Roxul ComfortBoard IS - Manual (Page 17)

INSULATING SHEATHING FOR RESIDENTIAL CONSTRUCTION APPLICATION GUIDE 15
Condensation Control
Air leaking outward through the enclosure wall in cold weather will contact the back of the sheathing in
framed walls and can form condensation. This condensation can accumulate as frost in cold weather, and
subsequently cause “leaks” when the frost thaws and liquid water drains down, or cause rot if the
moisture does not dry quickly upon the return of warmer and sunnier weather.
In walls with sufficient exterior insulation, the temperature at the back of the sheathing will be above the
dewpoint temperature of the interior air: therefore condensation due to air leakage cannot occur within
the stud space. If an assembly is shown by calculation to be safe against air leakage condensation (using
the ratio of exterior-to-interior insulation method described below – Table 1), then diffusion
condensation cannot occur, even if absolutely no vapor resistance is provided inside of the sheathing (i.e.,
no vapor barrier or other control layer), and even if the sheathing is a vapor barrier (such as foil-faced
insulations).
The interior conditions within a building during cold weather are critical variables in understanding the
risk of condensation, and must be known if predictions and calculations are to be made. Interior
temperatures are often in the 70 °F / 21 °C range, but relative humidity levels, and thus air moisture
content, can vary significantly. In most office, school, and retail occupancies, ventilation rates are high
enough that the RH during winter months is in the range of 25 to 35%. In some residential occupancy, the
interior moisture generation is higher and exterior air ventilation rate lower than commercial occupancies,
and hence the RH will often be higher. In special occupancies, such as swimming pools, both the interior
temperature and relative humidity levels will be higher (78°F/25 °C and 60%RH), resulting in very high
absolute humidity levels.
The moisture content of the exterior air always drops at very cold conditions as the maximum air
moisture content drops. As outdoor conditions become colder, the interior RH drops because interior
moisture is diluted by increasingly dry exterior air. This effect provides some protection against
condensation, as the coldest week of the year is likely to coincide with some of the lowest interior
humidity levels.
The interior moisture content is usually defined by a combination of temperature and relative humidity.
More direct metrics are absolute humidity or humidity ratio, usually expressed in grams of water per kg
of dry air (or grains of water per pound of dry air). However, practically speaking, the most useful
measure is the dewpoint temperature of the interior air.
Table 1 provides the level of insulation (sheathing plus airspace and cladding) that should be provided
outside of a stud space filled with air permeable insulation (i.e., batt or blown fibrous insulation) to
prevent cold-weather exfiltration condensation. It can be seen that mild temperatures and dry interior air
require little exterior insulation to control condensation, whereas a museum maintained at 50% in
Fairbanks, Alaska or Yellowknife, Northwest Territories should have essentially all of the insulation on
the exterior.