Literature Review on Building Envelope, Heating and ... - Beeshive.org

dimensional computer model (MOIST), which could only provide approximate average
moisture performance for actual three-dimensional physical conditions. The authors
noted that, during winter, more moisture might accumulate in the walls of prefabricated
houses than those built on site, because they are fabricated tighter and thus have lower air
infiltration rates and thus lower potential drying out of the wall. Table 1 compares the
assemblies of prefabricated walls and the proposed walls.
Table 1. Assemblies of Prefabricated Walls: 1995 Practice and Proposed Walls by
Burch et al.
1995 Practice
Wall Assembly
Proposed Wall # 1
(Variable-permeancecladdings
wall)
Proposed Wall # 2
(Sandwich panel wall with
low-permeability insulation)
Latex paint, vinyl wallpaper,
or kraft paper
7.9 mm (5/16 in.) gypsum
board
89 mm (3.5 in.) glass fibre
insulation
12.7 mm (½ in.) fibreboard
sheathing
Latex paint
7.9 mm (5/16 in.) gypsum
board
12.7 mm (½ in.) exterior
grade plywood
89 mm (3.5 in.) glass fibre
insulation
12.7 mm (½ in.) exterior
grade plywood
Latex paint
7.9 mm (5/16 in.) gypsum
board
11.9 mm (5/32 in.) OSB
89 mm (3.5 in.) expanded
polystyrene insulation
(molded beads)
11.9 mm (5/32 in) OSB
Aluminum siding Latex paint Lapped vinyl siding
Vapour Retarder Requirement
The 1993 Manufactured Home Construction and Safety Standards from the US
Department of Housing and Urban Development (HUD) include the following moisture
control practice rules for prefabricated homes.
1. Install an interior vapour retarder with a permeance of 57 ng/s m 2 Pa (1 perm) or less,
or
2. use permeable sheathing and siding that has a combined permeance higher than 290
ng/s m 2 Pa (5 perm), or
3. provide an outdoor ventilated cavity between the siding and wall insulation.
An interior vapour retarder is not required in prefabricated homes when one of the first
two options is used. Burch et al. (1995) decided not to consider the third moisture control
option, because literature showed that a ventilated cavity might provide poor moisture
performance in both hot and humid, and cold climates. The authors considered the
combined permeance of the fibreboard sheathing and the aluminum siding to be
considerably greater than the 5 perm required by option 2 of the HUD standards.
Therefore, the HUD standards do not require this wall to have an interior vapour barrier.
PERD-079: Task 2 - <strong>Literature</strong> <strong>Review</strong> 8