EnerPHit Retrofits

Can you retrofit your home to be a truly Passive House? 

There is a misconception that Passive House builds are only possible with new builds where there was an intention to create an energy-efficient building envelope. Fortunately, through a focus detailed plan joined with quality building components, ventilation with heat recovery, superior insulation, thermal bridge exclusion and airtight construction, deep energy retrofits are a viable option in attaining the Passive House Standard.  

In 2016, the Passivhaus Trust campaigned for the UK government to support deep energy retrofit projects by recognising and implementing the EnerPHit Standard as an exemplary refurbishment target, helping to inform better practices and methodologies.  

Campaigners and experts alike have acknowledged that whole house retrofits are not always viable, and an alternative approach is required. The EnerPHit standard is the solution developed for the problem, allowing for a phased strategy, which includes intermediate certification, to be implemented. 

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What is the EnerPHit Standard? 

Depending on the level and components used within a retrofit application it is possible for projects to reduce their heating and energy demand up to 90%. Regrettably, this is not a feasible option for more dated buildings. The EnerPHit standard acknowledges the challenges and constraints posed by working within the parameters of an existing build which has lower airtightness and heating demands. 

According to the UK Passive House Organisation the goal of the “Quality-Approved Energy Retrofit with Passive House Components” was to ‘create a standard for an economically and ecologically optimal energy retrofit. For old buildings that cannot achieve Passive House Standard with reasonable effort’. 

As such EnerPHit is a more lenient building standard for retrofits, referred to when conservation issues and the existing architecture mean that meeting the Passive House standard is not practicable, with the largest obstacle tending to be inherent thermal bridges which cannot be resolved. Whilst this standard is not as demanding as the Passive House Classic, Plus or Premium Standard, it does require a maximum heating demand of 25 kWh (m2a), or the consistent use of passive components throughout a project.  

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What are the benefits of a Passive House Retrofit? 

It is possible to make substantial energy savings through the implementation of passive design updates throughout existing buildings. The focus of Passive Housing are as follows: 

  • The reduction of thermal bridging where possible; 
  • Ventilation with improved heat recovery efficiency; 
  • Greater airtightness of a build; 
  • Enhanced thermal insulation, with specific regard being given to the quality of materials and labour undertaken; 
  • Utilising Passive House Standard windows which promote better heat retention; 
  • A decrease in the risk of surface condensation and mould growth by increasing surface temperatures and controlling moisture. 

The resulting cost savings made from reducing energy and heating costs can be substantial.

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What are the Criteria for EnerPHit Certification? 

There are two standards to meet when looking at EnerPHit Certification. The first being based upon a performance criteria, and the second based upon individual components. 

According to the Passivhaus Trust the performance criteria is as follows:  

Criteria  New Build  Retrofit 
QSpecific Space heat demand   max. 15kWh/ (m²a)  max. 25kWh/ (m²a) 
Pressurisation test result n50  ≤ 0.6 air changes/ hr @ n50  ≤ 1.0 air changes/ hr @ n50 
QP Entire Specific Primary Energy Demand  max. 120kWh/ (m 2a)  max. 120kWh/ (m²a) +((QH –15kWh/(m²a)) *1.2) 
Frequency of overheating (over 25 degrees)  max. 10%  max. 10% 
Water activity of interior surfaces aw    max. 80% 


The criteria required for individual components is: 


Building Component  Retrofit Criteria 
External Wall  External insulation U ≤ 0.150W/(m²K)  

Internal insulation U ≤ 0.300W/(m²K) 

External Door  UD installed ≤ 0.80W/(m²K) 
Roof or top floor ceiling  U ≤ 0.120W/(m²K) 
Windows  UW installed ≤ 0.85W/(m²K)                     g -1,6W/(m²K) ≤ Ug 
Ventilation  [Symbol]HR,eff ≥ 75%  / ≤ 0.45Wh/m3 
Thermal Bridges  No linear thermal bridges with [Symbol] > + 0.01W/(m²K) or punctiform thermal bridges with [Symbol] > + 0.04W/(m²K) 


It is advisable to contact a certifier early into the planning process, as any potential issues in a design can be identified early on and corrected before causing more long-term problems. Before the process begins, an overall plan ought to be completed ahead of time, considering the current retrofit steps, as well as any potential steps which may be undertaken in the future. This is the only possible way to ensure that every component update will fit together allowing the owner to have a comfortable building with low energy demands. 

The EnerPHit Retrofit Plan, available through the Passive House Planning Package, provides a methodology for this type of overall plan. To gain additional quality assurance, the PHI also will offer a pre-certification as an EnerPHit project. To gain this certification it is necessary to have a fully comprehensive plan, where the initial retrofit step has already been implemented with minimum energy savings of 20%. The pre-certification provides assurance to the owners and planners that following the installation of all the retrofit components, the desired Standard will have been reached. 

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Is it worth getting an EnerPHit Retrofit? 

Although the EnerPHit Standard does not match that of a Passive House, each retrofit project will guarantee higher levels of energy efficiency and will increase longevity through every passive component installed. This allows for home and building owners the option of not breaking the bank when looking to improve a property’s energy efficiency. This partial retrofit will often focus on a particular area of the structure which can be optimised e.g. the number of thermal bridges or insulation present.  

Having even one aspect of a build improved upon can provide a significant return on investment through lower energy bills received.  

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What Barriers can obstruct the Retrofitting of a property to reaching the EnerPHit Standard? 

Whilst new builds have the ability to eliminate thermal bridging and other potential hindrances from the outset, a retrofit is working within the confines of an already established build. Due to this, there are a number of pre-existing conditions which affect the smooth installation of various EnerPHit Standard friendly components. These can include: 

  • Building Foundations: As numerous standard builds do not account for thermal bridging it is unlikely that a retrofit will be able to resolve this issue without significant expense and inconvenience to the owner. 
  • External Wall Insulation: Often times it will not be possible to fit an adequate amount of additional insulation within the limited interior room, or space, or a property line. 
  • Property Orientation: Many properties utilise solar power in order to optimise energy efficiency during the colder months, and with a South facing building it is possible to install a solar PV system in an effective area which is facing the Sun. If a building is not already facing this direction it will be more difficult to install a system which produces the equivalent energy. 
  • Limitation of airtightness: Without a pre-planned design which aims to ensure the building envelope has sufficient airtightness, it will be difficult to introduce measures which can make a sizeable change to the airtightness of a property.  

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What other renewable services and products are available to owners? 

Here at UK Alternative Energy we aim to help individuals to meet their energy requirements through the most sustainable methods possible, rather than using wasteful energy options with provide little beyond immediate short-term demands. For example, our high-quality air source heat pumps can provide the possibility to offset any potential heating costs which may arise, even within Passive Houses during the coldest seasons.  

This would eliminate annual energy costs whilst providing owners with peace of mind knowing they are self-sufficient in relation to their heating demands. Along with Solar PV and Battery Storage you can further reduce your energy consumption, helping you become even more self-sufficient. 

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