PHPP Modelling for UK Architects: When to Use It, What It Costs, and How to Read the Output

Why PHPP matters for UK low-energy briefs

The UK architecture market has shifted decisively over the past five years. Clients arriving with a brief that explicitly references net zero operational carbon, EPC A targets, EnerPHit certification or LETI-aligned performance are no longer fringe. They are the new mainstream of mid-to-upper-end residential work.

The problem is that SAP — the modelling tool every UK architect knows — was never designed to support low-energy design. SAP exists to demonstrate Building Regulations compliance and to generate EPCs. It is reasonably accurate when fabric performance is average. It becomes systematically misleading when fabric performance is high.

PHPP fills that gap. It is the same tool used to design every certified Passive House and EnerPHit retrofit in Europe since 1998. APMBuild Ltd, based in Hereford and serving architects across the West Midlands and Wales, uses PHPP as a design input on every project where the heating target is below 30 kWh/m²a — whether or not the client pursues formal certification.

What PHPP actually is

PHPP is an Excel-based calculation tool published and maintained by the Passive House Institute in Darmstadt. The current release is PHPP 10, and it is updated roughly every 2–4 years. It is the official tool for Passive House and EnerPHit certification — but it is also used by tens of thousands of European architects who never seek certification, simply because it is the most accurate energy model available for low-energy buildings.

What makes PHPP different from SAP is not what it calculates but how. PHPP is a steady-state monthly balance model with substantial empirical calibration. It accounts for:

• Thermal bridges (ψ-values) at every junction.
• Real ventilation system performance, including duct losses and efficiency drift.
• Climate-specific solar gains, drawn from European meteorological datasets.
• Window-by-window orientation, frame factor, glazing g-value and shading.
• Internal gains based on residential occupancy patterns proven across 30 years.
• Summer overheating frequency under realistic warm-weather scenarios.

The output is a set of monthly heating and cooling demand figures, an annual heat demand in kWh/m²a, a primary energy demand, an overheating frequency, and U-values for every element of the envelope. If you understand the report, you can iterate the design with surgical accuracy.

PHPP vs SAP — the comparison

The practical implication: on a Passive House design, SAP frequently over-predicts heating demand by 20–50% compared with PHPP (see UK research summarised by the Passivhaus Trust UK). The architect specifies more heating capacity than the building actually needs, the client overpays for kit, and the building overheats. APMBuild has seen this exact pattern on three UK projects in the last 18 months where the original architect relied on SAP only.

When you need PHPP on a project

Not every UK residential project needs a PHPP model. Here is APMBuild's practical decision tree.

You need PHPP if any of the following are true:

• The brief specifies Passive House or EnerPHit certification.
• The target heating demand is below 30 kWh/m²a.
• The client wants verified net zero operational carbon.
• The project is highly glazed and there is any overheating concern.
• You are designing for a JV or development partner who will scrutinise the energy model during due diligence.
• You want to specify MVHR confidently rather than as a black box.

SAP alone is sufficient if:

• The project targets Building Regulations compliance only.
• EPC band C or D is acceptable to the client.
• The fabric performance is conventional (U-values ≥ 0.25 W/m²K).

If you are unsure, APMBuild offers a free 30-minute call to look at the brief together. See our PHPP service page for the standard pre-assessment package.

Inputs PHPP requires

PHPP needs more information than SAP. The trade-off is that it gives more reliable output. Here is what a competent PHPP assessor will ask the architect to provide:

Geometry

Treated floor area (the official Passive House definition is slightly different from GIA — known as TFA). External envelope areas broken down by orientation. Net glazing areas for every window. Internal volumes for ventilation calculations.

Fabric

U-values for every element of the envelope, including the build-up. Where the build-up is not yet finalised, the assessor can iterate. Thermal bridge ψ-values at every junction — the architect's details package needs to be specific enough to read these.

Glazing

Whole-window U-value (Uw), glass U-value (Ug), frame U-value (Uf), spacer ψ-value (ψ-spacer), glass g-value, frame factor. Reputable European window manufacturers — including the Polish and Austrian suppliers APMBuild works with — provide all of this on a single product datasheet.

MVHR specification

Heat recovery efficiency (per PHI testing, not manufacturer marketing). Specific fan power. Duct layout and length. Location of the MVHR unit relative to the thermal envelope.

Location and climate

Site postcode, altitude, ground conditions, orientation. PHPP includes UK climate datasets for all major regions.

How to read a PHPP report

A complete PHPP output runs to dozens of pages, but architects should focus on five core numbers. Once you can read these, you can interrogate the design.

1. Annual heating demand (kWh/m²a)

The headline number. Targets: Passive House <15, EnerPHit <25–30 depending on method, ambitious UK net zero <30–40. Anything above 50 means the design is not low-energy regardless of marketing.

2. Annual primary energy demand (kWh/m²a)

Accounts for all energy use including domestic hot water, lighting and appliances, weighted by primary energy factors. Passive House target: <120 kWh/m²a (or PER <60 for the new methodology — see Passipedia: Classic, Plus, Premium Passive House classes).

3. Heating load (W/m²)

Peak heating load on the coldest day. Passive House target: <10 W/m². This number drives heating system sizing. If you see 25 W/m² in the report and the architect has specified a 5 kW heat pump for a 200 m² house, the numbers do not match.

4. Overheating frequency (% hours over 25°C)

Passive House limit: <10% of hours. In UK practice with increasing summer temperatures, target <5%. If the report shows 15%+, the glazing strategy needs work — usually external shading or reduced south glazing.

5. Airtightness (n50, ACH)

Design target. PH: 0.6 ACH@50Pa. EnerPHit: 1.0 ACH@50Pa. UK Building Regs (Part L 2021): 8 m³/(h·m²) @ 50 Pa air-permeability backstop. The gap between the latter two tells you how much detail and discipline the build requires.

Every element table at the back of the report shows the U-value, the area, and the heat loss in W/K. This is where you find the weakest element to improve first.

Common pitfalls in UK projects

From projects APMBuild has reviewed for UK architects, three pitfalls recur.

Form factor under-estimation

Form factor — external envelope area divided by treated floor area — dictates how easy or hard the energy targets are. A form factor below 2.5 is comfortable; above 3.5 is hard. UK detached bungalows often hit 4+. PHPP will reveal this immediately; SAP-only models hide it.

Thermal bridges treated as defaults

UK practice often uses Approved Document L default ψ-values. PHPP wants real calculated values. The difference between "default 0.08" and "calculated 0.04" across 50 metres of wall-to-roof junction is 2 W/K — real money in energy and real risk of cold-spot condensation.

Glazing g-value chosen for daylight, not heat

UK architects often specify low-g glazing (0.4) for solar control. In a properly shaded south facade, a high-g glazing (0.6) gives free winter heat and PHPP will reward it. The right choice is project-specific and only PHPP shows the trade-off.

Working with a PHPP-trained contractor

A PHPP report is an instruction manual for the build. If the contractor cannot read the manual, the as-built fabric will not match the model. The two most common breakdowns:

• Insulation continuity at junctions — the contractor places a service void where the model expected continuous insulation.
• Airtightness layer punctured by trades unaware of its location — the model expects 0.6 ACH; the test result is 2.5 ACH.

APMBuild is PHPP-trained, with 20+ years of European construction experience. We read the model, build to it, and verify the result with a blower door test before handover. For UK architects this means the model you commission actually arrives in the finished building. Read more on our architect partnership page, or browse our materials catalogue to see the Polish, German and Austrian components we routinely specify with full PHI datasheets.

Typical costs and timelines

UK PHPP pricing varies by complexity but a useful rule of thumb:

• PHPP pre-assessment (APMBuild service): from £950+VAT. Indicative model used during early design to confirm the brief is achievable and to set fabric and MVHR targets. 1–2 week turnaround.
• Full PHPP model for design: typically £2,200–£3,200+VAT for a standard detached UK home. 2–3 week turnaround. Used to finalise fabric, glazing and MVHR specification.
• PHPP for Passive House or EnerPHit certification: £2,800–£3,800+VAT. Delivered via APMBuild's PHI-accredited modelling partner with full QA against PHI requirements. Allow 3–4 weeks plus certifier review.

These figures are 2026 indicative rates. They look high next to a simple SAP at £200–£400, but on a £400k+ build the PHPP cost is a fraction of one window cost and removes risk worth multiples of itself.