The role of the building envelope is to protect the occupants from the external environment and to provide thermal comfort to them. Unfortunately, the natural external elements (sun, wind, rain, snow, etc.) as well as the pollution affect the envelope and cause, with time, the deterioration of the components of this envelope.
Building envelope and theat transfer
When we talk about the building envelope, we designate all the walls that separate the building from the outside environment. It is not only the exterior facades (walls, siding, doors and windows, ceilings on the upper floor), but also the roof, the slab on the ground and the walls of foundations.
Moisture problem in the envelope
An intrinsic enemy can also attack the envelope from the inside and cause premature deterioration of the envelope, namely the condensation inside that envelope. The condensation phenomenon in the building envelope can be caused by the diffusion of water vapor into the warm air, indoors, or by air leaks and the heat transfer à through the envelope.
Water vapor and air leaks
There are therefore two causes that explain the presence of moisture in the building envelope, namely: 1) the diffusion of water vapor and 2) air leakage.
- La water Vapour in the hot air (inside the building), which passes through the envelope, will condense and turn into water droplets as the temperature gradually cools, in the envelope, through the layers towards the 'outside. By definition, water vapor is moisture in the air as a gas. Water vapor that exceeds the dew point (or saturation) turns into liquid, this is called condensation. Thus, the installation of a vapor barrier is necessary to prevent water vapor from passing through the building envelope.
- Moreover, the air leaks in the envelope convey water vapor to the interior of the structure also causing the condensation. For this reason, the installation of a continuous air barrier is essential to prevent air movement and avoid condensation in the building envelope.
In the building, during the winter, the warm air rises to the upper floor by the chimney effect (pulling effect) that occurs because of the movement of warm air that is lighter. This movement creates a negative pressure - on the lower floors - that draws cold outside air into the building. This is why this situation produces, in the lower part of the building, air infiltration coming from the outside towards the inside.
Thus, warm, moist air rises to the upper floor creating a high pressure that pushes warm air out through the casing. To balance the total pressure in the building, warm, moist air passes through the structure to the outside. In this structure, the temperature gradually drops, the closer you get to the outside, and the water vapor (in the hot air) condenses, because the air cools. If condensation of water vapor enters the structure (dew point), this will cause premature deterioration of the structure. Normally, condensation and decay of the structure are more common in the upper parts of the building, where warm, moist air accumulates.
To illustrate this point, take the example of buildings built before the 60 years. In these cases, the mechanical detachments of the bricks are more concentrated (bellies) in the upper parts of the external walls. These walls are made of planks and nails fixing the bricks with planks. Hot, moist air flows through the log walls to the outside, in the upper part of the building, and the water vapor condenses, causing corrosion of the nails securing the bricks to the planks. As a result, the bricks move outward in the upper part.
The vapor barrier and the a transfer heat
It is therefore important to install the vapor on the warm side of the outer wall and the air barrier on the cold side of the exterior wall. These two components are essential for controlling air infiltration and exfiltration, through the casing, and to prevent condensation and premature deterioration of the envelope structure.
Airtightness of the envelope
It is also necessary to add the insulation in the wall to delay the movement of air in the building envelope. However, as a first step, special attention must be paid to the airtightness of the casing at the time of construction, ie the air barrier must be properly installed. respecting the details, especially around openings (doors and windows).
When the air barrier is installed outside, we can then ensure good energy efficiency by minimizing heat loss and heat transfer by the outer shell. In addition, the correction of deficiencies relating to the installation of the air barrier is complicated and costly, because these correction work requires the removal of bricks or external coatings.
Exterior sealing joints, around the periphery of the outer casing and around the doors and windows, are also very important for sealing the outer casing and minimizing a transfer heat.