Roxul ComfortBoard IS - Manual (Page 25)

INSULATING SHEATHING FOR RESIDENTIAL CONSTRUCTION APPLICATION GUIDE 23
Recommendations for vapor control by climate zone
Different types of assemblies have different vapor control requirements. Although the requirements can
be developed through rational engineering analysis, a simplified summary of recommendations, many
from the US ICC (International Code Council) “I” codes, is presented below for the “normal” occupancies
described above. 
Houses with indoor relative humidity levels of over 40%RH when the outdoor
temperature is below 20°F (-7°C) for prolonged periods of time require special analysis and often unique
enclosure details to prevent moisture problems.
Vapor control recommendations are divided into two categories:
 Assemblies with all or most (more than 75% of the total) of the insulation value located outboard
of the structure (framing or solid)
 Framed assemblies with some insulation value outside of the framing or structure
These recommendations apply only to the wall assemblies addressed in this guide. Recommendations
for walls in each category are provided below.
Assemblies with all or most (more than 75% of the total) of the insulation value located outboard of the
structure (framing or solid)
This is the simplest and most robust wall to design with respect to vapor control. Such walls should
ideally have all moisture sensitive components and materials located on the inside of the insulation. In
this location, a Class I or II layer on the inside of all or most of the insulation value is acceptable and
recommended if all outboard components are moisture tolerant. A Class III layer on the interface of a
high permeance (more than 10 perms) insulation layer outboard of a moisture-sensitive structure should
only be used if warm weather and inward vapor drive condensation are not an issue or are controlled by
interior drying, ventilation of the cladding, or other means.
Condensation during warm (humid) weather and condensation caused by vapor driven inward from
wetted cladding heated by the sun will occur on the exterior of the vapor control layer: hence it is best
detailed as a drainage plane water control layer.
Framed assemblies with some insulation value outside of the framing or structure.
It is desirable to design for drying, especially in warmer climate zones (4 and 5 especially). The use of
insulation on the exterior of the sheathing increases its temperature in cold weather, thereby relaxing the
need to control cold-weather vapor diffusion. Exterior insulation made of stonewool fibers is highly vapor
permeable and when combined with vapor permeable sheathing and membrane layers, enclosures will
behave differently than less permeable sheathings/membranes: less R-value of such products are needed
to perform well than the rules in this category.
A Class III vapor retarder can be used instead of a Class I or Class II in zones 4c, 5, 6, 7, or 8 where any of
the criteria for the specific zone from the list below is met. These criteria may depend upon the climate
zone and the ratio of the insulation value in the stud space to insulation value installed outboard of the
sheathing.
The insulation value in the stud space is often a function of whether 3.5”, 5.5”, 6” or 8” framing is filled
with insulation. Batt products well supported on the open side DO NOT need to fill the whole stud space,
and thereby reduce the required insulation value of the exterior continuous insulation layer. Cavity
insulations need to be in tight contact on five surfaces: the sixth surface can be open to the interior.