Attic ventilation

We have already dealt in a previous article with the question of the condensation in the attic of a sloping roof. We will now address the issue of ventilation in the attic of a flat roof.

Ventilation and condensation 

To provide ventilation in the attic of a roof flat and to avoid condensation problems (which can cause premature deterioration of the roof and the formation of mold in the attic), it is therefore necessary:

  • Leave a free space above the insulation so that air can circulate freely in the attic;
  • Install aerators on the roof to ensure good air circulation in the attic (ventilation).

Two conditions

There are two important conditions to be observed to ensure that the flat roof - with an attic - is healthy and durable and to minimize heat loss through the top floor ceiling. This is to avoid condensation in the attic and associated problems:

The first condition relates toinsulating and to the vapor barrier: by insulating the existing attic adequately, it then favors the accumulation of snow, because it prevents the heat of the house from melting the snow on the roof. The roof structure must be calculated to support this extra weight. Before insulating the roof, it is therefore essential to have its structure calculated, by an engineer, so that it offers the appropriate solution in order to obtain the necessary structural resistance, particularly with the significant ice load in recent years. Insulation should be installed - between ceiling joists - to prevent warm building air from leaking into the attic. Additional insulation can be injected over the joists to achieve an adequate insulation factor. However, the insulation must leave a free ventilation space so as not to obstruct the air circulation under the roof. Thus, there will be no condensation and avoid rot problems;

Le vapor must be installed on the warm side of the insulation, that is, directly behind the gypsum board in the ceiling, to minimize the migration of hot and humid air into the attic.

The second condition relates to the ventilation: In old buildings, this circulation space did not exist, because the roof support covering was installed directly on the joists. Thus, each section of two consecutive joists forms a completely closed corridor. In this way, air circulation over the entire attic is impossible. It is for this reason that the structure must be modified (parts installed above the joists) in order to create a ventilation space free of any obstacle. This is to allow air to circulate in all directions over the insulation. It is necessary to consult an engineer to ensure that the snow load will be distributed directly on the joists, to avoid structural subsidence and accumulation of water on the roof.

Before closing the attic, with the support covering, it will be necessary to check if the ventilation is not blocked. Also, ventilators should be installed on the roof, according to the manufacturer's recommendations, to improve ventilation.

Finally, the elastomeric or other membrane must be laid, by a roofer, under the rules of the art and recommendations of the AMCQ (Association of master roofers of Quebec) by creating an adequate slope to ensure good drainage of the roof under study.


Lack of ventilation in the attic of a flat roof

Lack of ventilation in the attic of a flat roof

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