Passivhaus Retrofit (EnerPHit): Upgrading Existing UK Homes to Near-Passivhaus Standard

The UK has approximately 29 million homes, and fewer than 40,000 of them were built to Passivhaus standard. The EnerPHit certification — Passivhaus’s retrofit-specific standard — exists precisely because most of the housing stock will never be demolished and rebuilt. It provides a realistic route to near-Passivhaus performance in buildings that were never designed for it.

What EnerPHit is

EnerPHit is the Passivhaus Institute’s certification scheme for the energy retrofit of existing buildings. It recognises that retrofitting an existing building to the full Passivhaus standard is often impractical — you cannot rotate a Victorian terrace to face south, and you cannot add 300mm of external wall insulation to a property that sits on the pavement line.

EnerPHit relaxes certain Passivhaus criteria while maintaining the core principles:

Criterion	Passivhaus (new build)	EnerPHit (retrofit)
Space heating demand	≤ 15 kWh/m²/year	≤ 25 kWh/m²/year
Primary energy (renewable)	≤ 60 kWh/m²/year	≤ 60 kWh/m²/year
Airtightness	≤ 0.6 ACH @ 50 Pa	≤ 1.0 ACH @ 50 Pa
Window U-value (typical)	≤ 0.80 W/m²K	≤ 1.0 W/m²K

The heating demand is relaxed from 15 to 25 kWh/m²/year, and the airtightness from 0.6 to 1.0 ACH. These are meaningful concessions — 25 kWh/m² still represents an 80-90% reduction in heating demand compared to an average UK pre-1919 property.

Why EnerPHit matters for UK windows

Windows are the single largest source of heat loss in a typical UK retrofit. In a Victorian terrace with solid walls, the windows can account for 30-40% of the total fabric heat loss, even after wall insulation has been installed. Achieving the EnerPHit window U-value of 1.0 W/m²K (or better) is a non-negotiable part of the certification.

Glazing specifications for EnerPHit

The standard demands that windows achieve a whole-window U-value of no more than 1.0 W/m²K. In practice, this means:

Configuration	Whole-window Uw	EnerPHit compliant?
4-16-4 argon, low-E, warm-edge, uPVC	1.2-1.4	No
4-20-4 argon, low-E, warm-edge, uPVC	1.1-1.3	Borderline
4-16-4-16-4 triple, argon, low-E, warm-edge, uPVC	0.8-1.0	Yes
4-16-4-16-4 triple, argon, low-E, warm-edge, timber	0.7-0.9	Yes
4-18-4-18-4 triple, krypton, low-E, warm-edge, timber	0.6-0.8	Yes

Double glazing will not achieve 1.0 W/m²K in most frame materials. You need triple glazing with a warm-edge spacer and a well-insulated frame. uPVC triple-glazed systems are the most cost-effective route; timber frames make compliance straightforward but at a higher cost.

See our Passivhaus windows spec guide for the new-build Passivhaus glazing requirements — which are even tighter at 0.80 W/m²K.

The airtightness challenge

The most difficult aspect of EnerPHit for UK homes is the airtightness target of 1.0 ACH @ 50 Pa. A typical pre-1919 solid-wall house achieves 10-15 ACH — ten to fifteen times the EnerPHit limit. Even a 1990s house with double glazing typically achieves 7-10 ACH.

What airtightness means for windows

Every window opening in a wall is a potential air leakage path. The leak is not through the glass — it is at the junction between the window frame and the wall, and at the joints between the frame and the opening sash. To reach 1.0 ACH, every window in the property must be:

1. Taped to the wall using specialist airtightness tape (not just silicone — silicone is not airtight under pressure testing)
2. Installed with compriband (pre-compressed sealing tape) between the frame and the masonry
3. Tested individually before the whole-house air test

The cost of an airtightness-compliant window installation is approximately 15-25% more than a standard installation, because of the additional materials (tape, compriband, vapour barriers) and the time required for careful detailing.

The airtightness line

In an EnerPHit retrofit, a continuous “airtightness line” is drawn on the plans. This line must not be broken. Every penetration — windows, doors, services, flues — must be sealed to this line. Windows are the largest penetrations, and the taping detail at the window-to-wall junction is the most common failure point.

The EnerPHit process: what a retrofit involves

An EnerPHit retrofit is not a “swap the windows and add some insulation” project. It is a structured, phased programme that typically takes 12-24 months for a full house.

Phase 1: Assessment and modelling

A Passivhaus consultant creates a PHPP (Passivhaus Planning Package) model of the existing building. This is a detailed thermal model that calculates heating demand, primary energy, and airtightness. The model identifies which measures are needed and in what order.

Phase 2: Fabric first

The external envelope is upgraded:

• Wall insulation: External wall insulation (EWI) or internal wall insulation (IWI), typically 100-200mm
• Roof insulation: 300-400mm of mineral wool or rigid board between and above the rafters
• Floor insulation: 100-150mm of rigid insulation board above or below the slab
• Windows: Triple-glazed windows with Uw ≤ 1.0 W/m²K, installed to airtightness standard

Phase 3: Building services

• MVHR (Mechanical Ventilation with Heat Recovery): Essential in an airtight home — provides fresh air while recovering 85-95% of the heat from exhaust air
• Heating system: Often a heat pump, sized to the much-reduced heating demand (typically 2-4 kW — the size of a kettle)
• Hot water: Heat pump or solar thermal

Phase 4: Testing and certification

The whole-house air test is performed. If it passes (≤ 1.0 ACH), the project can be submitted for EnerPHit certification by the Passivhaus Institute.

Cost of an EnerPHit retrofit

EnerPHit retrofits are not cheap. The cost of a full retrofit on a typical 3-bed semi-detached house:

Measure	Approx. cost	Notes
External wall insulation	£8,000-£15,000	100-150mm EWI, rendered finish
Roof insulation	£3,000-£6,000	Loft or rafter level
Floor insulation	£4,000-£8,000	Depends on floor construction
Windows (triple, airtight installation)	£12,000-£20,000	8-10 windows, timber or uPVC triple
MVHR	£4,000-£7,000	Ducted system with heat exchanger
Heat pump	£6,000-£12,000	Air-source, 3-5 kW
Airtightness detailing	£2,000-£5,000	Taping, compriband, vapour barriers
Design and certification	£3,000-£6,000	PHPP modelling, consultant fees
Total	£42,000-£74,000

The window component is typically 25-30% of the total cost. Using triple-glazed uPVC windows rather than timber can reduce the window cost by £4,000-£8,000. See our cost of new build windows UK guide for the per-unit pricing of triple-glazed specifications.

Is EnerPHit worth it?

Energy savings

A typical pre-1919 3-bed semi in the UK spends approximately £1,500-£2,500 per year on gas heating. An EnerPHit retrofit reduces this to approximately £150-£300 per year — a saving of £1,200-£2,200. Over 25 years, that is £30,000-£55,000.

The capital cost (£42,000-£74,000) does not pay back within 25 years on energy savings alone. But energy prices are not static. If gas prices continue the trend of the last decade, the payback period shortens significantly.

Property value

EnerPHit-certified homes are rare. Estate agents report a 5-10% premium on properties with clear energy-performance credentials (EPC A, heat pump, MVHR). On a £300,000 property, that is £15,000-£30,000.

Comfort

The comfort argument is the one that converts most homeowners. An EnerPHit home has no cold spots, no draughts, and no condensation. The internal temperature is stable at 20-21°C year-round without the boiler cycling on and off. This is a qualitative improvement that energy-bill calculations do not capture.

The heritage problem

EnerPHit is hardest to apply to the homes that need it most: pre-1919 solid-wall properties in conservation areas. The same regulations that protect the character of the area — Article 4 Directions, listed building consent — prevent the external wall insulation and triple-glazed window replacements that EnerPHit requires.

In these cases, EnerPHit may be unachievable. The Passivhaus Institute recognises this in its “EnerPHit with exceptions” route, which allows higher U-values where heritage constraints prevent the standard specification. The alternative is a “fabric first” approach using internal wall insulation, secondary glazing, and draught-proofing — which can achieve significant improvements without reaching EnerPHit certification.

See our conservation area windows guide and secondary glazing for listed buildings guide for the heritage-compliant alternatives.

The AECB Silver Standard

For homeowners who find EnerPHit too demanding or too expensive, the AECB (Association for Environment Conscious Building) Silver Standard provides a middle ground:

Criterion	EnerPHit	AECB Silver	Building Regs Part L
Space heating demand	≤ 25 kWh/m²/yr	≤ 40 kWh/m²/yr	No specific target
Window U-value	≤ 1.0 W/m²K	≤ 1.2 W/m²K	≤ 1.6 W/m²K
Airtightness	≤ 1.0 ACH	≤ 3.0 ACH	≤ 5.0 ACH (new build)

AECB Silver is achievable with double glazing and standard construction methods, at approximately half the cost of EnerPHit. It is a practical target for homeowners who want a deep retrofit without the certification overhead.

Summary

• EnerPHit is the Passivhaus standard for retrofit — it relaxes new-build Passivhaus criteria to account for existing building constraints
• Window U-value must be ≤ 1.0 W/m²K — this effectively requires triple glazing
• Airtightness must be ≤ 1.0 ACH @ 50 Pa — this is the hardest requirement for pre-1919 homes
• A full EnerPHit retrofit costs £42,000-£74,000 for a 3-bed semi, with windows at 25-30% of the cost
• Heritage constraints in conservation areas and listed buildings may make EnerPHit unachievable
• AECB Silver provides a practical half-way standard at roughly half the cost