Building on a riverbank requires construction techniques that address both flood resilience and energy efficiency. A 250 sqm residence with 1158 cum gross volume on the Danube bank in Regensburg, Germany, demonstrates how passive house design in modern construction can coexist with flood-resistant concrete construction. The project, completed between March 2018 and May 2019 after a planning period from November 2016 to October 2017, uses a windowless waterproof concrete basement to house building services while lifting the main living spaces above the hundred-year high water mark.
Waterproof Concrete Construction and Flood Zone Foundations
The structural concept of the Danube bank residence relies on waterproof concrete forming both the basement enclosure and the visual identity of the building. This aligns with integrated design approaches for sustainable building where structural and environmental requirements shape architectural form simultaneously. The anthracite-colored wall panels lift the building above flood level while the smooth-grey concrete filler facade creates the monolithic appearance.
Waterproof Concrete Mix Design
Waterproof concrete (also called watertight concrete or white tanking) achieves its impermeability through mix design rather than external membranes. Specifications for flood-zone construction include:
- Maximum water-cement ratio of 0.50 to limit capillary pore formation
- Minimum cement content of 350 kg/cum for adequate paste density
- Maximum aggregate size of 20 mm to reduce segregation risk during placement
- Superplasticizer addition to maintain workability without excess water
- Construction joints fitted with hydrophilic waterstops that swell on contact with water
Curing Requirements for Watertight Structures
Proper curing determines whether waterproof concrete performs as designed. The Regensburg project required wet curing for a minimum of 7 days after pour, with temperature maintained between 10 and 30 degrees Celsius. Rapid drying causes shrinkage cracks that compromise watertightness. Covering with wet hessian and polyethylene sheeting, or applying curing compound, prevents moisture loss during the critical hydration period.
| Flood Protection Method | Initial Cost | Maintenance | Flood Height Capacity | Typical Application |
|---|---|---|---|---|
| Waterproof concrete basement | High | Low | 1.0-3.0 m above grade | Permanent flood zones |
| Elevated structure on piers | Medium-High | Low | Variable by design | Coastal flood zones |
| Flood barriers (deployable) | Medium | High | 0.5-2.0 m | Intermittent flooding |
| Dry floodproofing (sealants) | Low-Medium | High | 0.3-0.9 m | Low-risk zones |
Passive House Strategies and Building Envelope Performance
The Regensburg house incorporates multiple passive house strategies despite not being a certified passive building. The high-performance envelope uses Josko one windows and STO Milano concrete plaster, achieving low thermal transmittance. The building services package includes a gas condensing boiler with solar thermal system, KNX building automation, and controlled living space ventilation with heat recovery by F2 Architekten partners, demonstrating integrated mechanical design.
KNX Building Automation for Energy Management
KNX is a globally recognized standard for home and building control that integrates heating, ventilation, lighting, and shading into a single system. In this project, KNX controls:
- Solar thermal system operation based on storage tank temperature and solar gain
- Ventilation heat recovery bypass during summer nights for passive cooling
- Window actuator coordination with the HVAC system to prevent heat loss during open-window operation
- Lighting scheduling to reduce electrical loads during peak demand periods
Solar Thermal Sizing for Residential Applications
A solar thermal system for a 250 sqm residence typically requires 6-10 sqm of collector area and a 400-800 liter storage tank. The system covers 50-65% of annual domestic hot water demand and 15-25% of space heating demand, depending on orientation and local climate. The gas condensing boiler provides backup during periods of low solar gain, achieving combined seasonal efficiency above 90%.
Steel Walkway and Elevated Access Systems
Access to the raised living levels uses a steel walkway with grating covering, echoing the jetty theme of the riverbank setting. This approach to elevated access ensures the house remains accessible from all sides even at high tide. The design follows principles used in heritage-integrated passive house design where circulation routes double as outdoor living spaces.
Steel Grating Walkway Specifications
The walkway comprises galvanized steel grating panels supported on steel beams cantilevered from the concrete basement structure. Key design parameters include:
- Grating panel thickness of 30-40 mm with serrated bearing bars for slip resistance
- Span between supports limited to 1.2-1.5 meters for service load of 4.0 kN/sqm
- Hot-dip galvanizing to BS EN ISO 1461 for corrosion resistance in flood-prone conditions
- South-facing widened section functions as a sun terrace during dry weather
Facade Design for Privacy and Flood Resilience
The building facade balances three competing priorities: flood protection, privacy from adjacent streets, and views toward the Danube. The two residential floors open to publicly visible sides through small windows framed delicately in black, cut in harmonious proportions. Larger glass surfaces face the Danube, including a continuous floor-to-ceiling window front at the northwest corner terrace. This selective glazing strategy reduces both heat loss and visual exposure while maximizing the primary view corridor.
Three-Zone Outdoor Strategy
The design provides three outdoor areas with distinctly different qualities achieved through facade recesses and balcony positioning. A facade recess at the northwest terrace offers both privacy and weather protection simultaneously. A corner seat window in the upper-floor bedroom provides panoramic views of the river without compromising the closed facade treatment on the street side. A balcony with closed parapet shields the entrance while offering views of the old town.
Space Planning Within Tight Building Limits
Despite tight building limits, the house accommodates an above-average space program for a family of five. The ground floor is organized along two visual axes. The first axis extends from the entrance between street-side rooms and the central staircase to the eat-in kitchen and beyond through a floor-to-ceiling window. The second axis runs from the cooking area with kitchen island to the dining area and terrace, opening to the green Danube bank. A flexible room near the entrance serves as either a guest room or an office, demonstrating how multipurpose spaces maximize utility within constrained floor plans.
Multi-Use Room Adaptability
The room immediately next to the entrance is designed for adaptability. As a guest bedroom, it works with the bathroom opposite for single-level living. As an office, it provides visitor-friendly access without disturbing the private areas. The two upper-floor bedrooms each have separate dressing rooms, and two bathrooms plus a utility room complete the upper level. This zoning strategy concentrates daily living on accessible lower floors while reserving quieter sleeping spaces above.