EnerPHit is the Passive House Institute’s certified standard for retrofitting existing buildings. It relaxes the Passivhaus targets to a space-heating demand of 25 kWh/m²a or less and airtightness of 1.0 ACH at 50 Pa, because existing geometry and junctions constrain what a retrofit can reach. A component-based route and a staged, step-by-step pathway are also available.
Why does retrofit get its own standard?
Because an existing building arrives with decisions already made. The orientation is fixed. The foundations, party walls and junction geometry are what they are, and some thermal bridges can be reduced but never designed out. Holding retrofits to the new-build criteria, 15 kWh/m²a heating demand and 0.6 ACH airtightness, would rule out buildings that can still be transformed into excellent performers.
EnerPHit accepts those constraints and sets targets that are demanding but reachable within them. The methodology is otherwise the same discipline as new-build Passivhaus: a PHPP model, calculated rather than assumed details, a blower-door test to EN ISO 9972, and independent certification of the as-built evidence.
How does EnerPHit differ from Passivhaus Classic?
| Criterion | Passivhaus Classic | EnerPHit |
|---|---|---|
| Space-heating demand | ≤15 kWh/m²a (or heating load ≤10 W/m²) | ≤25 kWh/m²a, or the component route |
| Airtightness at 50 Pa | ≤0.6 ACH | ≤1.0 ACH |
| Compliance route | Whole-building energy balance in PHPP | Demand route or component-by-component route |
| Verification | Blower-door test, PHPP review, PHI certificate | Same process, retrofit criteria |
Demand route or component route?
There are two ways to certify. The demand route is the familiar one: model the building in PHPP and demonstrate a heating demand of 25 kWh/m²a or better. It suits buildings whose form and orientation give the energy balance a fair chance.
The component route exists for buildings that cannot get there honestly. Where a poor surface-to-volume ratio, fixed shading or protected facades cap what the energy balance can achieve, the building certifies instead by bringing each element it touches up to PHI component-quality criteria: insulation, windows, ventilation, airtightness, each done properly. The logic is that if every component is right, the building performs as well as that building can. Checking what a wall build-up achieves is exactly what our U-value tool is for.
Why is moisture the retrofit-specific risk?
This is the question that separates retrofit from new build. In a new building you choose the construction; in a retrofit you inherit it, and then you change how it behaves.
The classic case is internal insulation. Insulating a solid wall from the inside makes the room warmer and the existing masonry colder, which shifts the dew point into the original fabric. Warm, moist indoor air reaching that colder zone can condense inside the construction, invisibly, year after year. The wall that performed adequately for a century can be made to fail by a well-intentioned upgrade.
The screening sequence is well established. The Glaser method is the steady-state screen for interstitial condensation. Surface mould risk is checked against the f_Rsi threshold of 0.75 under EN ISO 13788. And for marginal or high-stakes cases, WUFI provides dynamic hygrothermal simulation that captures rain, sun and seasonal storage in a way the steady-state method cannot. Run your build-up through our condensation and dew point tool before committing to an internal insulation strategy, and treat a marginal Glaser result as an instruction to model properly, not a pass.
What is step-by-step EnerPHit?
Few owners can fund a whole deep retrofit in one contract, and PHI recognises this with a staged route: certification on the basis of an overall retrofit plan, delivered step by step over years. The plan is the critical artefact. It commits each stage to the end state, so that the roof insulated this year matches the wall insulation arriving in five years, and no step blocks or undoes a later one. Retrofitting piecemeal without that plan is how buildings end up with stranded work: a new roof that has to be reopened because the airtight layer was never planned through the eaves.
What order should a retrofit follow?
Every building needs its own plan, but a sensible default order looks like this:
- Assess the whole building and write the staged plan. Survey the fabric, screen the moisture risks, model the end state in PHPP, and sequence the steps so each one connects to the next.
- Fabric first: roof, walls, floor. Insulation and the airtight layer go in together, with the continuity of both planned across every junction before work starts.
- Windows with the walls. Replacement windows should be positioned and detailed with the wall insulation, because the reveal junction sets the thermal bridge and the airtightness connection. Done separately, the junction is done twice.
- Ventilation once the building is tight. A tightened building must have designed ventilation. MVHR with at least 75% heat-recovery efficiency turns the airtightness work into comfort and energy performance rather than stuffiness.
- Heating last, sized to the new demand. After the fabric steps, the heat load is a fraction of the original. Replacing the heating system first means buying a system sized for a building that is about to stop existing.
Where Mosart fits
Retrofit rewards the team that takes moisture as seriously as heat loss. If you are taking an existing building to EnerPHit, our Passivhaus certification service covers the design-stage and as-built reviews, and the Deep Retrofit Masterclass in our learning programme teaches the moisture-safe methodology behind it.