Roxul ComfortBoard IS - Manual (Page 18)

ROXUL
® The Better Insulation™ 16
Table 1: Ratio of exterior-interior insulation to control air leakage condensation
If the sheathing layers are very vapor permeable (e.g., ROXUL COMFORTBOARD IS over fiberboard or
gypsum sheathing, and housewrap) then very little insulation value is required outboard of the stud bay.
However, while these permeable layers can essentially eliminate vapor diffusion condensation risks with
lower exterior sheathing R-values, the risk of air leakage condensation does not drop as much: air
leakage may still deliver more water vapor to the back of the sheathing than can be removed by diffusion
through the sheathing, and hence condensation could still occur and accumulate.
For important projects or situations in which the design team has little historical experience,
investigation using widely available computer models such as WUFI Pro would be prudent if the
necessary time and skills are available.