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Understanding the Requirements and Benefits of Passive House EnerPHit and AECB CarbonLite Standards for Energy-Efficient Retrofits

Considering that the UK and Irish governments are committed to achieving Net Zero by 2050, energy-efficient retrofitting of existing buildings is an absolute must!

Retrofitting existing buildings is undoubtedly a huge challenge in supporting the reduction of carbon emissions but is a valuable approach to enhance the value and longevity of existing structures, whilst delivering a whole host of benefits for building occupants and owners alike.

With its multifaceted process of upgrading and improving functionality, efficiency, and sustainability, retrofitting transforms older structures into exceptionally energy-efficient, cost-effective, and sustainable spaces. It requires a holistic and forward-thinking approach which optimises resources, reduces carbon footprint, and improves occupant comfort. Furthermore, retrofitting projects can often have a wider impact on the environment and community, through job creation, economic growth, and community redevelopment.

In this article we explore two well designed standards applicable to energy efficient retrofitting –Passive House EnerPHit Standard and AECB (Association for Environment Conscious Building) CarbonLite Standard – focusing on their requirements, benefits and application.

There is a lot of common ground when looking at EnerPHit and the CarbonLite Standard. Both are for retrofit and founded on good building physics ensuring good indoor air quality including summer comfort. Each standard requires the adoption of the PHPP (Passive House Planning Package) and a detailed Quality Assurance process with photos and drawings supplied to show design intent. The emphasis for EnerPHit is low energy and comfort, whereas the focus for AECB CarbonLite is low carbon and healthy environment. However, the intent of both standards is to improve the energy efficiency of homes and buildings, ensuring comfort and well-being for occupants, whilst supporting the move to net zero buildings.

It is vitally important to have early up-front discussions around any retrofit project. It may be difficult to navigate which standard to follow, but this decision will likely be aligned to budget, ambitions for the project and company/individual values. Also, during these initial discussions, it may be that one of the standards appear more practical to achieve for a particular project. Both the Passive House Trust and AECB can offer technical support and detailed documentation to support you in achieving a successful retrofit project.

Passive House EnerPHit Standard

The EnerPHit Standard is derived from the Passive House Standard and can be described as “certified energy retrofits with Passive House Components”.  Passive House building physics principles apply to any building; however, it is unlikely the same results will be achieved from an existing building where you are not in control of the building’s orientation, structure, shape or amount of glazing.  The EnerPHit Standard has therefore been produced which takes account of these limitations and also relaxes some criteria to reflect this.  However, it is still a very demanding and rigorous standard and will typically result in a building that outperforms a new-build property both in terms of energy, thermal comfort, durability and cost effectiveness.

There are two ways to achieve the EnerPHit standard. The first is the Energy Demand Method, which is similar to that used to measure Passive House Standards but with one exception that the energy demand requirements are graded by climate zones. For example, a cool-temperate climate zone requires a max heating demand of 25kWh/m2/year, while a warm-temperate climate requires a max heating demand of 20kWh/m2/year. For the purposes of this article, we are focussing only on the cool-temperate climate zone which is relevant to the UK and Ireland.

The second way to achieve the EnerPHit standard is the Building Component Method. This is where building components are rated and must achieve certain criteria in order to comply. Components taken into account include interior insulation, exterior insulation, windows, ventilation and air tightness.

It is important to follow the advice laid out by the Passive House Trust to determine whether your project is suitable for retrofitting.

  1. Assess and evaluate the existing condition of the building fabric, including calculating U-values, initial surveys and air-tests to determine the building’s airtightness.       
  2. Set out all feasible upgrade measures for your building and what order they need to be executed. This prevents carbon lock-in which avoids choosing energy saving measures that may prohibit future improvements. The Passive House Trust recommends using PHPP.
  3. Consider a phased approach to spread costs and potential disruption.
  4. Aim to reach EnerPHit standards but be realistic that this may not be feasible for your project.
  5. If you are advised that EnerPHit is not achievable, adopt the recommended EnerPHit component values to achieve a very efficient and comfortable building.

Renovated existing buildings are certified according to the EnerPHit Standard and where necessary, new extensions can also be included, however, an EnerPHit certificate cannot be issued for entirely newly constructed buildings.

Comparison of Performance Targets for a European Climate for Passivhaus and EnerPHit

The table compares the targets for both Passive House and EnerPHit, based on a European climate. The minimum requirements for acquiring EnerPHit certification are annual heating of 25kWh/m2 for each treated area; and building airtightness is less than 1.0 air changes per hour.

Performance CriteriaPassivhaus  EnerPHit
Airtightness n50≤ 0.6 ACH @ 50 Pa≤ 1 ACH @ 50 Pa  
Space Heating Demand (SHD)≤ 15 kWh/m².a  ≤ 25 kWh/m².a (variable)*
Peak Heating Load (alternative criterion)                                               ≤ 10 W/m² 
Primary Energy Renewable (PER)  ≤ 60 kWh/m².a≤ 60 kWh/m².a (variable)*
Space Cooling Demand≤ 15 kWh/m².a  ≤ 25 kWh/m2. yr
Surface Temperature≥ 17 °C  ≥ 17 °C
OverheatingMax 10% > 25°C  Max 10% > 25°C

*For EnerPHit, the heating demand target varies according to climate zone, of which the UK spans three, most falling into 25 kWh/m².a.

PER varies by building use, and, in retrofit, by allowance for larger heating and cooling demand compared to a new build.

Source:  Passive House Trust https://www.passivhaustrust.org.uk/

AECB CarbonLite Standard

The AECB Standards are key in the Association’s aim in the deployment of environmentally responsible building practices and the creation of sustainable low energy, low carbon buildings. The first AECB standards were launched in the early 2000’s.  This was followed by the launch of the Passive House Trust in 2010 with the aim to make energy efficiency more mainstream as highlighted in the German Standard.  There are several Standards, each giving building owners and occupants flexibility in helping the country to reach net zero targets, plus these are supplemented with further building guidance on Water, Airtightness and Daylighting.

Focussing on low carbon, there are two AECB retrofit standards that, according to their lifetime carbon modelling, should deliver dramatically lower lifetime carbon emissions for homes, compared to not retrofitting. The AECB suggest that over time there will be two waves of retrofit, with some step-by-step projects progressing to full retrofit, depending on future policy and financial developments.

The CarbonLite Retrofit Standard can be carried out step-by-step with a minimum first step called ‘Step1’.  This recognises the practical and financial challenges faced by ‘early adopter’ retrofitters in the absence of a working UK retrofit programme and offers a pragmatic and flexible approach to retrofitting existing buildings. Completing a Step 1 CarbonLite retrofit does not lock out later works to take the building to a cost-effective full CarbonLite Retrofit. The Step 1 standard has been designed for a lighter fabric retrofit, effective ventilation and installation of an electric heat pump.

The full Retrofit standard allows retention of the existing heating system if required and is designed for a deeper fabric retrofit.

The energy and non-energy benefits of a deeper retrofit may justify adopting the full CarbonLite Retrofit standard for some projects, whereas the faster and lower capital cost Step 1 may be more appropriate for other projects.

AECB have devised a risk assessment that should be carried out prior to commencing the project considering flood risk, radon risk, exposure zone, building in a protected site or area, material risk (pre retrofit), conservation area and listed status.

The CarbonLite Standard has a number of key principles, focusing on energy efficiency, air permeability to minimise energy loss, which is calculated differently to the EnerPHit standard. There is a minimum healthy air permeability requirement, again measured differently in m3/m2.hr and not ACH as per the EnerPHit Standard. The standard focuses on summer comfort overheating with best practise being <3% with a limit of 10%.  A heat pump is to be installed either initially if using Step1 or at a later date if part of a full retrofit program, with a flow temperature of <45°C.  Ventilation can be provided by either MEV or MVHR and is important to ensure good indoor air quality.

Details of each Standards’ requirements are outlined in the table below in detail.

HEADLINE CERTIFICATION CRITERIA – updated SEPT 2023
CriteriaCarbon Lite Retrofit Step-by-StepCarbonLite Retrofit
Delivered space heating and cooling (kWh/m2/a)Report result≤ 50 kWh/m2.a (≤ 100 kWh/m2.a with certifier-approved exemption)
EITHER Primary Energy (PE, varies) OR Renewable (PER) (kWh/m2/a/annum)Report resultReport result
Ensure ventilationContinuous MEV or MVHR must be installed: follow PAS2035 Annexe C or as required by PART F of Building Regulations
Air tightness (q50)≤5.0 m3/m2.h≤2.0 m3/m2.h
Thermal bridgesN/A if some additional & significant fabric measures are being replaced or installed, certifiers will advise whether full Retrofit Standard requirements are applicableAssumed to be less than 0.01 W/mK, or else accounted for in PHPP or for retrofits a default heat loss factor may be used
Surface condensation (fRsi) assessedfRsi to meet criteria in PHPP, or 0.75 (as Building Regulations/PAS2035), or local standards, whichever is more onerous.
Heating systemChange existing fossil fuel (or direct electric) heating system to heat pumpExisting heating systems may be retained, but a practical plan to allow for future low carbon heating supply must be in place
Thermal comfortPHPP modelled overheating risk, <10% acceptable (guidance: <5%, good practice or <3% best practice)
Running cost comparisonMust be same/lower running costs than base case** 
WHERE A HEAT PUMP IS INSTALLED

Certifiers must liaise with the building owner and the MCS heating system designer in order to ensure that:

Maximum flow temperatures for the designed and installed heating system (space heating only)No great than 50 C: Best practice – heating system is designed and installed for flow temp < 45°C
ENERGY MODEL USING PHPP, showing:
Pre-retrofit BaselineStep 1 achievedRetrofit scenario showing how Full Retrofit could be achieved (≤50 – 100 kWh/m2.a)Retrofit Standard achieved (≤50 kWh/m2.a or ≤100 kWh/m2.a with certifier approved exemption)

Source:  AECB website https://aecb.net/aecb-carbonlite-retrofit/

CarbonLite Retrofit marks a move from the AECB’s fabric first approach, now looking for effective carbon reducing measures which can be actioned more quickly. The reasoning behind this is that realistically the culture and institutions in the UK are not able to change quickly enough to address climate change at scale and at pace. Therefore, this standard has been produced to enable carbon reduction in way that is less prescriptive than say the EnerPHit Standard and offers more flexibility for building owners.

Conclusion

Both the EnerPHit and CarbonLite standards are vital in supporting the move to net zero carbon buildings, offering alternative routes for the energy efficient retrofitting of existing buildings for the construction industry, but essentially with the same long-term aim.

So, the questions is, how do you make the choice about which standard to adopt – EnerPHit or CarbonLite? The decision will be based on client aspiration, type of project and its practicalities and budget. If the budget is available then generally the EnerPHit standard would be followed as the best route to a net zero carbon building, due its more rigorous and stringent set of performance criteria.

In some cases, the project itself will make it impractical to follow the EnerPHit Standard and therefore the CarbonLite Standard can be used as an alternative offering more flexibility, yet still addresses the move to net zero carbon buildings.  

References:

  1. https://aecb.net/
  2. https://www.passivhaustrust.org.uk/
  3. Criteria for Buildings – Passive House – EnerPHit – PHI Low Energy Building
    Version 10c | January 2023 | valid with PHPP 10

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