Roxul ComfortBoard IS - Manual (Page 23)

INSULATING SHEATHING FOR RESIDENTIAL CONSTRUCTION APPLICATION GUIDE 21
Vapor Control
Although most condensation problems occur because of air leakage, vapor diffusion can also
occasionally cause damaging amounts of wetting. However, vapor diffusion is also an important drying
mechanism, which may be an important part of a wall assembly design.
Vapor diffusion is the movement of water vapor molecules through the microscopic open pore structure
of porous materials (glass, solid plastics, and metals are not porous, wood, gypsum, and concrete are).
Vapor diffusion always moves from more to less vapor concentration, but practically this tends to mean
from the warm side to the cold side of an enclosure or material layer. As the process is relatively slow, it
usually requires weeks or months to move significant quantities of water vapor. Vapor permeance is used
to describe the ease of vapor diffusion through a layer of material. A Building Science Digest entitled
BSD-106: Understanding Vapor Barriers is available at buildingscience.com for more specific guidance on
this topic.
The need for a vapor control layer, and which class, depends strongly on the enclosure design, the air
permeance of the insulation layers, the interior conditions, and the exterior climate. The exterior climate
is divided into zones from 1 to 8 based on heating degree days (HDD) and cooling degree days (CDD) (see
Figure 9 above). The zones are further sub-divided into different exterior humidity levels, indicated by
appending the letters A to C (e.g., zone 6C).
The class of vapor control required can be prescribed for many common wall or roof assemblies for
interior conditions of normal residential, school, retail, and office occupancy. This means indoor
temperatures of around 72 °F (21°C) and indoor winter time relative humidity of less than 40% (less than
35% in zones 7 and 8). Special analysis and unique enclosure details may be required for buildings with
higher interior relative humidities.
Table 4: Vapor control layer classification; tested by ASTM E96 dry-cup (Method A)
Class Permeance Description
US perms
(gr/hr
.
ft
2.
inHg)
SI perms
(ng/s
.
m
2.
Pa)
I Less than 0.1 Less than 6 Impermeable
II 0.1 to 1.0 6 to 60 Semi-impermeable
III 1 to 10 60 to 600 Semi-permeable
none Over 10 Over 600 Permeable
Inward Vapor Drive
Inward vapor drive in building enclosures occurs when the cladding absorbs and stores rain water
(masonry, stucco, etc.), is heated by the exterior environment and solar radiation. This combination of
water and heat energy in the cladding results in an elevated vapor pressure and the moisture is driven
into the enclosure. Inward vapor drive is a design concern for buildings in Climate Zones 1 to 4 and for
some enclosures with absorptive claddings in other climate zones.
The amount of vapor diffusion is dependent on the vapor pressure gradient, that is, the difference in
vapor pressures between two points in the assembly, and the vapor permeance of the building materials.