Passive House in the US: PHI vs Phius Certification and Airtightness Targets
US teams pursuing Passive House certification have two routes: the Passive House Institute (PHI) in Darmstadt, Germany, and Phius in the US. Both are rigorous and respected, but they define performance differently, which changes how you design, model and test. This guide covers how they differ, what each requires for airtightness, and why airtightness is usually decided on site rather than on paper.
Two certification routes for Passive House in the US
PHI: one set of criteria for every climate
PHI publishes international building criteria that apply worldwide. For new construction, the Passive House standard limits heating demand to 15 kWh/m² per year (4.75 kBtu/ft² per year), with matching cooling criteria. It also sets a renewable primary energy (PER) limit, which defines three classes:
- Passive House Classic: the base standard, with no on-site renewable generation required.
- Passive House Plus: on an annual average, the building generates about as much energy as it uses.
- Passive House Premium: on an annual average, the building generates significantly more energy than it uses.
For existing buildings, PHI offers EnerPHit, a retrofit standard with relaxed, climate-dependent targets. Projects are modeled in PHI's Passive House Planning Package (PHPP) and certified by a PHI-accredited certifier.
Phius: climate-specific targets for North America
Phius sets space conditioning limits per square foot of conditioned floor area that vary by climate, building size and occupant density, so a home in Minnesota and one in Georgia get different heating and cooling targets. A source energy target sits alongside them. The program family includes:
- Phius CORE: the base new-construction standard. On-site renewables are not required, and fossil-fuel combustion is permitted.
- Phius ZERO: builds on CORE. Renewable energy is required and fossil-fuel combustion is not permitted.
- Phius CORE Prescriptive: a checklist-based path for some projects.
- Phius REVIVE: the retrofit standard, which also focuses on resilience.
Phius re-optimizes its standards every three years; the current version is Phius 2024, and the version a project follows depends on when its certification contract is signed. On-site testing is done by a Phius Certified Rater (single-family) or Phius Certified Verifier (multifamily and non-residential).
Airtightness: same goal, different metrics
Both programs treat airtightness as a pass/fail requirement confirmed by a whole-building blower door test. They measure it differently.
| PHI (Passive House) | Phius (2024) | |
|---|---|---|
| Metric | n50: air changes per hour at 50 Pa (ACH50) | Leakage per square foot of gross enclosure surface area (CFM50/ft²) |
| New construction limit | ≤ 0.6 ACH50 | ≤ 0.060 CFM50/ft² for most buildings (≤ 0.08 CFM75/ft² if tested at 75 Pa) |
| Variation by building type | Same limit for all new Passive House buildings | ≤ 0.08 CFM50/ft² for buildings of 5 or more stories built with non-combustible construction |
| Retrofit | EnerPHit: ≤ 1.0 ACH50 (extensive leak detection required between 0.6 and 1.0) | Covered under Phius REVIVE (check the current guidebook) |
For context, 0.060 CFM50/ft² is roughly 1.1 m³/(h·m²) at 50 Pa. The choice of metric matters:
- ACH50 divides leakage by volume. Large, compact buildings have more volume per square foot of enclosure, so 0.6 ACH50 is easier to reach on a big multifamily block than on a small house.
- CFM50/ft² divides leakage by enclosure area. Small and large buildings face a similar limit per square foot of wall, roof and floor, which can be tougher than an ACH50 target for big buildings.
Neither is "easier" across the board; it depends on size, shape and the program you're working with. Phius defines "non-combustible" as construction with no plant-based framing, sheet goods or insulation, so wood-framed mid-rise doesn't qualify for the relaxed limit. Confirm the exact protocol in the current PHI criteria or Phius Certification Guidebook.
Where Passive House is gaining ground in the US
Much of the US growth is in multifamily and affordable housing, supported by state and utility programs. In Massachusetts, Mass Save offers incentives and technical assistance for Passive House multifamily buildings of five or more units certified through either Phius or PHI, subject to all-electric and other eligibility rules. The Pennsylvania Housing Finance Agency added points for Passive House certification to its Low-Income Housing Tax Credit allocation plan in 2015, and other state housing finance agencies have followed. Program terms change, so check current details with the sponsor.
Why airtightness is usually the hardest target on site
Most Passive House targets are largely locked in at design: insulation, windows, thermal bridging and heat-recovery ventilation are modeled, specified and bought. Airtightness is different. You only find out when the blower door runs, and the result depends on many small details built correctly by several trades:
- Transitions: wall to foundation, wall to roof, window and door rough openings, wherever one material or trade hands off to the next.
- Penetrations: plumbing, electrical, ductwork, ventilation and fasteners, often added after the air barrier is installed.
- Substrate changes: concrete to CMU to wood framing to steel, each needing a detail that adheres to and bridges the junction.
- Sequencing: once drywall or cladding covers the air barrier, fixing leaks gets expensive.
The air barrier material is rarely the weak point; joints, laps and penetrations govern the whole-building number. A good strategy cuts the number of joints and makes the rest easy to execute and inspect. For how US energy codes treat air barrier continuity, see our guide to continuous air barrier code requirements.
How a certified fluid-applied air barrier helps
Passive Purple is a fluid-applied airtight, vapor control and smoke-tight membrane. It is certified as a Passive House component by PHI, with a Class A airtightness rating of 0.02 m³/h·m² at 50 Pa (EN 12114). Its certification is from PHI, not Phius; Phius project teams can still evaluate it on its tested performance. Key features:
- Continuous, seamless coverage. Passive Purple is sprayed (airless) or rolled across brick, CMU/concrete block, concrete, drywall and steel. For wood or OSB substrates, check fire and finish requirements for your project with us and your code official. It forms one continuous membrane across substrate changes, without the laps and taped seams of a sheet system.
- Detailing at penetrations. Passive Purple Brush is a fiber-reinforced brush grade for joints, penetrations and cracks of 2–5 mm (about 1/16–3/16 in). Elongation of roughly 219–350% (EN 12311-1) gives crack-bridging at moving joints.
- Intelligent vapor control. Its variable vapor resistance (Sd ~0.8–40 m, EN ISO 12572) helps manage moisture. It is applied on the warm side of the insulation in heating climates. The right vapor strategy depends on climate zone and assembly; see our guide to vapor retarder class by climate zone, and ask about our condensation risk analysis service.
- Documented US performance. Passive Purple has an air barrier material result of 0.02 L/s·m² at 75 Pa (ASTM E2178), an ASTM E84 Class A rating (flame spread 0), and a 10/10 mold resistance rating (ASTM D3273). Code acceptance is through the test reports, submitted to the code official for each project; ask us for copies.
- Speed. By spray, one installer can cover up to about 3,200 ft² (300 m²) per day. That makes it practical to finish the air barrier, test it and fix leaks before the next trade arrives.
For exterior airtight and water-resistive layers, Passive Purple External is a vapor-open, UV-stable façade membrane and is also a Passive House component.
Test early, test often
The final certification test is the wrong time to discover leaks. Schedule a blower door test as soon as the air barrier is complete and before drywall or cladding covers it (a pre-drywall test):
- Design the air barrier line. Draw one continuous line on every section and assign each transition to a trade.
- Install and inspect. Apply the membrane, detail penetrations and walk the building with the rater or verifier.
- Test pre-drywall. Depressurize and use smoke pencils or thermal imaging to find leaks while the air barrier is exposed.
- Seal and retest. A fluid-applied membrane is simply touched up at the leak.
- Protect it. Brief later trades and seal new penetrations as they're made.
- Final test. Run the PHI or Phius protocol with a known margin in hand.
PHI and Phius measure airtightness differently, but on site the work is the same: one continuous air barrier, built carefully and tested while you can still fix it.
Get support on your Passive House project
Intelligent Membranes supports US architects, specifiers and contractors with Passive House detailing, section editing, substitution requests, LEED documentation and condensation risk analysis. Download our CSI MasterFormat specifications, explore Passive Purple and Passive Purple Brush, or contact our technical team at +1 (347) 220 8403.