Folded roof house—Design solution to optimize environmental conditions with generic equipment and techniques

Abstract

We covered a rectangular site in a residential area oriented at a 45° angle to the north and rotated the roof of the second floor on top of the rectangular first floor; then, we created a large window on the side at the south corner. To achieve an open interior with a limited footprint, we created a single‐volume space connecting the first floor to the second floor with stairs. The timber frame of the second floor sits on the concrete box of the first floor. The stiff timber box consists of a folded roof and exterior walls to achieve a large pillar‐free space. We set the main windows higher to maximize the effect of natural light, placed eaves in front of the large window to block direct sunlight, and used slit windows between the first and second floors to promote natural lighting and ventilation.

Introduction

Specializing in architectural environmental engineering, we present a small house for a family of four. The primary design objective for this house was to optimize the indoor environmental quality within a limited site space and budget. Along with the social trend of sustainable and energy efficient private houses in Japan, a relatively high percentage of new houses has been equipped with sophisticated new technologies and energy creation systems. However, these systems are generally costly, and the design of houses is not harmonized with these technologies. We believe that the “folded roof house” is a design solution that optimizes pleasantness, comfort, and environmental conditions while using generic equipment and techniques; additionally, it achieves an architectural expression together with technologies. Figure  shows an exterior aerial view of our folded roof house in Kasuga.

Overall exterior aerial view of a folded roof house in Kasuga

Architectural concept Site planning

The single-family house is located in a suburban residential area where many private houses are situated side-by-side on similar housing lots (Figure ). The rectangular lot, which is 176.7 m², is oriented approximately 45° to the north. Because the carport, which has a depth of approximately 5.7 m, needs to be along the street, the house must be situated very close to the southern boundary, just a step behind the neighboring houses. We considered and implemented a solution involving the rotation of the second floor by 45° on top of the first floor to open a large window at a higher level facing south (Figures  and ).

Overall view from the street

Site plan

Roof study model

Floor planning

We produced a single continuous interior volume to create an open indoor environment with a limited footprint of 79.2 m² and accommodate different living spaces for the daily life of the family. The floor of the main living space was raised approximately 800 mm from the entrance and the ground floor level, on which a bathroom, toilet and kitchen are located. A large and symbolic dining table as the meeting place of the family is located next to the kitchen, and its height is just 300 mm above the kitchen counter top (Figure ). The main living space is connected to the second floor with large steps that can act as benches or shelves. The second floor is divided by furniture into two spaces for bedrooms. These different levels help maintain a distance between various places in the volume (Figures ).

Overall interior view

1F and 2F plan

1F corridor view

2F bedroom view

Structure system and roof construction

The foundation and exterior walls of the first floor are made of 250 mm thick concrete, while the structure of the second floor and other components are all made of 120 mm timber materials, which are prevalent on the market. We also used conventional techniques and generic steel joints for timber construction, which a local carpenter can afford. The stiff timber box structure, which consists of the folded roof panels and the exterior wall panels, achieves a pillar-free space of 7.7 m × 10.2 m. While we set the section of the folded roof at a right angle to make the construction easier, the whole roof slants approximately 10° for drainage and scale adjustment of the interior space. There were some joints with unusual angles at the meeting points of the folded roof and walls, and we made 3D models and drawings to explain them for carpenters (Figures  and ).

Structure concept diagram

Timber frame construction

Natural lighting and ventilation

We designed the house to have sufficient natural lighting and ventilation. Meanwhile, the area of the window surface was minimized to reduce thermal loss. We set four main windows at a higher level to maximize the natural lighting effect, from which the light comes in along the pitched ridges of the folded roof. The large window has eaves to block direct sunlight during summer and two ventilation windows lit from both sides of the central ridge, and the small triangular skylight facing north is at the peak of the roof (Figures  and ). The second-floor timber box was set on arms popping out from the first-floor concrete box. We used the slit between the two boxes as horizontal windows. Ventilation windows perforated the house in many directions and at different levels; this was effective at not only exploiting various wind directions but also realizing ventilation caused by temperature differences (Figure ).

Living room view

Triangular skylight

Horizontal slit-type windows for natural ventilation and ambient daylight

Insulation and air conditioning

We set the insulation specifications to optimize thermal performance. The first-floor concrete was covered with insulation and worked as heat storage to stabilize the interior thermal conditions. The second-floor timber panels were filled with onsite forming insulation to prevent heat bridges and condensation. It is generally difficult to achieve an effective heating system for an interior volume with a high ceiling during winter. The underfloor heating system consists of an energy efficient air-conditioning unit, and a circulation fan keeps the first floor warm. The air flow along the folded roof created by the circulator fan minimizes the non-uniformity of air temperature in the interior space. Because of the exterior heat-insulated concrete wall, which has a large heat capacity, and air-conditioning system beneath the floor, it is possible to maintain comfortable indoor thermal conditions all year-round (Figure ).

Section/environmental design concept

Conditions of sustainability

Owing to these environmental design solutions, the observed data showed mostly expected results with regard to energy consumption. If solar cell panels are introduced on the roof in the future, the house will be a zero-energy building. The introduced generic technologies and construction systems were quite effective to keep the building cost reasonable. However, sustainable building issues are not dependent only on technologies. Although the house looks quite closed from the street, the inhabitants can see the sky and truly experience the outside environment. The distinctive folded roof, altered light conditions, and stepped floor levels differentiate the character of each place in the continuous interior space. These places can be used flexibly according to future changes in the lifestyle of the family. It is crucially important for a house to be sustainable to stay attractive to residents. We generally expect the house to accommodate their happy and comfortable life for longer period.

Engineering concept

The design of a personal residential house from an engineering point of view might be similar to school education. The success or failure of education becomes apparent by degrees obtained over a long period of time and has a dominant influence on life. Individual abilities and demands are diverse, but there is also an averaged-common basis to adapt to society and culture. In this regard, failure is not permitted in school education. It becomes difficult to challenge existing experiences and standardized theories of school education. The design of a personal residential house, especially an environmental design, may be the same.

A house, along with the family living there, exists on the same time scale as life. Introducing state-of-the-art environmental control systems and equipment can improve the initial environmental performance, but these technologies may become obsolete within a few years and may need to be replaced with new state-of-the-art technology. The basic strategy of a small residential house design may be to adopt a reasonable structure on firm ground, reduce the external heat load by improving the thermal insulation and airtightness, and utilize sunlight, outdoor air, and groundwater as necessary. In subsequent phases, we should consider introducing reasonable and efficient active-type environmental facilities from a comprehensive viewpoint.

The owner, who is an expert in environmental design, as well as heating, ventilation and air-conditioning design, shares in this belief.

On the basis of this engineering design concept, we introduced the following environmental control devices/methods. (i) Utilization of well water; (ii) Introduction of horizontal slit-type windows for natural ventilation and ambient daylight; (iii) Materials-integrated photocatalytic oxidation reaction activated under visible light; (iv) Floor heating system with an underfloor chamber using a heat pump-type air conditioner; (v) Air circulator fan on the ceiling to manage heterogeneous air and temperature distributions; and (vi) Exterior heat insulation finish.

The following performances were analyzed with environmental simulation techniques: (i) Optimization of the window opening position by solar radiation and indoor light environment analysis; (ii) Natural/cross-ventilation design supported by computational fluid dynamics (CFD) simulation; (iii) Optimization of opening position of horizontal slit-type windows using wind pressure coefficient analysis with CFD; and (iv) Annual heat load calculations to optimize the thermal insulation and natural/mechanical ventilation.

Since the numerical simulation/analysis tools provide deterministic information for environmental conditions, the flexibility of design might decrease greatly when aiming at environmental optimization. Against this trade-off problem, collaboration between the architect, environmental engineer, and resident, from the design stage until completion, enables harmonization of architectural design and the environment.

Conclusion

For this residential house, the indoor air/thermal environment and utility demands (e.g., electricity and tap water usage) have been continuously measured immediately after the families moved in. The owner, an expert on environmental engineering, has been investigating ways of environmentally optimizing the lifestyle of the family. Continuous efforts to optimize the indoor environmental quality immediately after the completion of construction make this residential house pleasant.

Project data

• Main structure: Reinforced concrete (RC) structure + Wooden (in part)
• Site area: 176.76 m²
• Building area: 79.20 m²
• Gross floor area: 102.49 m²
• Completion: August 2015
• Location: Kasuga-shi, Fukuoka
• Construction company: Kawakita Construction, Co. Ltd.

Disclosure

The authors have no conflict of interest to declare.