PHI or PHIUS? What's right for my project?

Passive Houses (PH) are defined by following two characteristics, regardless of the certification standards:

· thermally comfortable indoor environment

· significantly lower operating energy as compared to the existing building stock

Passive Houses in North America, regardless of building use, are certified by one of the two organizations, unlike rest of the world.

· Passive House Institute (PHI) – European Standard

· Passive House Institute US (PHIUS) – American Standard

While both standards have the same building science principles, they differ in the performance targets required to certify a building. My thesis is that an agnostic approach to modeling reveals each building’s natural predication for PH certification. This blog compares the two PH standards and presents an opinion on how to think about modeling and certifications on various types of projects.

New Construction vs. Existing Buildings

PHIUS has the same standard and thus the same criteria for new construction and existing building retrofits i.e. PHIUS+ Standard. However, PHI offers different certification standards:

· Passive House Standard – New Construction

· EnerPHit – Existing Buildings

PHI also provides an option of certifying buildings with PHI Low Energy Building Standard if a building cannot comply with Passive House criteria for one or more reasons.

Certification Criteria

Both Passive House organizations require compliance with a combination of multiple quantitative performance criterion, while PHIUS has some additional prescriptive, qualitative design and field inspections requirements.

Quantitative Performance Criteria

PHI and PHIUS outline their building performance certification requirements around following categories of criteria.

· Heating – Load and/or Demand

· Cooling – Load and/or Demand

· Airtightness

· Energy

The categories are the same but the combination of compliance criteria leading to certification are different, as seen below.

Criteria Category	PHI	PHIUS
Heating	Load OR Demand	Load AND Demand
Cooling	Demand OR Demand AND Load	Load AND Demand
Airtightness	Based on volume	Based on envelope area
Energy	Renewable Primary Energy Demand (PER)	Net Source Energy Demand

Heating and Cooling Criteria

One of the biggest differences between PHI and PHIUS is that PHIUS’ criteria for heating and cooling are based on envelope area, floor area, occupant density and location of the project. PHI has the same heating/cooling load/demand criteria for all climates worldwide, except the dehumidification demand is dependent on climate, air change rate and internal heat and moisture loads. However, PHI has different sets of values depending on the applicable certification standard i.e. Passive House, EnerPHit or Low Energy Building.

Airtightness

PHI’s airtightness requirement is based on “air changes per hour at 50Pa (ACH50)” while PHIUS uses “cfm/ft2 of envelope area at 50Pa” as a unit of measure. Air tightness requirements by PHI vary depending on the certification standard pursued by a given project. EnerPHit and Low Energy Building Standards have more relaxed air tightness requirements as compared to Passive House Standard for new buildings while, PHIUS has a same target for both new construction and existing retrofits.

Energy

For energy, PHIUS has net source energy demand (kWh/person/yr) as criterion with different requirements for residential and commercial buildings, based on the certification tier chosen. Each of the three tiers vary based on the amount of renewable energy generated to meet the source energy criterion.

· PHIUS+ Core

· PHIUS+ 2018

· PHIUS+ Source Zero

PHI uses Renewable Primary Energy (PER) (kBTU/ (ft2.yr)) as the metric to evaluate building’s energy performance. PER factors describe how much more renewable energy must be supplied in order to cover final energy consumed at the building, including losses incurred along the way. PHI also offers three categories for certification, depending on PER demand and generation of renewable energy.

· Passive House Classic

· Passive House Plus

· Passive House Premium

Energy Modeling Tool Summary & Recommendation

PHIUS uses WUFI Passive as a tool to inform design decisions and for certification while, Passive House Planning Package (PHPP) is used for projects pursuing certification with PHI.

The feasibility of pursuing one standard versus another standard depends on a multitude of factors unique to a project. At AUROS Group, we use different modeling platforms to determine the standard that provides the easiest path to achieve performance and certification. To identify which standard would yield the best performance at the lowest cost for your project, reach out to AUROS Group.