Moisture Safety in Cold Attics with Thick Thermal Insulation

Abstract

Damage caused by moisture and mold in attics has increased in Sweden during recent decades. The main reasons are thicker thermal insulation between the living space and the attic and lack of a warm chimney. Very thick thermal insulation will result in attics that are more sensitive to convection from the living space. With only a thin layer of thermal insulation, the attic will be warmer as a result of convective heat transfer from the living space. With a warmer attic during the winter, a certain amount of vapor can enter into the attic from the occupied area without causing any damage. With very thick thermal insulation, however, vapor cannot be allowed to enter the attic. Without a chimney, the temperature in the attic will decrease (especially in winter), which will result in higher relative humidity and lower ventilation owing to a lack of buoyancy. A warm chimney will also increase the natural ventilation in the occupied area and hence decrease the amount of vapor entering into the attic by convection. The conclusions are that both thicker thermal insulation and lack of a chimney will decrease the temperature in the attic but also affect the ventilation rate in the living space. To prevent moisture damage in modern attics, the floor structure also has to be very airtight. The construction in modern buildings is more sensible for sustained air leakage from the interior space into the attic, but in addition, the effects from heat radiation from the roof to the sky could result in condensation and mold problems in the attic. This paper presents the results from a project with modern Swedish attics where different measures have been taken to try to achieve a dryer climate in a highly insulated attic. Field measurements and computer simulations on attics with reduced ventilation, internal heat sources, permeable felt, and a thin layer of thermal insulation in the outer roof are discussed.