the competition brief, as well as providing a case study through which to understand the ins and outs of EnerPHit.


Watch a recording of this webinar on our youtube channel here

Next up, speakers Trish Andrews, Gervase Mangwana, Andy Simmonds do the same for the AECB standard


EnerPHit resources

enerphit resourcesPassivhaus Retrofit in the UK

Why Choose Passivhaus?

Summary for Policymakers

LETI Climate Emergency Retrofit Guide