Proper Roof Ventilation

Where comes the water from we are fighting against

Human body, as it contains of 70% of water, shares this water with the environment. During any activity (even sleeping) people produce some amount of vapor. These vapors permeate through the attic to the thermal insulation if we do not prevent them from doing that. Roof ventilation conveys all the vapors under the roofing layer where the air circulates and away from this space through the exhaust roofing vents.

Natural and forced ventilation

The roof ventilation (air circulation) under the roofing layer (shingles, tiles) is automatic, as the warmer air always rises up, is exhausted by the exhaust vents and as there is the under-pressure in this airspace, new, cold air is sucked in via the intake soffit vents. Only in case that the dimensions of the intake vents is inadequate to the total dimension of the exhaust vents, forced (active) ventilation has to be applied.

why we use the vapor barriers

As we mentioned before - one of the main tasks of the properly vented roof is to prevent the vapours from permeating to the thermal insulation (mineral wool, cellulose insulation, foam insulation). In order to achieve that, the vapor barrier is placed under the thermal insulation layer, which is the non-permeable PVC or asphalt based felt or sheet that stops the vapors.

Why we use the vapor permeable membranes

If – in spite of that – the vapors get into the thermal insulation the continue on their way up, meet the cold roofing layer and start to condensate. The small water drops fall back to the thermal insulation layer where they later cause problems (degradation of the material, loss of insulating properties). To stop the condensated water, we can apply another „vapor permeable membrane“ which lets the vapor through but stops the water which condensates and convey it away from the roof.

The exhaust and intake roof vents

As the exhaust vents, we can use the vent pipes with caps (mushroom vents) or the roof ridge vents.
The intake vents usually are continuous soffit vents, eave vents or drip edge vents (in all cases it refers to continuous roof opening running the whole roof length partly covered to let the air in but prevent insects, debris and windows_without_spending_too_much_money.php">birds from being sucked in).