A guide to conceive a bioclimatic house

A guide to conceive a bioclimatic house

 

A bioclimatic house is a building designed in a way that allows occupants to benefit from natural advantages of its site. This leads to maximum thermal comfort and energy savings.

Bioclimatic design is the first step towards having a passive house (heating energy consumption beneath 15kWh/m²/year).

Residential buildings are responsible for 26% of the total French end-use energy consumption (2015, ADEME). This is more than the energy consumption of the industrial sector and almost as much as for transportation.  When constructing new buildings, the potential for energy savings and therefore limiting greenhouse gas emissions is very high.  These savings also translate to financial savings.

What you can do:

  • Take advantage of the sun’s heat
  • Avoid overheating
  • Keep the heat

TAKE ADVANTAGE OF THE SUN’S HEAT

Orientation of windows

A bioclimatic house’s essential characteristic are well-oriented windows. Windows are both, a source of heat gain (and of light) and of heat loss. Indeed, glazings and window frames are less insulating than walls. Therefore, it is crucial to properly position and quantify windows.

In the Northern hemisphere, walls oriented towards the south are those benefitting the most from the sun (towards the north in the southern hemisphere). Therefore, positioning windows on the southern side of the building helps letting in the sun to provide natural heating and light.

The sun rises in the east and sets in the west. Windows oriented towards the east and the west provide heat especially in summer. Thus, it is better to avoid excessively large windows to the east and west where the sun is too low to be blocked by overhangs above.

Putting too many windows towards the north is unwise as the yielded solar gains would be widely surpassed by the heat loss through them in winter.

It is recommended to have a total window surface equal to 20% of the living area, distributed as follows : 50% to the south, between 20 and 30% to the east, 15% to the west and between 0 and 10% to the north.

For buildings in warm climates such as in the south of France, it is better to limit the windows to the west (0 to 10%) and to add some to the north (10 to 20%).

 

Indoor layout

To maximize heat gains from the sun, the optimal indoor layout is as follows: rooms not requiring large heat gains at the north (corridors, staircases, hallway) and living spaces to the south, east or west (living room, kitchen, bedrooms).

AVOID OVERHEATING 

Excessive solar gains can lead to overheating in summer, especially in warm regions. As a result, it is important to plan the installation of shading devices:

  • Overhangs: for a window towards the south, the ideal length of a overhang is half the length between the bottom of the window and the bottom of the overhang.

 

Overhang length

  • External venetian blinds: to install on west and east windows. Overhangs are not sufficient as solar rays arrive horizontally and pass under the overhangs.
  • Vegetation: well-chosen and well-placed trees can help reduce solar gains in summer and let in solar rays in winter. Example: deciduous trees to the south and evergreen trees to the north and east. Trees can also reduce the outside temperature by 1-2°C and create a microclimate around themselves.

Windows’ positions relative to the wall can also help monitor solar gains. Windows aligned with the inside of the wall are better protected from the sun than windows aligned with the outside of the wall.

>> For a passive house:
It is important that the number of hours with an inside temperature over 25°C does not exceed 10% of the year (in the case of a house without a cooling system).

KEEP THE HEAT

Compact building shape

Building compactness is an important element in avoiding heat losses through walls. The more compact the shape of a building is (block shaped), the lesser the heat losses. When designing your house, think of minimizing the ratio between exterior surface and indoor volume of the house. The ideal would be a cube shaped house.

Airtightness

A house can keep the heat inside if it does not let it escape through holes in the building’s envelope. A lot of precaution is necessary when assembling the house to avoid any holes in corners, near window frames, around the passing of wires and pipes… The building’s leakage rate is measured during an infiltrometry test when the structure is finished so that faults in the envelope can be rectified. The French thermal regulation (RT 2012) requires the air leakage rate to stay below 0,60 m3/h.m² for a single-family home.

>>For a passive house:
A house can be qualified as passive if the air changes does not exceed 0.6 vol/h under a 50 Pascals pressure differential

Windows

  • Airtightness

The major part of heat losses are caused by window frames. It is crucial to choose windows that do not break the structure’s airtightness. As a result, it is better to install casement windows rather than sliding ones.

  • Energy efficiency

Heat losses through windows can also be reduced by choosing high energy efficient windows. The windows’ heat transfer coefficient (U-value) should not exceed 1.2 W/m².K.

>> For a passive house:
A house has a higher chance of attaining the passive house level if its windows’ heat transfer coefficient is below 1.1 W/m².K. The window frame also plays its role. Choose windows with less frame surfaces and one glass pane rather than 2 smaller ones.

Thermal inertia

PopUp House’s structure is composed of light materials. Thermal inertia is therefore lower than what is generally encountered in buildings.

Despite this low inertia, summer comfort can be guaranteed. The dephasing of walls takes approximately 9 hours, which is a comparable value to the one for concrete buildings.

Low heat storage has several advantages :

  • It allows to reduce the period of natural nighttime ventilation to cool the house
  • In a PopUp House walls and inside air are at the same temperature. This has a positive impact on comfort.
  • This temperature uniformity enables the reduction of the heating temperature and therefore generates savings while maintaining comfort (1°C less = 10% reduction in heat consumption).

The main element in keeping a high comfort-level in summer is to properly manage solar gains with hoverhangs and blinds.

Ventilation

Once it is warm inside heat has to be well distributed among the rooms. Controlled Mechanical Ventilation (CMV) plays an essential role for low-energy houses. To keep the energy required for heating low, it is recommended to install a ventilation system with heat recovery (dual-flow ventilation). This system uses the heat (or coolness) of the exiting air to increase (or reduce) the temperature of the air coming in. Consequently dual-flow ventilation systems:

  • Minimize the heating and cooling needs
  • Reduce the power rate of heating and cooling systems installed in the house
  • Improve indoor air quality (better air flow rate than simple-flow ventilation)

 

A dual-flow ventilation system does not prevent you from opening windows. A natural nighttime ventilation (opening windows at night and opened CMV bypass) is very efficient to renew indoor air and cool down the house in summer.

>> For a passive house:
A dual-flow ventilation system is mandatory to attain the passive house level. Ground-coupled heat exchangers have the capacity to reduce heating and cooling needs even more.

Heating

A heating equipment in bioclimatic houses is useful for supplemental heating but is still necessary. In the case of a PopUp House, inertia is low. As a result, the chosen heating and cooling systems have to be sufficiently responsive to changes in temperature. When the heating system is well-managed, this responsiveness leads to energy savings.

In regions with warm winters, a heat pump (for heating) with a dual-flow ventilation system can be enough to maintain thermal comfort and minimum consumption.

Keep in mind that the best way to choose the right equipments for your new home is to conduct a complete thermal study.  A thermal dynamic simulation enables you to know the thermal behavior of your house precisely and to anticipate its summer and winter energy needs.

Indeed, low-energy buildings react differently than more traditional buildings. In order to design and build energy-efficient buildings, all the building experts who met in Marseille in September 2016 recommended to conduct a thermal dynamic simulation.