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 hot air wants to
rise upwards, some of that moist air can flow into the attic through leaks at attic joist level.
Nowadays, plastic wrapped joists are available in order to create a seal but some leakage always
occurs.
Moisture in the attic can also come from other sources. Incoming moisture via the attic's ventilation
can condense when moisture content and temperature differences occur between the climate
outside and in the attic space.
Building moisture that comes from materials that are naturally moist during the construction period
such as wood and concrete. If too much moisture leaks into the attic, there is a risk that the humidity
will be so high that there may be problems with mould and rot on the tongued and grooved timber
or plywood of the outer roof structure. The risk of growth is often greatest in particularly cold areas
where the relative humidity is highest: for example at a northern corner, far down towards a
ventilated eave. If it gets too cold, there is a large risk of the moisture precipitating as condensation.
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.
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