Condensation problem in the attic

Often the roof structure is deteriorated and blackened and black mold is found on the wooden frame. Generally, the degradation of this structural component results from:

  • A condensation problem caused by air leakage from the ceiling;
  • Insufficient insulation and;
  • Poor ventilation in the attic.

During the winter, the greatest heat loss occurs through the last ceiling in the building (the one that separates the interior space from the attic), which promotes the formation of humidity and condensation in the attic.

Attic condensations

© Condensation in the attic

Attic condensations (or attic)

A condensation problem forms when warm, humid air flows into the attic through the unsealed holes in the last ceiling. In fact, hot air leaks through these holes are the main cause of condensation in the attic or in the roof space. The problem is worsened when there is insufficient insulation to prevent heat from passing through the ceiling of the building, i.e. this isolation has a weakness thermal resistance..

Solution to the condensation problem

To eliminate the problem of condensation in the attic and the associated deficiencies (rot of the structure, mold formation, water infiltration problem, premature deterioration of the roof covering, etc.), it is therefore necessary:

  • Ensure a good airtightness between the house and the attic;
  • An insulation with a thermal resistance sufficient to minimize heat loss;
  • A ventilation made in such a way as to evacuate hot, humid air into the attic outwards. The attic must remain dry and cool as outside.

Chimney effect and or pulling effect

Why do the air leaks are they more important at the last ceiling towards the attic? In winter, the movement of air in a building resembles the movement of air in a chimney. During the winter, heated air tends to flow up the building. This situation creates a negative pressure in the lower part of the building, which causes a cold air infiltration, in the lower part of the outer shell, and theexfiltration of hot air by the upper part.
The last ceiling of a building is the element most exposed to the hot air rising from the bottom to the top. In fact, hot, humid air tends to escape to the attic through the openings. That's when the warm air meets the cold air in the attic. In the case where there is no adequate ventilation system to evacuate the humid air, condensation forms due to the hot and humid air.

Reduction of air leaks

To minimize hot air leakage from inside the building to the attic, make sure the last ceiling is airtight. Here is a list of places that are likely to leak and need to be sealed. It is thus necessary to seal the junctions between the partitions and the ceiling in the following places:

  • Around the plumbing column and other pipes at the ceiling level;
  • Around wires or light fixtures that pass through the ceiling;
  • The access hatches to the attic;
  • Around the fireplace;
  • At the junctions between the partitions and the last ceiling;
  • Around ducts, fans, etc.
Areas prone to air leakage to the attic

Figure 1: Areas of air leakage to the attic (source: Manitoba Hydro)

Deflector to prevent insulation from blocking soffits (source: Natural Resources Canada)

Figure 2: Deflector to prevent insulation from blocking soffits (source: Natural Resources Canada)

Diagram 3: The cold air infiltrates in the lower part and exfiltrates in the upper part of the building (source Natural Resources Canada)

Ventilation of the attic

Concerning the insulation in the attic, care must be taken that the insulation does not block the perforated soffits at the edge of the roof. This situation prevents the circulation of fresh air in the attic and promotes condensation. To always ensure a space between the insulation and the bridging of the roof, install deflectorsat the base of the eave, so as not to block the soffits.

He is careful about the houses of the 50 and 60 years, because the soffits are blocked by wooden planks. So, cut the excess boards so that perforated soffits are functional.

The presence of perforated soffits, at the edge of the roof, is not enough to create an air circulation in the attic. Again, the chimney effect is used in the attic to remove hot, moist air and replace it with dry, fresh air from the outside.

The chimney effect can be created in the attic by installing one or more aerators at the top of the roof. This allows fresh air to enter the perforated soffits and warm, moist air out through the vents.

Solution to the condensation problem

In summary, three elements are needed to prevent premature deterioration of roof structure and coating and to prevent mold growth, water infiltration problems, ice damming and costs. high heating.

The solution is to respect the following three conditions: 1) insulation with sufficient thermal resistance; 2) ventilation ensuring good air circulation in the attic and; 3) a good airtightness at the last ceiling to prevent air leakage to the attic.

Ventilation of the attic (source: Natural Resources Canada)

Figure 4: Attic ventilation (source: Natural Resources Canada)

Attic condensations

© Excessive condensation on the structure and the insulation in the roof space

Attic condensations

© Condensation on the roof deck and blackened wood

Attic condensations

© Lack of ventilation in the attic causes the formation of ice dams