RE-2020 is a legislation introduced by the French government to make buildings Energy Positive & to reduce its Carbon Footprint (Batiment a Energie positive & Reduction Carbon). It stipulates that in order to get a building permit for new construction one need to submit proof that certain energy criteria are met.  For that purpose they have created a number of energy coefficients e.g. Bbio, DH, CEP.

Bbio (Besoin Bioclimatique) is a coefficient that represents the energy efficiency of a building regardless of heating or cooling systems. It entails entities like orientation with regards to the sun, compactness of the bounding surfaces/masses like wall structures, protection against the sun, thermal mass, control of solar gain during all seasons. 

DH (le nombre de Degres Heures) is a measure of comfort over time during which the building is heated up or cooled down based on real weather data and the local climate zone. What is taken into account is how well the building is protected against the sun in terms of window shutters, home automation, exterior insulation, green roof, draft flows, and passive cooling.

CEP (Consommation d'Energie Primaire) is a measure of the building's primary energy consumption (e.g. heaters, aircos). It concerns imported energy only i.e. energy that is bought in like electricity from the utility company. It does not include exported energy like the electricity from domestic solar panels. 

To obtain the Bbio and DH coefficients it is essential to know what the thermal envelope of the building looks like. In general a thermal envelope defines the internal space that is enclosed by the floors, the walls, and the roof of the structure. It consists of materials like wood, insulation, glass, each of which has a thermal mass and heat coefficient (see next section). That combined with the surface area and the volume gives us the data that is needed for the Bbio and DH coefficients.

This project deviates from that in the sense that the upper envelope is defined by the attic floor rather than by the roof itself. So in essence the thermal envelope corresponds to a rectangular box and the roof will act simply as a rainscreen to protect the thermal box below. 

That way the entire floor of the attic can be used to lay down an almost continuous slab of insulation material thereby minimizing the number of thermal breaks (rather than placing insulation material between the rafters).

Minimizing thermal breaks is considered as a positive contribution to both the Bbio as well as the DH coefficient.

An envelope profile is an assembly of parts that make up a floor or wall. The image on the left shows a wall profile that consists of 4 parts/layers. The first 2 grey layers on the outside of the structure represent a slab of staggered insulation (e.g. Rockwool). The third layer represents a substrate of OSB and the fourth layer represents another slab of insulation that is on the inside of the structure. 

What is not shown in this profile is the Water Resistant Board & Air (WRBA) barrier/membrane that sits on the outside of the OSB substrate. This WRBA membrane is a major component of the permeability of the thermal envelope. An additional benefit of having a box shaped envelope is that this WRBA membrane (e.g. Delta Vent SA) can be wrapped around the junction where wall and floor meet. That way one could have a continuous sealed membrane around the substrate of the structure thereby minimizing the number of thermal breaks.

External insulation is considered as a positive contribution to the DH coefficient of the building.

The outside of the thermal envelope has a protective layer against rain, wind, and sun. In the context of this project, the walls are protected by a ventilated rain-screen that consists of panels (e.g. wood, bamboo, rockpanel) and Venetian blinds (volet battants) which sits in front of the windows.  

Behind the rain-screen is an airgap that acts as a air-insulation layer as well as a ventilation layer so that any incoming moisture can be removed.    

The traditional venetian window slats play a role in regulating the incoming thermal radiation of the sun. During the summer it prevents the house from overheating while during the winter it allows the sun to enter and heat the interior space. 

The Venetian blinds also form a passive ventilation system that produces a natural draft through the house, similar as in a traditional Sechoir a Tabac.

Both ventilated rain-screen as well as the Venetian blinds are considered as positive contributions to the DH coefficient of the building. 

Passive ventilation - The rectangular shape of the building lends itself for passive cooling by means of cross ventilation i.e. placing one side of the building perpendicular to the prevalent wind direction, creating an low-pressure interior, and using the slats of the Venetian blinds to control the subsequent natural draft. Additional passive means could be the use of a cooling tower, external foliage to provide shade.   

Active ventilation and heating could be provided by a double flow CMV system (Ventilation mécanique contrôlée). 

A double flow CMV is a ventilation system that captures fresh air through an external air intake and then passes it through a heat exchanger. The purpose of the exchanger is to recover part of the calories before rejecting them.

The air is then distributed to the different rooms through a network of ducts (also called ductwork). To be precise, there are two duct networks, each of which is equipped with an electric fan. The first one is used to blow fresh air into the most occupied rooms. As for the second one, it is used to extract the polluted air from the service or wet rooms (wc, bathroom, kitchen).

Thanks to these components, the double flow CMV ensures the extraction of stale air and the supply of fresh air into the house. Additionally, it proceeds to filter the incoming air and preheats it in the event of a drop in outside temperature.