Phius Certified Passive House Consultant (CPHC) cheat sheet

Passive House targets a space-conditioning demand so low that a conventional heating/cooling distribution system becomes unnecessary; the classic metric pairs ~15 kWh/(m²·yr) heating and cooling demand with a peak load near 10 W/m².

The three pillars are a continuous, airtight, thermally bridge-free envelope; minimized internal and solar gains managed by orientation and shading; and balanced mechanical ventilation with heat recovery for fresh air.

Specific space heat demand qH = QH / A_ref, where A_ref is the treated floor area (TFA), not gross floor area; every demand metric in Phius work is normalized to TFA.

Peak heating load is governed by transmission through the envelope plus ventilation losses minus internal gains at design conditions; a low peak load is what allows omitting a central heating plant.

Thermal bridges are quantified by a linear thermal transmittance ψ [W/(m·K)]; the junction penalty is ψ·L added to the UA of the assembly.

Internal heat gains are typically modeled at roughly 2.1 W/m² of TFA for residential occupancy; overestimating gains lowers predicted demand on paper but risks real underheating.

Superinsulation shifts the annual balance so that overheating risk and cooling demand often dominate design decisions in temperate and warm climates; shading and night flush become primary strategies.

Airtightness is expressed as n50, the air change rate at 50 Pa, or as q50 per envelope area; passive targets are typically n50 ≤ 0.6 h⁻¹ for the whole building.

UA of an assembly = Σ (A_i / R_i) + Σ (ψ_j · L_j) + Σ (χ_k); reduce UA by raising R and by eliminating repeated junctions, not by adding insulation in one place.

Continuous exterior insulation keeps the structural layer warm, raises the dew point outside the sheathing, and reduces thermal bridging at studs, slabs, and rim joists.

Rigid insulation must be thick enough to keep the condensing surface above the interior dew point; the ratio of exterior to cavity R needed depends on climate and cladding permeance.

Windows dominate losses: use whole-window U-values (frame plus spacer plus glazing), not center-of-glass; a good passive window is roughly U ≤ 0.80 W/(m²·K) with a high solar heat gain coefficient for heating climates.

The glazing-to-frame ratio and spacer type drive edge-of-glass temperatures; cold edge temperatures cause condensation and comfort complaints even when the center-of-glass U is excellent.

Airtightness layers must be continuous and on the warm side or protected by a vapor-permeable, airtight membrane; penetrations for ducts, wires, and pipes are the usual leak paths.

Vapor control is climate-specific: in cold climates place a vapor retarder on the warm side; in mixed and warm-humid climates use vapor-open assemblies and control inward vapor drive from cladding.

Roof and slab junctions are the highest-frequency thermal bridges; a cantilevered balcony or a slab-on-grade edge can each add several W/(m·K) if left unmitigated.

WUFI Passive couples a hygrothermal assembly model with a whole-building energy balance; envelope assemblies are built from layers with material properties, then referenced by area in the building model.

Model geometry is defined by zones with conditioned, unconditioned, and adjacent classifications; only conditioned TFA enters the demand metrics, so zone boundaries must be drawn deliberately.

The certification metrics are annual specific demand (kWh or kWh/(m²·yr)) plus peak load (W or W/m²); both must pass, so a design can fail on peak load alone.

Ventilation is modeled as a balanced system with a heat recovery efficiency and a frost-protection strategy; the recovered heat is credited in the space-conditioning balance.

Shading is modeled with horizon and overhang objects; the solar gain through a window is reduced by the shading factor and by the glazing g-value (SHGC), not by orientation alone.

Thermal bridge values are entered as ψ per junction length or as a point χ; unentered junctions default to uncorrected and will overstate or understate performance depending on the default.

Model results must be sanity-checked: compare predicted specific demand against the assembly U-values and the glazing area; a very low result usually means an unmodeled zone or a wrong area.

The model is the certification artifact; the as-built building must match the modeled envelope, ventilation, and systems, and deviations require a documented update.

Balanced ventilation supplies and exhausts equal air volumes; an unbalanced system drives pressure differences that push moist air into assemblies and increases infiltration losses.

Heat recovery efficiency (sensible, and total where an enthalpy core is used) directly reduces ventilation heat loss; a recovery of ~75-85% is typical for certified units.

Ventilation airflow is sized from occupancy or floor area; a common residential basis is roughly 30 m³/(h·person) or a per-area rate, and the design rate must be documented.

Frost protection strategies include a preheater, a recirculating defrost damper, or supply-air tempering; the chosen strategy must be represented in the energy model.

Ducts inside the conditioned envelope avoid losing recovered heat; ducts in unconditioned attics or garages negate much of the recovery benefit and must be insulated and sealed.

A post-heater is permitted but its energy counts against the demand metrics; the design intent is that ventilation air alone, tempered by recovery, meets most of the load.

Domestic hot water is a major remaining load in passive buildings; drain-water heat recovery and compact units with distribution inside the envelope reduce it.

Cooking and bathroom exhaust must be interlocked with or balanced against the HRV/ERV so that the building does not depressurize and pull humid air into the envelope.

Phius certification has multiple tiers; the core passive requirement is a very low specific space-conditioning demand plus a peak load low enough to omit a conventional distribution system.

Phius standards are climate-specific: the source energy and demand limits are set by climate zone, so a design that passes in one location may fail in another.

Source energy limits account for the efficiency of the grid and of on-site generation; all-electric designs and heat pumps are favored over fossil combustion.

Renewable energy can be credited against the source energy limit; the credit depends on the renewable type and whether it is on-site or off-site.

The Phius certification process requires a certified consultant, a submitted energy model, and third-party quality assurance; the certificate is issued to the building, not the designer.

Prescriptive and performance paths exist; the performance path uses the WUFI Passive model, while prescriptive paths constrain assemblies and systems directly.

Airtightness and ventilation commissioning are verified on site; a passing model without verified airtightness does not yield certification.

Recertification and continuing education keep the consultant credential current; lapsed credentials cannot submit projects.

Blower-door testing measures n50 at 50 Pa; the result is extrapolated to natural conditions with an n-factor, so the reported natural infiltration is always lower than n50.

The airtightness test must be done after the air barrier is complete but before finishes conceal it; a failed test at that stage is still fixable.

Thermal imaging during a blower-door test reveals missing insulation and air leaks; a temperature difference of at least ~10 K between inside and outside is needed for useful images.

Ventilation commissioning verifies design airflow at each terminal, the balance between supply and exhaust, and the measured heat recovery efficiency.

A quality-assurance checklist tracks envelope continuity, window installation, penetration sealing, and system commissioning; each item is signed off by the responsible trade.

Documentation includes the as-built model, test reports, commissioning records, and photographs of concealed air-barrier and insulation work.

Common failures are unsealed penetrations, discontinuous air barriers at floor and wall junctions, and ducts outside the envelope; each is caught by inspection before drywall.

The QA reviewer is independent of the design team; the reviewer's role is to confirm that the built building matches the certified model.