When a team of architecture students from Delft University of Technology (TU Delft) set out to retrofit a 1960s Dutch rowhouse, they did not simply add insulation and swap out windows. They wrapped the entire structure in a glass “second skin” that turned a drafty, energy-hungry home into a net-zero building powered entirely by solar energy. Their project, Prêt-à -Loger (French for “Home with a Skin”), earned third place at the Solar Decathlon Europe 2014 in Versailles, France, demonstrating how the Solar Decathlon model for energy-efficient home building can address real-world retrofit challenges. For builders and developers working with aging housing stock, the lessons from this project offer a blueprint that goes far beyond the competition.
The Existing Rowhouse Problem
Millions of rowhouses across Europe and North America share a common set of performance problems. Built in the mid-20th century, these structures were designed before modern energy codes existed. The original 1960s rowhouse in Honselersdijk, a small town in South Holland, exhibited all the classic symptoms: high energy consumption, poor thermal comfort, moisture issues, and limited adaptability.
Why Retrofitting Matters More Than Demolition
The TU Delft team made a deliberate choice to retrofit rather than demolish and rebuild. This decision carries significant implications for the construction industry.
- Embodied carbon is preserved. Demolishing an existing structure and pouring new foundations releases years worth of operational carbon savings before the new building breaks even.
- Existing neighborhoods maintain their character. Rowhouse retrofits keep urban fabric intact and avoid the disruption of full-scale redevelopment.
- Occupants can stay in place. The Prêt-à -Loger team prioritized minimal disruption during construction, allowing residents to remain in their homes.
- Cost effectiveness. While a full replacement can be prohibitively expensive in dense urban settings, a targeted retrofit with prefabricated components can deliver near-zero performance at a fraction of the price.
Key Performance Deficits in Postwar Rowhouses
The team identified four critical shortcomings in the original structure:
- Thermal bridging through the structural frame and uninsulated masonry, causing heat loss and cold spots on interior surfaces.
- Air leakage through gaps in the original envelope, leading to drafts and high heating demand during Dutch winters.
- Moisture accumulation from inadequate ventilation and poor vapor control, resulting in mold and condensation inside wall cavities.
- Inflexible floor plans that could not adapt to changing household needs or seasonal use patterns.
The Second Skin Concept: Passive Design as a Starting Point
The heart of the Prêt-à -Loger solution is a transparent glass structure attached to the southeast facade of the existing rowhouse. This “second skin” is not a simple sunroom addition. It functions as a thermal buffer zone, a year-round garden space, and a structural frame for a building-integrated photovoltaic (BIPV) system. The approach follows a principle that TU Delft professor Andy van den Dobbelsteen calls “smart and bioclimatic design” making optimal use of local conditions through passive measures before adding active technology.
How the Second Skin Works Thermally
The second skin creates a transitional zone between the interior living space and the outdoors. During winter, the glass enclosure traps solar radiation and warms the air inside the buffer zone. This preheated air reduces the heating load on the original building fabric. During summer, the operable glass doors open fully, transforming the zone into a ventilated outdoor room that shades the original facade and prevents overheating.
The team added an extra layer of insulation to the northwest side of the house the “cold” side that faces prevailing winds and receives minimal direct sunlight. This asymmetric approach treats each orientation differently, a strategy that aligns with high-performance building envelope design principles that prioritize climate-responsive solutions over one-size-fits-all assemblies.
Moisture Management in the Retrofit Assembly
One of the hidden achievements of the second skin design is its approach to moisture control. The original rowhouse suffered from condensation and mold because the building fabric could not dry effectively. By adding the ventilated buffer zone on the warm side and continuous insulation on the cold side, the team shifted the dew point outward. This meant moisture could no longer accumulate within the wall assembly, eliminating the conditions that had previously supported mold growth. The glass structure itself acts as a rain screen, preventing wind-driven moisture from reaching the original facade while allowing vapor to escape through natural ventilation.
Year-Round Garden Functionality
For many Dutch households, the garden is an essential part of daily life. The glass structure preserves this connection by maintaining temperatures conducive to plant growth even during winter months. The buffer zone also functions as:
- A passive solar collector that preheats ventilation air for the main house
- A sheltered outdoor living space in spring and autumn when temperatures are marginal
- A natural ventilation stack when the glass doors are opened, drawing cool air through the house
Active Systems and Energy Generation
Once the passive design measures were in place, the team addressed active systems and on-site energy generation. The project achieves net-zero energy performance through a carefully integrated combination of solar generation, heat recovery, and efficient lighting.
Building-Integrated Photovoltaics
The BIPV system consists of photovoltaic panels mounted directly on the aluminum frame that supports the glass structure. A light steel skeleton carries the loads, with primary beams following the 1.2-meter grid pattern of the original rowhouse so the new structure aligns with the existing foundation layout. The main beams transfer roof loads through a portal frame at the facade, which also supports the glass skin’s operable door and additional PV panels.
| System Component | Description | Function |
|---|---|---|
| BIPV panels | Photovoltaic modules on aluminum frame | Generate electricity from solar radiation |
| Steel skeleton | Primary and secondary beams on 1.2m grid | Structural support aligning with existing rowhouse |
| Portal frame | Facade transfer structure | Roof load distribution and door support |
| Solar water heating | Two panels with heat pump integration | Domestic hot water from glass structure heat |
| TDDs | Tubular daylighting devices through roof | Daylight delivery to main living space |
| LED lighting | Solid-state fixtures with RF adapters | Wireless-controlled efficient illumination |
HVAC and Mechanical Systems
The stone wool insulation strategies used in the original building upgrade reduced heating demand to the point where a compact mechanical system could handle the remaining load. A fan and heat exchanger located in the transitional zone allow occupants to adjust temperatures as needed. The mechanical system was placed against the exterior wall in the first-floor bedroom after the attic was removed to meet Solar Decathlon solar envelope requirements. Acoustic concerns from this placement were addressed with wood flooring, interior vegetation, and fabric shading panels that absorb sound within the glass structure. The team also installed additional acoustic baffles in the mechanical system’s ductwork and specified resilient channel mounting for the equipment to minimize vibration transfer through the original structure. These sound-mitigation measures demonstrate that energy retrofits need not compromise interior comfort when careful attention is paid to noise paths.
Daylighting and Electrical Design
Tubular daylighting devices (TDDs) channel sunlight through the roof into the main living space, reducing the need for artificial lighting during daytime hours. All interior lights use solid-state LED fixtures with radio frequency (RF) adapters, enabling wireless on-off control. This eliminates much of the wiring that would normally be required, cutting installation time and material costs.
Water Systems and Energy Recovery
The Prêt-à -Loger project treats water as seriously as energy. The plumbing system includes separate fresh water and wastewater tanks, a rainwater recovery system for non-potable uses, and a heat-recovery system that captures warmth from the glass structure and transfers it to heat pumps.
Rainwater Harvesting and Greywater Strategy
Rainwater collected from the glass roof is stored in dedicated tanks and used for toilet flushing and garden irrigation. This reduces demand on municipal water supplies while also managing stormwater runoff a consideration that becomes more important as cities tighten stormwater management requirements for renovation projects.
Heat Recovery from the Buffer Zone
The solar water heating system uses two panels that capture heat accumulating inside the glass structure during sunny periods and transport it to the home’s heat pumps. This integration of passive solar gain with active mechanical systems is a hallmark of the net-zero energy design approach that the team applied throughout the project.
Prefabrication and Applicability for Builders
One of the most practical lessons from Prêt-à -Loger is its reliance on prefabricated components. The glass structure, steel frame, insulation panels, and mechanical systems were all designed for off-site fabrication and rapid on-site assembly. This approach minimized disruption to occupants and reduced construction waste.
What North American Builders Can Learn
Professor van den Dobbelsteen advises that the same basic approach can work for U.S. residences. His two-step framework is worth restating:
- Identify dominant housing typologies with the greatest improvement potential. In the United States, this means postwar suburban homes, older rowhouses in East Coast cities, and the vast stock of pre-1980 housing that lacks modern insulation and air sealing.
- Apply smart and bioclimatic design. Understand local climate conditions, solar orientation, and existing construction methods before proposing interventions. Passive strategies come first, active technology second.
Cost Implications and Market Viability
While the second skin approach requires a higher upfront investment than basic weatherization, the long-term operating savings offset the premium over time. Key factors that improve the business case include:
- Reduced energy bills from near-zero heating and cooling demand
- Increased usable living space from the conditioned buffer zone
- Extended building lifespan through improved moisture management
- Higher property value from verified energy performance
The Prêt-à -Loger project remains on the TU Delft campus as a living laboratory, hosting tours, demonstrations, and ongoing research. It proves that creative retrofit solutions can preserve what makes older homes valuable while eliminating their energy waste. For builders willing to look beyond conventional renovation methods, the second skin concept offers a path toward housing stock that is comfortable, durable, and genuinely sustainable.