EnerPHit Retrofit Certification: A Homeowner's Guide

Introduction

EnerPHit is the Passive House Institute's certification standard specifically designed for deep energy retrofits of existing buildings — as distinct from the Passivhaus standard which applies to new-build projects. The EnerPHit standard acknowledges that retrofitting an existing building to full Passivhaus standards is technically very difficult (particularly for solid-wall Victorian and Edwardian properties) and sets performance targets that are achievable in practice while still representing a dramatic improvement over the original building's energy performance. This guide explains the EnerPHit standard, how it applies to north London period property renovation, and the process for achieving certification.

EnerPHit Performance Criteria

EnerPHit can be achieved via two pathways, both certified by the Passive House Institute (PHI):

Pathway 1: Demand Method

The building must achieve a space heating demand of no more than 25 kWh/m²/year (significantly higher than the Passivhaus threshold of 15 kWh/m²/year, reflecting the constraints of retrofitting existing buildings). The primary energy demand threshold is 120 kWh/m²/year.

Pathway 2: Component Method

Each building component meets defined EnerPHit quality criteria independently — the U-values for walls, roof, floor and windows each achieve specified thresholds; the airtightness target is met; and MVHR is installed and functioning. This pathway is more flexible where meeting the overall demand target is prevented by unavoidable thermal bridges or geometric constraints.

Airtightness

EnerPHit requires an airtightness of ≤1.0 air change per hour at 50 Pa (n50) — slightly less demanding than the Passivhaus 0.6 ACH threshold, reflecting the difficulty of achieving extremely low air permeability in existing building fabric.

EnerPHit Component Criteria

The component-method U-value targets for EnerPHit in a central European climate (the PHI reference) are:

• Opaque external walls: ≤0.15 W/m²K (achievable with 150–200mm of high-performance insulation)
• Roof: ≤0.15 W/m²K (achievable with 300mm mineral wool)
• Floor slab: ≤0.15 W/m²K
• Windows: ≤0.85 W/m²K (triple glazing with thermally broken frames)
• External doors: ≤0.80 W/m²K

The EnerPHit Certification Process

The EnerPHit certification process follows these stages:

1. PHPP energy modelling: The Passive House Planning Package (PHPP) — the Passive House Institute's energy modelling tool — is used to model the retrofitted building's performance. A Certified Passive House Designer (CPHD) or Certified Passive House Tradesperson (CPT) typically prepares the PHPP model. This is the primary design tool for the retrofit design, informing specification decisions and predicting performance.
2. Design certification: The PHPP model and design drawings are submitted to the PHI for design certification. The PHI reviews and issues design certification if the criteria are met.
3. Construction monitoring: During construction, an EnerPHit-certified building certifier monitors critical details — insulation continuity, thermal bridge construction, window installation position, and airtightness layer integrity.
4. Airtightness test: An accredited blower door test is performed on completion to verify the achieved airtightness meets the ≤1.0 ACH target.
5. As-built certification: The as-built PHPP model (updated to reflect the actual constructed condition) and the airtightness test result are submitted to the PHI for as-built certification. On approval, the EnerPHit plaque is issued.

EnerPHit in Victorian Terrace Houses in London

Victorian solid-wall terrace houses present specific challenges for EnerPHit certification:

• Solid wall insulation: The solid brick wall (typically 220mm) must be insulated to achieve U≤0.15 W/m²K — requiring approximately 150mm of mineral wool or 100mm of rigid PIR insulation as internal wall insulation (IWI). In conservation areas where external wall insulation (EWI) is not acceptable, IWI is the only option — reducing floor area in each room by approximately 300mm × 2 = 600mm per room width.
• Thermal bridges at wall/floor junctions: The solid brick wall typically runs continuously under the floor slab — creating a thermal bridge at the perimeter that is very difficult to eliminate in a retrofit. The component method accommodates this by allowing individual components to meet their targets independently even if a few unavoidable thermal bridges remain.
• Original windows in conservation areas: Where original sash windows must be retained, they cannot achieve the U≤0.85 W/m²K window target. The window component may fail certification unless secondary glazing or listed building consent for window replacement is obtained.

Cost of EnerPHit Certification

These costs are in addition to the construction cost of the EnerPHit-specification retrofit works themselves — see our Passivhaus retrofit guide for construction cost ranges.

Conclusion

EnerPHit certification provides independent, third-party verification that a deep energy retrofit has been designed and built to a defined high standard — a credential that is increasingly valued in the north London premium residential market and that provides genuine assurance of the building's energy performance. For homeowners committing the significant investment required for a deep fabric-first retrofit, the additional cost of EnerPHit certification is modest relative to the total project budget. An architect who is a Certified Passive House Designer can provide both the energy modelling and the design guidance needed to achieve EnerPHit, integrating performance targets into every design decision from wall specification to window installation detail.