Passive House Retrofit: The EnerPHit Standard and Deep Energy Upgrades

The majority of buildings that will exist in 2050 are already built. Meeting climate targets requires dramatically improving the energy performance of existing buildings through deep retrofits. The EnerPHit standard, developed by the Passive House Institute, provides a proven framework for achieving world-class energy efficiency in retrofit applications.

Why Existing Buildings Matter

Scale of Opportunity:

80%+ of 2050 building stock already exists
Current retrofit rate < 1% annually
Must reach 2.5%+ annually by 2030
Existing buildings account for largest share of building energy use

Challenge: Existing buildings face constraints that make achieving full Passive House certification difficult:

Fixed foundation systems
Party walls shared with neighbors
Historic preservation requirements
Budget limitations
Occupied buildings requiring phased work
Existing spatial limitations

EnerPHit was created to accommodate these realities while still achieving dramatic energy improvements.

The EnerPHit Standard Explained

Purpose: Quality-assured certification for renovation using Passive House components and principles adapted for existing buildings.

Target Performance:

75-93% reduction in heating/cooling energy demand
Exceptional thermal comfort
Superior indoor air quality
Structural protection and durability

Key Principle: "If you do it, do it right" - Avoid lock-in of poor solutions

Performance Requirements

EnerPHit offers two compliance pathways:

Component Method: Meet specific U-value requirements for each building element:

Walls: U ≤ 0.15 W/m²K
Roof: U ≤ 0.12 W/m²K
Floor: U ≤ 0.15 W/m²K
Windows: U ≤ 0.85 W/m²K
Airtightness: n50 ≤ 1.0 air changes per hour
Ventilation: Heat recovery ventilation required

Energy Demand Method: Meet overall building energy targets:

Heating demand: ≤ 25 kWh/m²/year
Primary energy: ≤ 120 kWh/m²/year (renewable) or ≤ 60 kWh/m²/year (non-renewable)
Airtightness: n50 ≤ 1.0 ACH

Both methods deliver similar deep retrofit results but offer flexibility based on project constraints.

Core Retrofit Strategies

Thermal Envelope Improvements

Walls:

Exterior Insulation (Preferred):

Add 8-16 inches of continuous insulation
Eliminates thermal bridging
Protects structure from thermal stress
Preserves interior space
Rain screen cladding over insulation

Interior Insulation:

When exterior access limited
Historic facades requiring preservation
4-8 inches typical (space constraints)
Requires vapor control analysis
Reduces usable floor area

Materials:

Mineral wool (fire-safe, breathable)
EPS or XPS foam boards
Wood fiber insulation boards
Spray foam (airtightness and insulation combined)

Roof:

Add insulation above roof deck (flat roofs)
Insulate at rafter level (sloped roofs)
R-60 to R-80 targets
Ventilation strategy depends on assembly
Airtightness layer continuous

Foundation/Basement:

Exterior insulation where accessible
Interior insulation more common in retrofits
Thermal break at floor perimeter
Moisture management critical
R-20 to R-30 typical

Windows:

Replacement:

Triple-pane, low-E coatings
U-value 0.85 W/m²K or better
Install in plane of insulation layer
Integrated airtightness and flashing
Thermal break at rough opening

When Replacement Not Feasible:

Secondary glazing (interior storm windows)
Insulated shutters for night insulation
Heavy thermal curtains
Address infiltration around existing frames

Thermal Bridge Mitigation

Common Problem Areas:

Balcony connections
Roof-wall junctions
Window reveals
Floor-wall connections
Structural penetrations

Solutions:

Continuous insulation layers
Thermal break elements
Detailed 3D thermal modeling
Specialized connection products
Careful execution during construction

Airtightness

Target: n50 ≤ 1.0 ACH (compared to typical existing: 6-15 ACH)

Strategy:

Continuous air barrier system
Seal all penetrations
Connect insulation layers airtight
Test with blower door during construction
Iterate to achieve target

Common Leakage Points:

Rim joists
Window and door frames
Attic hatches
Electrical and plumbing penetrations
Top and bottom plates
Service chases

Sealing Methods:

Spray foam at penetrations
Airtight tapes and membranes
Liquid-applied air barriers
Careful sequencing of trades

Mechanical Systems

Ventilation with Heat Recovery:

HRV/ERV recovering 75-95% of heat
Balanced ventilation ensuring fresh air
Filtration for improved air quality
Continuous low-level ventilation
Boost modes for high-occupancy

Heating and Cooling:

Post-Retrofit Loads: Dramatic envelope improvements reduce loads:

Heating demand: 75-93% reduction
Much smaller equipment needed
May enable elimination of fossil fuel heating

Systems:

Heat pumps (air-source or ground-source)
Point-source heating (limited distribution)
Hydronic systems with low-temperature delivery
Integrated ventilation and heating
Solar thermal for hot water

Phased Retrofit Approach

EnerPHit Retrofit Plan: Allows step-by-step retrofits while ensuring components work together toward EnerPHit performance.

Phases:

Phase 1 (Year 1-2):

Roof and attic insulation
Air sealing
Ventilation system installation

Phase 2 (Year 3-5):

Window replacement
Basement/foundation insulation

Phase 3 (Year 5-10):

Wall insulation (exterior or interior)
Heating system replacement
Final airtightness improvements

Benefits:

Spreads costs over time
Aligns with natural maintenance cycles
Allows homeowner to experience benefits incrementally
Reduces disruption for occupied buildings

Critical: Phases must be planned to avoid incompatible components or missed opportunities (e.g., insulating before window replacement complicates window installation).

Case Studies

Wilmcote House (UK)

Building:

1960s residential building
100 three-bedroom maisonettes
Local authority social housing

Retrofit:

Exterior wall insulation
Roof insulation upgrades
High-performance windows
Target: ≤ 20 kWh/m²/year heating demand

Challenges:

Complex existing geometry
Multiple balcony levels
Occupied during work
Budget constraints

Results:

Significant energy reduction
Improved comfort for residents
Protected building structure
Demonstrated EnerPHit viability for large, complex buildings

Single-Family Home Retrofits (Various)

Typical Results:

Heating energy reduction: 80-90%
Utility bill savings: $1,500-3,000/year
Improved comfort (even temperature distribution)
Elimination of drafts
Better indoor air quality
Moisture problems resolved

Investment:

$40,000-100,000 for comprehensive retrofit
Varies by climate, building size, existing condition
Payback: 15-30+ years on energy savings alone
Value proposition includes comfort, health, resilience

Multi-Unit Residential Buildings (Europe)

Amsterdam, Vienna, Frankfurt:

Numerous large apartment buildings retrofitted
Exterior insulation systems
District heating connections
Balcony thermal breaks

Performance:

75-85% heating reductions achieved
Retrofit costs: €150-350/m² ($150-350/sf)
Government subsidies support economics
Asset value preservation and enhancement

Benefits Beyond Energy

Comfort:

Warmer interior surfaces (no cold walls)
Elimination of drafts
More even temperatures
Better humidity control

Health:

Continuous fresh air ventilation
Filtration removes pollutants and allergens
Mold prevention through moisture control
Improved sleep and productivity

Resilience:

Passive survivability during power outages
Temperature stability during extreme weather
Protected against moisture damage
Extended building lifespan

Financial:

Dramatically lower utility bills
Increased property value
Reduced maintenance costs
Protection against energy price increases

Challenges and Solutions

High Upfront Costs:

Financing programs (on-bill, PACE, green mortgages)
Government rebates and incentives
Phased approach spreading costs
Value beyond energy (comfort, health, resilience)

Disruption During Construction:

Phased approach for occupied buildings
Exterior work minimizes interior disruption
Clear communication with occupants
Temporary relocation if needed

Historic Buildings:

Interior insulation strategies
Preservation of facades
Vapor control analysis required
Balance heritage and performance

Technical Complexity:

Engage Passive House professionals
Detailed energy modeling
Quality control and commissioning
Training for trades

Policy and Program Support

Government Incentives:

United States:

Federal tax credits for efficiency upgrades
State and utility rebate programs
Low-interest financing (varies by state)

Europe:

Substantial subsidies (30-50% of costs common)
Mandatory retrofit standards emerging
Property tax reductions
Low-interest loans

Canada:

Canada Greener Homes Grant
Interest-free loans
Provincial programs

Program Design:

Performance-based incentives
Technical assistance
Workforce training
One-stop-shop service models

Training and Certification

Passive House Retrofit Training:

PHI (Passive House Institute) certified courses
PHIUS (US-specific training)
Online and in-person options
Focus on retrofits and EnerPHit

Energy Advisor Certification:

BPI (Building Performance Institute)
RESNET Home Energy Rater
Blower door testing certification

Trade Skills:

Airtightness installation techniques
Window installation in retrofit
Continuous insulation systems
Heat recovery ventilation

Future Outlook

Market Growth:

Policy mandates driving retrofits
Energy costs increasing retrofit ROI
Climate goals requiring action
Aging building stock needing upgrades

Technology Development:

Improved insulation products
Better HRV/ERV systems
Integrated controls
Prefabricated retrofit panels

Financing Innovation:

On-bill repayment programs
Green mortgages and loans
Utility-led programs
Property-assessed clean energy (PACE)

Conclusion

EnerPHit demonstrates that existing buildings can achieve world-class energy performance. With 75-93% reductions in heating and cooling demand, deep retrofits deliver on climate goals while providing superior comfort, health, and resilience.

The path forward requires scaling retrofit delivery through:

Workforce training and capacity building
Financing innovation making retrofits affordable
Policy mandates and incentives
One-stop-shop service models
Phased retrofit planning tools

Most buildings standing today will serve us for decades to come. Transforming them into high-performance, low-energy structures is essential to meeting climate commitments. EnerPHit provides the roadmap; implementation is the challenge and opportunity before us.