Moisture safety in a highly insulated cold attic

Damages, caused by moisture and mould, in attics have increased in Sweden during the last decades. The main reasons are thicker thermal insulation between the living space and the attic and lack of the heat and ventilation effects from a warm chimney. Cold attics are ventilated to avoid that heat leakage from the living space heats up the outer roof with snow melting and icicles as result. Using traditional ventilation the climate in the well insulated attic becomes close to the outside climate. The original main purpose of the ventilation is therewith not valid any more. There can be other reasons to ventilate the attic, for example to take away the humid air that can be transported to the attic from the living space trough diffusion and moisture convection. Also water from small and occasional water leakage can be removed from the attic by the ventilation. There can also be disadvantages with ventilation. The wind pressure against the building can sometimes course a under pressure in the attic, which increases the moisture convection from the living space into the attic. During clear cold nights the heat radiation from the roof into the sky could lead to that the temperature at the inner side of the roof gets lower than outside, which increases the risk for condensation. This paper presents the results from a project where a newly developed commercial vapour retarder has been used to achieve a dryer climate in a highly insulated cold attic. Field measurements during two years are presented and comparisons with other measures two achieve a dryer climate in highly insulated cold attic are given.