Attic condensation: water that comes from the house

The passages through the ceiling

Warm air takes advantage of the smallest opening in the top-floor ceiling. Four spots come up often.

• The attic hatch, when it has neither weatherstripping nor insulation.
• Recessed light fixtures.
• Holes drilled for plumbing pipes and electrical wires.
• The duct of a bathroom fan or range hood, if it ends in the attic or has come loose.

Once up there, this moisture has only one way out: the attic ventilation, which must carry it outside. When the ceiling lets too much moisture through, the airflow is no longer enough.

A ceiling that drips without rain

In late February, the roof is under snow, it has not rained for weeks, and yet a stain appears on a bedroom ceiling.

During the deep cold, frost thickened under the deck and on the nail tips poking through it. Then the temperature rose, and everything melted within a few hours. The water followed the rafters or dripped onto the insulation, sometimes down to the ceiling.

The roofing is not punctured. The water comes from inside.

When the wood no longer dries

A little frost during a cold snap is nothing to worry about if it disappears quickly. The trouble starts when the wood goes from one season to the next without drying.

• Dark stains mark the deck, especially on the north slope and toward the bottom of the roof.
• Mould spreads over the wood and the surface of the insulation.
• Soaked insulation compresses and holds heat less well, and the attic warms up more.
• The deck panels soften and deform between the rafters. From the street, the roof seems to wave.

If the hatch can be reached without climbing, a flashlight is enough to see the frost or stains from the opening. If the hatch is in the ceiling, out of reach without a stepladder, leave it closed. The stain during a thaw, without rain, is enough to call a contractor, who will go and look. Do not enter the attic, and do not touch granular insulation that looks like vermiculite: it can contain asbestos.

Close the entrance before opening the exit

First, the moisture supply is cut off. Then, you check that it can get out. Everything done in the attic belongs to a contractor.

1. Your part. Keep the house's humidity at a reasonable level during the winter. This is the only step that falls to you.
2. The ceiling. The contractor plugs the air leaks from above, in the attic.
3. The hatch. The contractor adds weatherstripping and a layer of insulation to it.
4. The fans. The contractor runs their duct, insulated and well fastened, to an outside exhaust.
5. The air. We then make sure nothing blocks the path between the soffits and the ridge.

Vents added above a leaking ceiling can make the problem worse. An attic that draws harder pulls more air from the house.

Frequently asked questions