PHPP predicts how much energy a building will use. DEAP and SAP demonstrate that a design meets building regulations. Both are legitimate tools, but they answer different questions, which is why a dwelling can hold an A-rated BER or a strong SAP score and still cost far more to heat than the paperwork implies.
What is each tool actually for?
DEAP, the Dwelling Energy Assessment Procedure, is SEAI’s official Irish methodology for calculating the energy performance of dwellings. It produces the BER certificate and performs the compliance checks for Part L of the Building Regulations. SEAI’s own introduction is precise about what it is: an asset rating, calculated “under standardised operating conditions”, deliberately independent of how any particular household behaves. Occupancy and hot water demand are derived from floor area. Heating runs for fixed periods to fixed temperatures, 21°C in living areas and 18°C elsewhere. That standardisation is a feature, not a flaw: it lets a buyer compare two houses on a like-for-like basis.
SAP, the Standard Assessment Procedure, is the UK equivalent: the government’s methodology for estimating the energy performance of homes, developed and maintained by the BRE, used to demonstrate Part L compliance and to generate EPCs. The current version is SAP 10.3, with a successor, the Home Energy Model, in development partly to improve accuracy.
PHPP, the Passive House Planning Package, was built for a different job. It is a prediction engine: a monthly energy balance assembled from the real building, its real components and the real local climate, refined against measured data from completed buildings for three decades. Its output is not a rating band. It is a number you can check against a heat meter.
Why can a building pass compliance and still underperform?
Because demonstrating compliance and predicting performance are different exercises, and the gap between them has a name and a literature. A peer-reviewed study by Mitchell and Natarajan at the University of Bath, published in Energy & Buildings and hosted by the Passivhaus Trust, summarises the UK evidence bluntly: most new and retrofitted buildings use as much as 250% more energy than design-stage models predicted, and field testing has found fabric heat losses 50 to 60% above design predictions.
The causes are mundane. Compliance models accept default values where measured ones are missing. Thermal bridges can be covered by standard allowances rather than calculated. The assumed airtightness need never meet a fan. And once the certificate is issued, nothing in the regime goes back to check whether the finished fabric matches the file. None of this is dishonesty; it is a methodology doing exactly what it was designed to do, which is regulate, not forecast.
We see the consequence in the files that cross our desks: a design that sails through DEAP while its PHPP, built from the same drawings, shows a space-heating demand two or three times the Passivhaus limit. Same building, different question.
How does PHPP avoid the gap?
By refusing unverified inputs and then verifying the outputs. Climate data is local and monthly, not a national average. The floor area denominator is the treated floor area, measured to strict PHI rules rather than gross conventions, so the kWh/m²a results cannot be flattered by area. Psi-values are calculated, product values are declared and certified, ventilation efficiency comes from the tested unit, and the airtightness figure in the final model is the blower-door result, not an aspiration.
The same Bath study tested whether this discipline pays. Across 97 UK Passivhaus dwellings on 13 sites, measured mean space-heating demand was 10.8 kWh/m²a against a predicted 11.7, no statistically significant difference, in a country where the average home runs at about 145 kWh/m²a. Buildings modelled in PHPP perform the way the model said. That is the entire argument, in one result.
It holds beyond the lab. At Whitehaven, social housing we monitor in use, the measured performance tracks the PHPP that certified it. Monitoring is unglamorous work, but it is the only place where a model’s reputation is actually earned.
When do you need both?
On every Irish or UK Passivhaus project, without exception. DEAP and SAP are statutory: no BER, no sale or rental in Ireland; no SAP, no Part L sign-off in the UK. PHPP is what makes the performance real and, on certified projects, contractual. The tools are not rivals. They are run in parallel, and the workflow matters:
- Build the PHPP at feasibility, before form and glazing are frozen. It steers the design.
- Run the compliance model from the same geometry and specifications when the regulatory submission falls due. A Passivhaus design passes DEAP or SAP with room to spare.
- Reconcile deliberately. Keep compliance defaults out of the PHPP, and document why the two models show different numbers for the same building. They will, and the reasons should be known rather than discovered.
- Feed the as-built evidence, especially the blower-door result, back into the final PHPP and the final BER or SAP, so both records describe the building that exists.
What are the differences at a glance?
| PHPP | DEAP / SAP | |
|---|---|---|
| Purpose | Predict real energy use; Passivhaus certification | Demonstrate regulatory compliance; BER / EPC |
| Question answered | What will this building need? | Does this design satisfy Part L? |
| Climate data | Local, monthly | Standardised national conventions |
| Occupancy and heating | Realistic continuous comfort | Fixed periods, fixed set-points, occupancy from floor area |
| Floor area | Treated floor area to PHI rules | Gross conventions |
| Inputs | Declared, calculated and tested values | Defaults permitted where data is missing |
| Verification | Independent certifier checks design and as-built evidence | Assessor lodges the calculation; no in-use check |
Where Mosart fits
Both worlds are daily work at Mosart: PHPP energy modelling carried from feasibility through to the certification-ready workbook, reconciled against the compliance model along the way. If you want the skill in-house, PHPP is the core of the Certified Passivhaus Designer course, and the treated floor area tool shows in five minutes why the two methodologies measure the same building differently.