Moisture Safety in Cold Attics with Thick Thermal Insulation

Damages, caused by moisture and mould, in cold 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. With reduced heat flux from the living space the original purpose of the ventilation is 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 cause a negative 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 or high relative humidity.

This paper presents field measurements and results from four different concepts to increase the moisture safety in cold attics. Attics with reduced ventilation, internal heat source, thin thermal insulation of the outer roof, and reduced ventilation combined with new types of underlay with reduced resistance to moisture flux are discussed and compared with calculations and a reference house.