This is a brief description of a prize-winning entry at the

'Ecological Dwelling Ideas' competition

sponsored by the Greek Ministry of Environment, Planning & Public Works
in April 1999




General features General features
Energy data Energy data
Solar data Solar data
Solar roof Solar roof
Storage Storage
Distribution Distribution
Summer features Summer
Conclusions Conclusions
Solar study Solar study
Drawings Drawings
e-mail e-mail
The proposal refers to a solar heating system implemented on a detached house near Athens. Solar energy is collected by the metal roof which is utilized as an air collector. The warm air enters indoors through the floor, warming the structural mass for night use. The same route at the opposite direction allows for summer cooling. The system is based on a simple air handling unit that includes a blower & twin dampers.

P l e a s e   n o t e :
All competition entries will be displayed in Zappeion between 9-19/12/99.

An oral introduction by the designers will be held in the exhibition area on Wednesday,
15/12, 12:00-14:30.

A special catalogue will be available during the event.

This page has been edited with Netscape Composer & MS FrontPage 2000 for a 1024x768 display
Last edited 18.07.01
Fonts enlarged 28.6.03
General features Top
This proposal refers to a real detached house in Loutsa, Attica, whose construction was suspended at an early stage.
The building accommodates a common architectural brief (parents with two children); it consists of a basement and two floors on a square plot of negligible slope.
The scheme has been based on fundamental principles of environmental design.
  • The main volume of the building is a compact block to reduce the surface to volume ratio (F/V).
  • The building is placed adjacent the very northern border of the plot to maximize solar access of the southern elevation -and for better use of the unbuilt area.
  • The enclosed spaces are aligned with the contour of the plot, while the features related to the sun (windows & overhangs) adhere to the orientation.
  • Most of the openings are on the southern side, while the northern one is just a blind party wall.
  • Summer shading of the southern windows is achieved by a large covered verandah and a local recess of the elevation, while small overhangs protect the windows on the eastern and western sides. Most of the openings are protected by rolling shutters too.
  • The covered verandah of the ground floor can easily be converted into a sunspace by the addition of sliding glass panels.
The materials and method of construction are conventional:
  • Reinforced concrete frame insulated with extruded polystyrene sheets,
  • Masonry of solid insulating bricks,
  • Insulating plaster.
The only feature that is not common in Greek dwellings is the corrugated metal roof, which is the key element of the design.
Energy data Top
Envelope insulation data
  A U-value A*U
  (m2) (W/m2K) (W/K)
212 0.664 141
37 3.732 139
74 0.611 45
72 0.782 57
Total envelope
396 0.870 345
Heated volume (m3)
Surface to Volume ratio

Daily heat loss from fabric & ventilation (at 1 ach) is 12.06 kWh/K.
Part of the losses is offset by internal gains (people & appliances), assumed at 13.5 kWh daily.

Monthly heat losses & heating needs for Ti=18oC (kWh)
1 2 3 4 5 .. 10 11 12
264 224 196 85 10   29 96 206
Heat losses
3183 2701 2363 1025 121   350 1157 2484
Internal gains
419 378 419 405 419   419 405 419
Heating required
2765 2323 1945 620 -298   -69 752 2065

The required heat is supplied by a conventional under-floor water heating system fuelled by oil.
Additionally, a thermodynamic fireplace warms indoor air.
In practice these mechanisms will add up to the solar roof air heating system during long overcast & cold periods.

Solar heating data Top
The table below summarizes horizontal irradiation & heating needs monthly data in Athens. It is clear that the solar energy incident upon the roof -assumed horizontal- exceeds heat losses of the building. Therefore a solar system that utilizes the roof as a collector can -in theory- cover all heating needs, even without solar gains from the rest of the building envelope.

In reality this is not so because:

  • The performance of solar systems is far less than 100%
  • Solar irradiance is not constant, having periods of reduced or even zero intensity (overcast sky or night)
  • Heating needs vary due to ambient temperature fluctuations.

But even if the hypothetical solar system has a performance coefficient of only 20%, the energy that can provide is enough to reduce the auxiliary heating period from 6 to 3 months (December to February, plus a small amount in March).


Monthly solar irradiation & energy contribution by solar roof (kWh)

1 2 3 4 5 .. 10 11 12
Horizontal solar irradiance (kWh/sqm)
54 73 118 154 199   105 70 53
Irradiation on roof
4024 5429 8763 11431 14698   7761 5170 3886
Energy input by roof (20%)
805 1086 1753 2286 2940   1552 1034 777
Heating needs
without solar roof 
2765 2323 1945 620 -298   -69 752 2065
with solar roof
1960 1237 192 -1666 -3237   -1621 -282 1288
Heating needs with [red] & without [black] solar roof (kWh)
Solar roof Top
The metal roof of the dwelling is used as a solar collector for ventilation preheat.
The roof consists of two metal trusses covered by two layers of corrugated metal sheets, with slabs of extruded polystyrene between them. The corrugations of the lower metal sheets act as structural elements bridging the trusses, whilst those of the top layer function as air ducts.

Solar roof cross section

  • The solar rays [1] heat up the top metal sheets [2], warming the air gap below.
  • The warm air rises [3] towards the ridge, heated further through a transparent cover [4], then is collected in the air tank [5].
  • Fresh air enters the gap [6], repeating the cycle.
  • The air handling unit [7] propels warm air through vertical pipes [8] into a horizontal channel across the edge of the floor [9].
  • The air travels on the concrete floor to the opposite side of the room via the corrugations of embedded metal sheets [10] , then exits through an outlet [11] to the room.
  • According to temperature conditions, the warm air can be sent outdoors through a top vent outlet [12].

Storage Top
The warm air travels across the concrete floor through embedded ducts. These are formed by corrugated metal sheets 5 cm thick placed over the bare concrete slab. The sheets are covered by lightweight concrete (which contains the conventional central heating water pipes).
The 20x5 cm channels between the metal sheets & the wall at each end connect all the corrugations at each end.
The warm air enters one connecting channel, travels through the corrugations warming the concrete mass and exits to the opposite channel.

Air flow through embedded corrugations


For an even distribution of the heat across the whole floor, the air follows a zig-zag path, formed by blocking the channels at selected spots with curved metal strips or mortar.


Connecting channels detail

Distribution Top
Following its travel via the floor, the warm air finally exits into the inner space through outlets above or below the floor slab. The first option is better in terms of air flow in the room, but it is usually obstructed by furniture; the second option facilitates the extraction of warm air during summer.
An essential part of the system is the air handling unit, located in the air tank. It consists of a blower that drives air between two metal boxes, always at the same direction.
The first box is connected to the embedded air ducts & the ambient atmosphere and the second with the air tank & the indoor air; the connections are materialized by vertical PVC or metal pipes of 10-14 cm in diameter.
Each box contains a damper moving in two positions, thus providing four alternative air routes.
The first damper moves twice a year (winter-summer) and the other one daily (day-night). This is done manually, via a long chain, or automatically, by a small motor with a thermostatic switch.
In a hybrid version of the system, a heat coil between the boxes can adjust the air temperature whenever necessary.

Air handling unit operation modes

The warm air is propelled from the tank into the floor where it deposits heat before entering the inner space.

The warm air of the tank escapes outdoors either by natural draught or assisted by the blower, sucking more air through the floor on its way.
The air in the tank is cool & idle; warmer air is sucked from the top of the inner space, then it is blown through the floor from where it extracts heat stored during the daytime.

The warm indoor air escapes outdoors either by natural draught or assisted by the blower, drawing air & heat from the floor on its way.
Summer features Top
Summer cooling is achieved by shading via overhangs and shutters, as well as by ventilation.
  • Protection from direct rays is adequate, as demonstrated by solar simulation. But due to the high diffuse and reflected radiation it is necessary to use rolling shutters with ventilation slots.
  • Part of the heat absorbed by the mass of the building is removed by night ventilation:

  • The warm air rises up to the large void over the ground floor. The resulting stack effect facilitates the removal of the air through the roof and through the large southern window of the top floor.
    Structural cooling is achieved be sucking air through the embedded ducts as described above.
For watering the garden in summer, the basement contains a cistern where rainwater is collected. The system includes an electric pump in the cistern and a pressure tank in the attic.
Conclusions Top
The solar roof system offers the following advantages
  • Favourable solar access conditions.
    • The roof offers a quite large collecting surface, with no shading problem. Due to its low slope, the influence of orientation is negligible.
      Solar irradiation on a south facing vertical plane is considered higher than on a horizontal one.
      This is true in winter and under clear sky (under overcast sky the horizontal irradiance exceeds all other directions), and also assuming unobstructed solar access.
      In practice the difference is reduced due to the unavoidable shading by adjacent buildings, especially in dense or high rise areas.
      Furthermore, the size and performance of south collecting surfaces is restricted by functional & other factors which do not apply to the roof.
      Therefore, in spite of the lower solar irradiance, the roof can provide more irradiation due to its lack of shading and large size.
  • Construction savings.
    • The proposal utilizes the roof, a standard element in all buildings, using cheaper materials than usually (metal sheets instead of roof tiles). The air tank is in fact just the attic, the vertical air ducts are of simple and cheap materials, and so is the embedded duct network.
      Actually the only special component is the air handling unit, but even this is fairly simple technology.
      The complexity increases if we attach thermostats & automatic damper switches, heat exchanger for additional heating and/or cooling, filters etc. Such a hybrid air heating system can completely replace the conventional under-floor water heating system which is suggested here as a safeguard against particularly cold & cloudy periods.
  • Use of air as heat carrier.
    • The air requires no special piping, presents no hazard from potential leaks and has no corrosive properties.
  • Independence from building.
    • The solar roof and air tank are separated from the rest of the building, therefore, in case of  failure,  the system can be shut down without negative effects indoors; this is not the case with the usual means of passive heating (glazing & conservatories).
  • Uniform heat distribution.
    • The system heats the indoor air as well as the building mass, thus providing optimum thermal comfort conditions.
  • Use in summer.
    • The system is intended mainly for winter heating, but it may also be used for summer cooling, accelerating the removal of warm air from the interior and of heat from the structural mass.
The main drawback of the proposal is that -being an untried innovation- its performance & detailing are unclear.
However, the fact remains that the anticipated energy saving is significant, with a small construction expenditure, minimal operational cost and negligible maintenance.
Solar study Top
Winter solar access & summer shading were tested through solar simulation for 38N latitude, based on hourly solar views during Summer & WinterSolstice and Spring Equinox.
The model was projected along the solar rays with a LISP routine developed by the author.
In each view, all the hidden parts of the building are shaded.
This type of study excludes diffuse & reflected irradiation and does not provide energy data.
Selected solar views
9:00 12:00 15:00
Drawings Top
Click on each thumbnail for a 1600x1200 JPG version (about 100kB each)



Ground floor

Top floor











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