A common problem: in a cold attic, condensation or frost forms and the walls get wet. Simple ways to prevent it

Once one of the readers wrote a comment in one of the articles:

Dear Samostroischik, can I ask you for an article on the correct insulation and vapor barrier of the ceiling under the roof, so that the condensate does not wet the walls when it melts in the winter thaw, otherwise I got caught. Is a spacious attic volume always needed in this regard?

I promised to do an article on this topic. But only now, by my own example, can I show how this problem can be avoided. But first, I propose to figure out why condensation can form in the under-roof space of a cold attic?

If condensation forms, then there is a source of moisture. In humid climates, this source can be atmospheric air. And when the temperature drops, something from it condenses. But mostly, humid air comes out through the wood floor.

It is impossible to make a 100% vapor barrier in wooden breaks. There are places where it adjoins the walls. In theory, the vapor barrier film must be glued all over the seams and glued to the walls (or plaster it in them). Not everyone does this, and along the perimeter of the walls, some percentage of the air from the premises and moisture with it comes out through the ceiling. It condenses on the waterproofing film (if the roof is made of metal tiles) or on OSB sheets (if the roof is made of flexible tiles).

When condensate flows down or when frost melts, walls and beams will get wet. And the tree may even start to rot.

Condensation can be prevented by ventilating the cold attic.

If the roof is made of metal, then there are cracks in the ridge. Air can escape through them, and enter the attic through the cornices. But the problem is that there is a waterproofing film under the metal tile. And condensation accumulates under it. In theory, the windproof film should not cover the ridge part. But during construction, the roof is completely closed and overlapped with this film.

In the area of ​​the ridge, everything is hermetically closed with a hydro-vapor barrier. And in the area of ​​the ridge there are boards. In general, air does not come out of the attic through the ridge. Ventilation remains through a gable window or lattice. And if they are not there (hipped roof, for example)? Roof aerators need to be installed.

I will show by my example the installation of such aerators, but only in the roof of flexible shingles.

Type of shingle roof aerators and their installation. Two such elements are installed on each edge of the ridge of the hip roof. And the ventilation of the attic works as in the diagram at the beginning of the article. The air from the cornices rises to the ridge and carries with it the air saturated with water vapor. In strong winds (or in sunny weather when the roof warms up), this is especially effective. In addition, I have a monolithic attic floor that does not allow air to pass through.

There is also a ridge aerator. A ridge flexible tile is fixed on it.

There is also an addition to this: also the cornice should not be tightly hammered with boards. It is better if it is finished with perforated soffits.

Air enters the small holes and rises up to the ridge, ventilating the attic.

I hope this information will be useful to someone ...