Certified Passive House Designer / Consultant common mistakes
Certified Passive House Designer / Consultant · Understand the traps and how to avoid them.
Treating the Passive House criteria as a single 'energy-efficient building' threshold and quoting only the 15 kWh/(m²a) heating demand, while forgetting that certification also requires the heating load (10 W/m²), cooling demand, primary energy (renewable and non-renewable limits), airtightness (0.6 h⁻¹), and overheating frequency to be met simultaneously.
Candidates memorise the headline number from introductory material and never build a complete criteria checklist, so under exam pressure they answer with the one figure they are sure of and omit the others.
What to do instead
Write out the full criteria table from memory once a day for a week: heating demand ≤15 kWh/(m²a), heating load ≤10 W/m², cooling demand and cooling load limits, primary energy renewable and non-renewable limits, pressurisation test ≤0.6 h⁻¹ at 50 Pa, and the summer overheating criterion. For each, note the unit and whether it is demand, load, or a measured value. Practise stating explicitly that all criteria must be satisfied at once.
Confusing heating demand (kWh/(m²a), an annual energy quantity) with heating load (W/m², an instantaneous power quantity), and using them interchangeably in calculation and multiple-choice answers.
Both are expressed per square metre of treated floor area and both have '10' or '15' attached, so the numbers blur; candidates also rarely see a worked load calculation during study.
What to do instead
Anchor the distinction physically: demand is energy over the year, load is the worst-case power needed on the design cold day. Practise converting between them using annual full-load hours, and work at least three load calculations by hand using the transmission and ventilation loss terms at design outdoor temperature. Check units in every answer before selecting.
Using gross external floor area, or the wrong reference area, in PHPP-style calculations instead of the treated floor area defined by the standard, and then reporting results that are not comparable with the criteria.
Treated floor area is a specific definition based on internal dimensions with defined deductions, and candidates default to the architect's gross area or the energy certificate area they are used to.
What to do instead
Learn the treated floor area definition and its deduction rules, and practise deriving it from a simple plan with internal dimensions. In every practice question, state the reference area and its units before doing arithmetic, and treat any answer whose area basis is unstated as suspect.
Ignoring or underestimating thermal bridges, or treating the Ψ-value as negligible, when assembling the envelope heat loss calculation.
Thermal bridges are invisible in a simplified U-value calculation and candidates assume good detailing removes them; they also confuse the linear thermal transmittance with the U-value and drop the length term.
What to do instead
Practise the formula H = Σ(U·A) + Σ(Ψ·l) + χ until the length multiplier is automatic. Memorise typical Ψ-values for common junctions (balcony slab, window reveal, floor slab edge, party wall) and learn the difference between a certified thermal bridge catalogue value and an estimate. Be able to explain why a point thermal bridge from a penetrating fixing matters.
Describing airtightness as 'good draught-proofing' and confusing the airtightness layer with the vapour control layer or the wind barrier, or assuming a single material must perform all three functions.
In lightweight construction these layers often coincide, so candidates generalise that they are always the same, and they have not seen a detail where the airtightness line must be drawn separately.
What to do instead
For a wall, roof, and floor detail, draw the airtightness line, the insulation line, and the vapour control line in different colours and explain what each does. Learn the 0.6 h⁻¹ n50 criterion, how the blower door test is performed (pressure stages, leakage finding), and why airtightness protects the structure from convection-driven moisture as well as saving energy.
Selecting glazing on U-value alone, ignoring the g-value, and then failing to balance solar gains against overheating risk and winter heat loss.
U-values are the most familiar window metric and candidates treat lower as always better, forgetting that high-performance triple glazing can have a lower g-value and that south-facing area is a design trade-off.
What to do instead
Practise paired reasoning: for each orientation, state whether winter solar gain or summer overheating dominates and choose U-value and g-value accordingly. Learn typical values for triple glazing (U around 0.6–0.8 W/(m²K), g around 0.5–0.6) and for frames and spacers, and be able to explain the effect of the installation thermal bridge and of shading on effective solar gains.
Designing ventilation around the heat recovery unit alone and neglecting duct design, pressure drops, or the balance between supply and demand, then being unable to explain why a certified system still underperforms.
Heat recovery efficiency is the memorable number, while duct sizing, silencers, and commissioning are seen as installer detail rather than designer responsibility.
What to do instead
Learn to size ducts for low air velocity (roughly 1–3 m/s in main ducts) and to explain the consequences of undersized ducts: fan power, noise, and imbalance. Be able to state typical heat recovery efficiency, the role of frost protection, and the importance of commissioning with measured air flows. Link ventilation to indoor air quality: CO₂ and humidity targets, and why extract-only or window ventilation fails the comfort and moisture criteria.
Calculating summer overheating with a single worst-day snapshot or ignoring internal and solar gains, rather than applying a dynamic or monthly balance over the cooling period.
Steady-state winter calculations are the mental default, and candidates carry that method into summer, where thermal mass, night ventilation, and gain timing dominate the result.
What to do instead
Practise the summer balance conceptually: solar gains through glazing (area × g-value × shading factor × irradiation), internal gains, and the ability of mass and night purging to remove them. Learn the overheating frequency criterion and be able to explain why shading, orientation, and exposed thermal mass change the answer more than the wall U-value does.
Designing the heating system around a conventional boiler output and radiators sized for high flow temperatures, instead of the very low load that a Passive House envelope permits.
Candidates reason from conventional practice, where load drives plant size, and have not internalised that a 10 W/m² load allows small heat emitters, low flow temperatures, and heat pump or post-heater solutions.
What to do instead
Work through a load calculation to see how small the residual is, then practise matching it to generation and distribution: low-temperature heat pump performance, domestic hot water demand often dominating the peak, distribution losses from pipes outside the thermal envelope, and the role of a small post-heater in the ventilation supply. Be able to explain why oversizing harms efficiency.
Treating certification as a paperwork step at the end of the project and being unable to describe the sequence of quality assurance: design review, construction supervision, airtightness testing, commissioning, and documentation.
Candidates study physics and calculations and leave the process to the last weeks, so they cannot place the blower door test, the ventilation commissioning, or the documentation submission in the right order or explain who is responsible.
What to do instead
Build a one-page timeline from concept to certification: PHPP at design stage, detail review against thermal bridge and airtightness requirements, site inspections at key stages (before closing walls, before services are covered), pressurisation test, ventilation commissioning with measured flows, and final documentation. For each step, note the evidence produced and the most common failure at that step.