PHPP vs EPC: A Design Model vs a Rating
PHPP vs EPC.
Reviewed by George Sora, Certified Passive House DesignerUpdated June 2026Quick answer & key takeaways
4 min read- Bottom line: PHPP is a design-accurate energy model; an EPC is a simplified rating.
- When PHPP is enough: You are designing to a performance target
- When EPC is the better choice: You need the legal certificate to sell or let
- When you need both: You need the certificate and a real design
- Biggest misconception: “A good EPC means a low-energy house.” — EPCs use simplified methods and defaults; a good rating does not guarantee low real-world energy use.
- Retrofit IQ’s approach: We model in PHPP to predict how a home will really perform and direct spend accordingly, using the EPC only for the compliance role it was built for — never as a design target, because its defaults hide the very losses that matter.
Quick answer
PHPP is a design-accurate energy model; an EPC is a simplified rating. PHPP predicts real performance closely because it models thermal bridges, ventilation and site climate in detail; an EPC uses RdSAP defaults for consistency across millions of homes and is not intended as a design prediction. For an EPC you need RdSAP; to design a building that genuinely performs, you use PHPP. Treating the EPC band as a design target is a recipe for the performance gap.
At a glance
| Attribute | PHPP | EPC |
|---|---|---|
| Purpose | Design-accurate prediction | Rating for compliance/marketing |
| Thermal bridges | Modelled explicitly | Defaulted |
| Climate data | Site-specific | Standardised |
| Accuracy vs real use | High (validated) | Indicative |
| Required for | Passive House/EnerPHit, serious design | Selling/letting, compliance |
| Detail needed | High | Low (defaults) |
What is PHPP?
The Passive House Planning Package — a detailed, validated energy model that predicts real performance from fabric, thermal bridges, ventilation, shading and climate. It is a design tool, accurate enough to design to a target.
What is EPC?
An Energy Performance Certificate produced with RdSAP, largely from visual data and defaults. It rates a home A–G for compliance and marketing, but is indicative rather than a precise prediction of energy use.
What each method measures — and what it doesn’t
PHPP
- Predicted heating demand and load
- The effect of fabric, bridges and ventilation choices
- Whether a design meets a performance target
- It is not the legal certificate for selling or letting
EPC
- An efficiency band (A–G)
- Indicative running costs and CO₂
- Generic recommended measures
- Design-accurate energy use
- Thermal bridges and real airtightness
- The specific causes of poor performance
The building science
PHPP is validated against measured Passive House buildings, so when its inputs are accurate it predicts real energy use closely. It models thermal bridges, ventilation heat loss, shading and site-specific climate, and it is detailed enough to design to a heating-demand target with confidence. It is, in short, an engineering tool.
An EPC is produced with RdSAP, the reduced Standard Assessment Procedure. RdSAP exists to rate homes consistently across the whole stock, so it trades precision for standardisation, filling gaps with defaults — including airtightness and often construction details a surveyor cannot see. The output is a useful rating band, not a precise prediction of a particular home's energy use.
This is why EPCs and energy bills frequently disagree, and why the EPC band is the wrong thing to design to. The simplifications hide real losses such as thermal bridging and infiltration, producing the performance gap. PHPP closes that gap by modelling those losses explicitly — which is exactly what you need when the goal is genuine performance rather than a certificate.
The two are not in competition; they serve different needs. You need an EPC to sell or let a home and to demonstrate compliance. You need PHPP to design a building that will actually perform. The error is confusing the rating with the design tool — treating a good EPC as proof of a low-energy home, or expecting PHPP to be the legal certificate.
Key differences
- PHPP predicts real performance; the EPC rates for compliance.
- PHPP models thermal bridges and site climate; the EPC defaults them.
- You need the EPC legally; you need PHPP for design.
- A good EPC band does not guarantee low real-world energy use.
Common misconceptions
Myth: A good EPC means a low-energy house.
EPCs use simplified methods and defaults; a good rating does not guarantee low real-world energy use.
Myth: PHPP and the EPC should agree.
They have different purposes and methods; PHPP is the design-accurate one, the EPC is a standardised rating.
Myth: I can design my retrofit to the EPC band.
The EPC is not a design tool. Designing to it invites the performance gap; PHPP predicts what will actually happen.
Real-world situations
You need to sell or let a home
An EPC is the required certificate; PHPP is not needed for that purpose.
You want a retrofit that genuinely performs
PHPP to model the fabric, bridges and ventilation and design to a real target.
Deciding between competing upgrade options
PHPP, to compare options on predicted energy and comfort rather than a rating band.
EPC says efficient but the home feels cold
The rating hid real losses; a PHPP-based assessment would reveal and quantify them.
Which do you actually need?
When PHPP is enough
- You are designing to a performance target
- You need accurate predicted energy use
- You are pursuing Passive House or EnerPHit
When EPC is the better choice
- You need the legal certificate to sell or let
- You want a standardised efficiency band
- Compliance is the requirement
When you need both
- You need the certificate and a real design
- You are retrofitting and must satisfy both
What Retrofit IQ checks on site
We model in PHPP to predict how a home will really perform and direct spend accordingly, using the EPC only for the compliance role it was built for — never as a design target, because its defaults hide the very losses that matter.
- PHPP modelling of fabric, thermal bridges and ventilation
- Site-specific climate and shading inputs
- Prediction of heating demand against a target
- Comparison of upgrade options on real performance
- Clear distinction between the design model and the EPC rating
- Measured inputs (airtightness, U-values) to keep the model honest
What a Certified Passive House Designer recommends
An EPC and PHPP are not two versions of the same thing. The EPC is a standardised rating for selling, letting and compliance; PHPP is a design model accurate enough to commit to a performance target. Confusing them is how people end up designing to a band and then wondering why the house is cold.
When the goal is a home that is genuinely warm and cheap to run, I model in PHPP — because it tells the truth about thermal bridges and ventilation losses. The EPC remains useful for what it is, but it was never meant to be a design prediction.
— George Sora, Certified Passive House Designer, Founder, RetrofitIQ
Reviewed using current building physics principles and Passive House methodology.
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Read comparisonFrequently asked questions
Is PHPP the same as an EPC?+
No. PHPP is a design-accurate energy model; an EPC is a simplified rating produced with RdSAP for compliance and marketing.
Why doesn't my EPC match my energy bills?+
Because RdSAP uses default assumptions for consistency, so the rating is indicative rather than a precise prediction of your actual use.
Do I need PHPP for an EPC?+
No — EPCs are produced with RdSAP. PHPP is for design-accurate performance modelling, not the legal certificate.
Can I design my retrofit using the EPC?+
Not reliably. The EPC is a rating, not a design tool; designing to it invites the performance gap. PHPP predicts real performance.
Is PHPP recognised in the UK?+
Yes — it is the international Passive House modelling standard, widely used for serious low-energy design in the UK.
What is the performance gap?+
The difference between modelled and actual energy use, often caused by simplifications that hide thermal bridging and infiltration.
Can PHPP be used on existing homes?+
Yes — it is the modelling tool behind EnerPHit retrofits and detailed retrofit design.
Does a good EPC guarantee comfort?+
No. The rating does not capture thermal bridges or real airtightness, both of which strongly affect comfort.
Who carries out PHPP modelling?+
A Certified Passive House Designer trained in building physics, so the model reflects how the building will genuinely perform.
Need professional advice?
A comparison like this helps you understand the theory, but every property behaves differently. The only reliable way to establish the real cause in your home — rather than guessing — is professional building performance diagnostics. At RetrofitIQ we verify buildings using the appropriate combination of investigations:
- Thermal imaging
- Blower door testing
- Moisture investigation
- Building physics assessment
- Passive House methodology