VentoVind PRO Special user manual
© Corroventa Avfuktning AB
2020.12
10 (39)
Moisture problems in cold attics
Moisture problems in cold attics can be due to one or several causes, usually a combination of several factors
that determine how serious the moisture impact on the attic space is. Moisture problems usually occur in
connection with additional insulation or during the transition from oil boiler to geothermal or district heating.
The underlying cause, however, is the natural ventilation used in cold attics, as ventilation at the wrong time
leads to humidification of the attic space. According to a 2009 investigation
Så mår våra hus (This is how our
houses feel)
carried out by the Swedish National Board of Housing, about 300,000 buildings in Sweden suffer
from mould, mildew odours or high moisture levels that can be directly traced to problems in the attic. 84% of
these attics are cold attics with natural ventilation.
Disadvantages of natural ventilation
Problems most often occur during the colder half of the year when the relative humidity of the outside air is
highest, the solar radiation less intensive and the days shorter. The temperature of the attic spaces is then also
significantly lower than during the summer and the air can bear much less moisture. During nights with a clear
sky, the attic space can even become colder than the outside air, which results in condensation on the
underside of the roof. Water droplets can build up and in some cases can run down the inside of the roof. This
is because the outer roof of the attic is "facing" the sky and is therefore cooled by the colder layers of air higher
up in the atmosphere. This is the same phenomenon we see on cars, where we often have to scrape ice off the
angled windscreen, which is "facing upwards", but not off the side windows.
Moisture movement from the residence
An adult human being evaporates approximately 40 - 50 grams of water per hour when at rest at room
temperature and a family can easily produce 10 litres of water per day that evaporates into the indoor air.
Other sources of moisture production in a residence are baths/showers, drying laundry and cooking. Most of
the moisture is transported away
by the residence’s ventilation but when the hot indoor air wants to rise
upwards, some of that moist air can flow into the cold attic through leaks at attic joist level and then condense.
Even joists wrapped in plastic sheeting intended to seal against the residence leak somewhat.
Changing heat source and additional insulation
The residential ventilation, which mostly consists of natural draft or a mechanical exhaust air system, helps to
reduce moisture leakage up to the attic by creating a negative pressure in the residence. Today, however, it is
quite common to change heating systems from firing with heated chimneys to, for example, geothermal heat
pumps or district heating. In such a situation it is important to check that there is still sufficient ventilation,
since the driving force of the natural draft has been removed. There is otherwise a great risk, among other
things, of increased transport of moist air up to the attic. In the event of additional insulation, the cold attic's
temperature and the air's capacity to carry moisture are reduced. If it gets too cold in the attic, there is a large
risk of the moisture precipitating as condensation.
Building moisture
Building moisture that comes from materials that are naturally moist during the construction period but also
leaking water have a negative effect on cold attics. Concrete and aerated concrete are examples of materials
that contain a lot of building moisture and that need a long time to dry out. If large amounts of building
moisture leak into the attic during the construction period, there is a risk that the humidity will be so high that
there may be problems with mould and rot on the outer roof structure during, or soon after the construction is
completed.
