Retrofitting to Passive House Standards: EnerPHit Explained for Retrofit Professionals

Why Retrofit Matters More Than New Build

New build gets the attention, but the carbon problem sits in the homes that already exist. Existing buildings produce close to 40% of annual greenhouse gas emissions, and roughly two-thirds of today’s buildings will still be standing in 2050. Most of the UK’s net zero work will therefore come from retrofitting occupied homes: owner-occupied houses, private rented stock and social housing that hasn’t been touched in decades.

For Retrofit Coordinators and Domestic Energy Assessors this is the day job. It helps to understand the Passive House end of the market because it shows what “doing it properly” looks like, and many of its principles already appear in PAS 2035 thinking: fabric first, moisture control, ventilation and quality assurance.

Tip: Retrofit is mainly about cutting energy demand by improving the building fabric. Renewables such as solar PV are worthwhile, but they come after the fabric work, not instead of it.

Shallow Retrofit vs Deep Retrofit

The industry usually talks about two levels of intervention:

• Shallow retrofit: a few isolated measures, such as topping up loft insulation or fitting a more efficient boiler. It is quick and cheap, and the improvement is limited.
• Deep retrofit: a whole-house overhaul covering walls, floors, roof, windows, doors, airtightness, ventilation and often the heating system as well. It is disruptive and expensive, and the change in performance is large.

The hard part is working out how far to go and what each measure will actually deliver. That is why whole-house planning matters. A string of single measures installed without a plan can leave a home worse off, with a new thermal bridge or a sealed room that has no ventilation. Our guide to PAS 2035 and PAS 2030 explained covers how the six stages, the roles and the risk pathways stop that from happening.

EnerPHit vs Passivhaus: The Key Numbers

Full Passivhaus is almost always physically impossible to reach in a building that wasn’t designed for it. Architects are limited by the existing geometry, orientation and structure, and some thermal bridges can’t be removed completely. The Passive House Institute therefore publishes a separate refurbishment standard, EnerPHit, which relaxes the targets while keeping the same rigour.

Criterion	Passivhaus (new build)	EnerPHit (retrofit)
Space heating demand	15 kWh/m² per year	25 kWh/m² per year
Airtightness	0.6 air changes per hour	1.0 air change per hour
Design tool	PHPP	PHPP
Certification	Accredited Passive House certifier	Accredited Passive House certifier

Did you know? Passive House airtightness is expressed in air changes per hour at 50 Pa (n50). UK Building Regulations normally express air permeability in m³/(h·m²) at 50 Pa. These two figures measure different things, so don’t compare them directly. The difference is worth knowing for exam questions on airtightness testing.

Insulation: Position Matters as Much as Thickness

EnerPHit calls for a high level of insulation in the walls, below the ground floor and in the roof. Where that insulation goes needs careful thought, because a poorly positioned layer can cause problems elsewhere in the building.

The classic example is internal wall insulation. Fix insulation to the inside face of a wall and the room gets warmer, but the wall itself gets colder. If the wall is a porous material, such as some bricks or blockwork, you may cut off its ability to dry out, and damp and mould can follow. On a very exposed elevation, external insulation with a rendered finish is often the safer choice, although it brings its own detailing challenges at eaves, reveals and ground level.

That trade-off is at the heart of the moisture risk assessment in PAS 2035. We cover the external option in more depth in External Wall Insulation Cold Bridges: Why Moisture Management Decides Whether EWI Works.

Remember: Warmer rooms mean colder walls behind internal insulation. Always ask where the dew point now sits and whether the wall can still dry out.

Windows, Airtightness and MVHR

After the opaque fabric come the openings. A typical deep retrofit specifies:

1. High-performance triple glazing and external doors.
2. Careful insulation and taping around window and door frames, reveals and cavities, so the new units don’t become weak points.
3. A continuous airtightness layer, with membranes and tapes sealed around every penetration and junction.
4. MVHR (mechanical ventilation with heat recovery) to supply controlled fresh air once the envelope is sealed.
5. A low-carbon heating system, often an air or ground source heat pump, sized for the much lower demand.

The order matters. Once a leaky home becomes airtight, uncontrolled infiltration no longer does the ventilating, so a planned ventilation strategy is essential. This is exactly why PAS 2035 treats ventilation as part of the whole-house design rather than an optional extra.

PHPP, Certification and Quality Assurance

EnerPHit projects must be designed using the Passive House Planning Package (PHPP), a spreadsheet-based calculation tool for heating, cooling and primary energy demand. The finished building has to be certified by an accredited Passive House certifier. Nobody can self-declare.

The main benefit of using EnerPHit as the benchmark is that it forces quality assurance. Much of the airtightness work gets covered up once it’s finished, so it has to be right first time. That usually means an experienced design and build team, no hidden shortcuts, and testing to prove the airtightness target has been met.

Retrofit Coordinators will recognise the same principle in PAS 2035’s emphasis on design, installation evidence, handover and post-installation monitoring. EnerPHit and PAS 2035 are separate frameworks, but they share one goal: closing the performance gap between what was designed and what was built.

Costs and Outcomes

Costs vary a lot by building and region. As a rough rule of thumb, UK figures quoted for EnerPHit-standard deep retrofit are:

Item	Indicative cost
Overall deep retrofit budget	£800–£1,000 per m²
MVHR installation (average house)	About £10,000
Triple-glazed windows and doors	£400–£600 per m²

Most of the remaining cost is labour: installing insulation, membranes and the detailed taping around junctions and penetrations. Grants and incentives can make a big difference. In Ireland, for example, the Sustainable Energy Authority has subsidised up to 50% of homeowners’ retrofit costs.

The results can be dramatic. A deep retrofit can take a damp, leaky F or G-rated home up to an A rating, cutting energy demand by around 80% and carbon dioxide emissions by around 90%. Occupants report a steady temperature all year, clean fresh air from the ventilation system and very low heating bills. For social housing tenants living with persistent cold and damp, the health benefits can be significant too. Our guide to damp, mould and condensation explains the mechanisms behind those problems.

What This Means for DEAs

For Domestic Energy Assessors, keep the calculation methods separate. EPC ratings come from SAP or RdSAP, which rate a dwelling on modelled energy cost. PHPP is a different methodology that focuses on heating demand per square metre of treated floor area. A home can improve dramatically on both, but the numbers are not interchangeable.

What DEAs do need to record accurately on a deeply retrofitted home is every change: wall and floor insulation, glazing type, the ventilation system and the heating system. Get those entries wrong and the EPC won’t reflect the investment the homeowner has made. For a refresher on how a domestic assessment runs from start to finish, see The Ultimate Guide to an EPC.

How RetrofitReady Fits Into This

Deep retrofit ties together everything the retrofit exams test: fabric, moisture, airtightness, ventilation, heating systems and quality assurance. The RetrofitReady apps let you practise each of those areas before you meet them on site.

• RetrofitReady: Coordinator has 600 PAS 2035 questions for Level 5 Retrofit Coordinator preparation. They cover domestic retrofit, risk, fabric, services, ventilation, MTIP and monitoring, which is exactly the whole-house thinking a deep retrofit needs.
• RetrofitReady: DEA has 360 DEA questions covering safe practice, preparation, assessment, EPC production and focused supplement units, so you can record retrofit measures accurately and understand how they change a rating.

Across the full suite there are 1900+ questions in 4 apps, covering Assessors, Coordinators, DEAs and Installers.

Tip: After reading about a measure such as internal wall insulation or MVHR, answer a short topic set straight away. Then explain each wrong answer in your own words before moving on.

Start with the free quizzes, browse the recommended books and tools for building physics references, or explore the full RetrofitReady app suite.