Every certified Passivhaus building rests on one file: its PHPP model. PHPP, the Passive House Planning Package, is the energy model that predicts whether a design will hit its space heating and primary energy targets before anyone breaks ground. A PHPP consultant is the person who builds that model, keeps it honest as the design changes, and reads what it is trying to tell the team. On a serious low energy project they are not an added cost. They are the reason the number on the certificate matches the number on the meter.
What the model actually does
PHPP assembles a monthly energy balance for a building from its real geometry, fabric, junctions and services. It is a spreadsheet in the sense that a structural analysis is a spreadsheet: the format is familiar, the discipline behind the inputs is not.
A consultant working in it is doing four things at once. They are quantifying heat loss through every element and, crucially, through the thermal bridges at the junctions where elements meet. They are testing whether the ventilation strategy and the MVHR unit can recover enough heat to make the balance work. They are checking summer conditions so the building does not solve winter and create an overheating problem. And they are pricing all of it, because a model that reaches the target at a cost no client will pay is not a solution.
The output people remember is the space heating demand, 15 kWh/m²a or less for full certification. The output that saves projects is everything the model reveals on the way there.
When a project needs one
Not every building needs a dedicated PHPP consultant, and it is worth being honest about that. A single certified house designed by a practice that models in-house does not. The threshold is usually one of three things.
The first is intent. The moment a project commits to Passivhaus, EnerPHit or a genuine near zero energy target, PHPP stops being optional. It is the tool the standard is written around, and no other compliance model substitutes for it.
The second is scale or repetition. A build to rent block, a school, a housing scheme with eight house types: here the model is not just proving compliance, it is protecting a decision that gets repeated hundreds of times. A wrong assumption in a single dwelling is a disappointment. The same assumption across a scheme of hundreds of homes is a budget.
The third is doubt. When a design team suspects a building will struggle, on a tight urban site, a difficult form factor, a heritage retrofit, an early model turns suspicion into a number and a number into a plan. That is far cheaper at concept stage than at tender.
What PHPP catches that compliance tools miss
Ireland’s DEAP and the UK’s SAP are compliance calculators. They answer a regulatory question: does this building pass. PHPP answers a physical one: how will this building behave. The gap between those two questions is where most performance is lost, and we have written about why PHPP and DEAP or SAP are not the same tool.
Three examples come up on almost every project. Compliance tools apply default or simplified thermal bridge values; PHPP wants the real psi-values, and the difference between an assumed junction and a calculated one can move the heating demand by a full band. Compliance tools treat form loosely; PHPP is unforgiving about surface area to volume ratio, which is why it will tell a client early that their articulated, many cornered design is going to cost more to heat than a simpler one. And compliance tools rarely stress summer; PHPP models overheating hour by hour, which is now a live risk under warming summers and tighter fabric.
None of this is the tool being clever. It is the tool declining to average away the things that decide whether a building works.
What to hand your consultant
The quality of a PHPP model is set by the quality of what goes into it. A consultant can work from very little, but the model gets sharper, and the fee gets smaller, when the inputs are real. In practice that means dimensioned drawings rather than sketches, proposed build-ups with actual products, the intended window system with its frame and glazing data, and an honest description of the ventilation strategy. Early on, assumptions are fine and expected. What matters is that they are visible assumptions the team has agreed, not defaults hidden inside a calculation.
Where Mosart fits
We have run PHPP on buildings from single houses to schemes of thousands of homes, and we teach the tool to the people who go on to use it. PHPP energy modelling is one of the core services in our consulting practice, and it rarely travels alone: the same model that proves the heating demand feeds the airtightness strategy, the thermal bridge work and, on certified projects, the certifier’s review.
If you want to feel how the model thinks before you commission one, the treated floor area tool is a small piece of the same logic, the reference area every PHPP result is normalised against. It is a fair first look at why the standard measures buildings the way it does.