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

• Thermal and vapour properties of building materials should be placed side by side in
the same table.
• Insulated impervious building envelope assemblies should include adequate drainage
and venting to foster evaporation and drainage of condensate should it occur.
Are Vapour Barriers Necessary in Norway?
Thue et al. (1996) discussed whether a vapour barrier is necessary for wood frame walls
and ceilings in cold climate regions. Noting that the function of the vapour barrier is to
minimize the diffusion of indoor water vapour into the wall/ceiling assembly (or outdoor
water vapour in hot humid climate regions), some believe the so-called “breathing
constructions” (i.e., wall assemblies with vapour-permeable components) have as good a
moisture performance and contribute to better indoor air quality. Thue et al. argued that
there are no general design recommendations regarding the requirements for vapour
barriers in wood frame construction. For example, what limits of water vapour resistance
are recommended for barriers on the cold and the warm sides of the insulation?
Norwegian houses without a vapour barrier did not suffer moisture damage when the wall
assembly was airtight [Comment: this is probably because the wall finish, e.g., a painted
gypsum board performed the function of the vapour barrier, and indoor air leakage was
minimized by the airtight assembly.] In Norway, a typical wood frame wall assembly
consists of (inside to outside) the gypsum board or wood paneling wall finish; 0.15 mm
(6 mil) poly vapour barrier or equivalent, placed on the warm side of insulation; 150 mm
minimum thermal insulation (rock wool, glass wool, or loose fill cellulose fibres) in the
stud space; a wind barrier (building paper, fibreboard, or gypsum board), and the
ventilated exterior wood paneling. (Thue et al. noted that plywood sheathing, commonly
used in North America, is not used in Norway.)
Because of the limited results of only two laboratory tests, Thue et al. (1996) could not
provide a firm answer to the question of whether a vapour barrier is necessary for wood
frame walls and ceilings in cold climate regions. The authors made the following
conclusions.
• A wood frame building envelope with a 0.15 mm (6 mil) polyethylene vapour barrier
on the warm side of the insulation could be considered “moisture safe” if it has no air
leakage.
• The tests indicated that two parameters have to be considered in wood frame
construction in cold climates: the vapour diffusion resistance (or vapour permeability)
at the warm side of the insulation, and the ratio between vapour diffusion resistances
at warm and cold sides (the vapour resistance ratio).
The values of these two parameters depend on the thermal resistance of the wall. Higher
insulation levels would emphasize the requirements for a vapour barrier as potential
condensation increases in the wall as the temperature decreases in the components on the
cold side of the wall. A high vapour resistance ratio might compensate for a low vapour
diffusion resistance on the warm side. Thue et al. presented the following design rules,
but with caution because of the limited results.
PERD-079: Task 2 - <strong>Literature</strong> <strong>Review</strong> 6