Planning Air Conditioning for a Roof Addition in Residential Construction
New Construction Meets Existing Structures
Professional article 01.10.2026When new apartments are built within an existing building, the new construction and the existing structure come into direct contact with one another. For specialist planners, architectural firms, housing companies, and project developers, this means that air conditioning cannot be an afterthought. It plays a key role in determining whether the new living space can be used comfortably, efficiently, and reliably over the long term.
The focus is on four questions: How is the building heated? How is it cooled in the summer? How is the necessary air exchange ensured? And where can outdoor and indoor units be integrated in such a way that technology, noise, maintenance, and architecture all work together harmoniously?
Why Roof Extensions Pose Special Challenges
A roof addition makes use of existing building structures. This is precisely where the planning challenge lies. Structural engineering, utility routing, the existing heating system, sound insulation, utility spaces, and the provision of services to the existing apartments set the framework. At the same time, new, well-insulated rooms are created with their own requirements for heating, cooling, and fresh air.
Summer comfort, in particular, deserves early attention. Roof surfaces, new facade sections, and larger window areas can cause significant heat gain. Insulation, shading, and window design remain the foundation. In addition, active cooling may be necessary if the apartments are to remain comfortable even on hot days.
- Planning Heating, Cooling, and Ventilation Together
- Which systems are suitable for roof extensions
- Air-to-Air Heat Pumps
- Reference: Dinslaken
- Air-to-Water Heat Pumps
- Reference: Bilker Bunker
- Don't Treat Ventilation as a Minor Issue
- Air-to-air or air-to-water
- Conclusion
- In a nutshell: Air Conditioning for Roof Extensions
Planning Heating, Cooling, and Ventilation Together
PlanningWhen deciding on a system, it is not enough to simply consider the heating load. Roof extensions require a comprehensive building services plan. The following points should be clarified before selecting a product:
- Utility Supply Plan: Will the new apartments have their own independent utility supply, or will they be integrated into an existing system?
- Cooling Concept: Are structural measures sufficient, or is active cooling required?
- Ventilation Concept: Does a centralized or decentralized ventilation system make sense, for example, due to an airtight building envelope, an interior location, or the building's usage profile?
- Installation: Where should outdoor and indoor units be placed to ensure optimal technical, visual, and acoustic performance?
- Route Planning: What routes are possible without placing an unnecessary burden on existing infrastructure?
- Policy: Do individual rooms or apartments need to be controlled and billed separately?
- Maintenance: Are all components easily accessible during operation?
This early coordination is particularly worthwhile when adding stories to a building. When architectural and building systems designs are planned together, it’s easier to integrate utility spaces, reduce the length of piping and wiring, and carry out future maintenance work more easily.
Which systems are suitable for roof extensions
Roof ExtensionsThe right solution depends on the building, its use, and the planned heat transfer. In practice, three system approaches are the most common.
Air-to-Air Heat Pumps
Air-to-Air Heat PumpsDirect Room Heating for New Apartments
Air-to-air heat pumps are particularly well-suited for directly heating and cooling rooms. Mitsubishi Electric’s M Series is designed for small to medium-sized rooms and covers a capacity range of 1.5 to 18.0 kW for cooling and heating. Depending on the unit type, additional comfort features are available, such as quiet operation, filter functions, or sensors for adjusting airflow.
For roof-top additions, this system approach is particularly advantageous when a water-based heat distribution system is not to be installed, or when the new units need to operate as independently as possible from the existing building. In multisplit applications, multiple indoor units can be connected to a single outdoor unit. This allows for temperature control in different rooms within a unit without having to install a separate outdoor unit for each room.
Reference: Dinslaken
Reference: Dinslaken
New Living Space with Air-to-Air Heat Pumps
The project in Dinslaken demonstrates how a roof addition and decentralized room climate control can be combined. There, an apartment building dating from 1954 was expanded to include eight new apartments. At the same time, the existing apartments were renovated and their floor space was increased.
Mitsubishi Electric’s M-Series air-to-air heat pumps are being used as a complete heating solution in the new apartments. In the existing apartments, reversible air-to-air heat pumps supplement the existing radiators. In this way, the project meets several requirements: additional living space, more efficient heating, and cooling in the summer.
For planners, the most interesting aspect of this is the transferable logic: If an addition needs to be built quickly, if changes to the existing apartments must be kept to a minimum, and if the new units are to be heated or cooled on a room-by-room basis, an air-to-air heat pump can be a suitable solution.
Air-to-Water Heat Pumps
Air-to-Water Heat PumpsWater-based solution for larger projects
An air-to-water heat pump is a good choice when heat or cooling is distributed via underfloor heating, radiators, or other water-based systems. This approach is particularly well-suited for larger areas, clearly planned hydraulic systems, and projects in which multiple building zones are supplied via cascades.
Depending on their design, reversible air-to-water heat pumps can not only heat but also cool. In underfloor heating systems, cool water is circulated through the system for this purpose. This reduces the heat load in the room and can significantly improve comfort during the summer. Proper planning of heat transfer, control, dew point monitoring, and operating mode is crucial.
Reference: Bilker Bunker
Reference: Bilker Bunker
Heating and Cooling Using Ecodan Cascades
The Bilker Bunker in Düsseldorf offers a new perspective on rooftop additions. Five duplex apartments with a total of 670 m² of DIN-compliant living space were built on the roof of the former air-raid shelter. The apartments are heated via underfloor heating and can be cooled in the summer using the same systems.
Two reversible Ecodan air-to-water heat pumps, configured as a two-unit cascade, provide heating and cooling for these apartments. When outdoor temperatures are low, the outdoor units supply heat to the floor heating system. In the summer, cool water can be circulated through the floor heating system to remove heat from the rooms.
The project also demonstrates how important the placement of outdoor units is. At the Bilker Bunker, the units were installed on the exterior facade due to the inner-city setting. The locations are accessible for maintenance work, provide good air circulation, and have been integrated into the project both visually and acoustically.
Don't Treat Ventilation as a Minor Issue
VentilationThe more densely new apartments are built, the more important a robust ventilation system becomes. Window ventilation alone is not sufficient in many projects because it depends on how the space is used and can introduce additional heat loads in the summer. Mechanical ventilation ensures a predictable air exchange rate and can reduce the load on the HVAC system.
Mitsubishi Electric's Lossnay ventilation systems supply fresh air and utilize heat recovery. For residential buildings, both centralized and decentralized solutions are available, depending on the project. Centralized systems are particularly suitable for new construction or extensive renovations. Decentralized units can be used to supplement individual rooms, apartments, or homes and can also be flexibly incorporated into modernization projects.
For roof extensions, this means that ventilation should be planned in conjunction with heating and cooling. This allows for air flow rates, heat and moisture recovery, filtration, sound, and maintenance to be taken into account from the very beginning.
Air-to-air or air-to-water
Air-to-air or air-to-waterHow is the decision made?
The decision is not based on the product itself, but on the building. An air-to-air heat pump is particularly suitable when individual rooms need to be heated or cooled directly, when short construction times are a priority, and when no water-based heat distribution system is planned.
An air-to-water heat pump is a suitable choice when underfloor heating, radiators, or convectors are planned, and when multiple units or building zones are connected hydraulically. Cascade solutions expand the modulation range, increase operational reliability, and facilitate adaptation to varying load conditions.
In complex existing buildings, a combination of approaches may also be advisable. Dinslaken and the Bilker Bunker illustrate two different approaches: direct room temperature control using air-to-air heat pumps and a water-based system using air-to-water heat pumps. Both approaches can provide both heating and cooling. The appropriate approach depends on the floor area, building use, building envelope, piping routes, sound insulation, and desired comfort level.
Conclusion
ConclusionA roof addition requires air conditioning as part of the overall design
Roof extensions create additional living space without requiring new construction sites. For this living space to be practical in everyday life, building systems must do more than just provide heat in the winter. They must ensure comfort in the summer, integrate seamlessly into the existing structure, operate quietly, remain easily accessible, and function reliably.
The Dinslaken and Bilker Bunker projects demonstrate two viable planning approaches. In Dinslaken, the focus is on direct space heating and cooling using air-to-air heat pumps. At the Bilker Bunker, a water-based Ecodan system handles the heating and cooling of the new apartments. This does not impose an “either/or” rule on the planning process. The key factor is which system is best suited to the building, its intended use, and the desired level of comfort.
In a nutshell: Air Conditioning for Roof Extensions
FAQReversible heat pump solutions are particularly well-suited for roof extensions. Air-to-air heat pumps heat rooms directly. Air-to-water heat pumps are especially well-suited for water-based systems such as radiant floor heating, radiators, or convectors.
New penthouse apartments are more affected by solar radiation and summer heat loads than many existing apartments. For this reason, cooling, shading, the building envelope, and ventilation should be planned together at an early stage.
An air-to-air heat pump is a good choice when individual rooms need to be heated and cooled directly and a water-based heating distribution system is not being installed. This can offer advantages in compact attic apartments and when construction time is short.
An air-to-water heat pump is a good choice when heat or cooling is distributed via water-based systems. This is often the case with larger building additions, underfloor heating systems, or more complex building designs.
Ventilation ensures a predictable air exchange rate and can reduce the load on heating and cooling systems through heat recovery. For roof extensions, it should therefore be planned in conjunction with the heating and cooling systems.
