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

Burch et al. (1995) called Wall # 1 the “variable-permeance-claddings wall,” because the
moisture behaviour of the plywood varies with ambient relative humidity. When the
relative humidity is below 50%, the plywood behaves as a vapour retarder. When the
moisture content of the plywood approaches saturation, it becomes very permeable.
Computations showed that, for Madison, Wisconsin, a manufactured wall assembly, built
to 1995 standards without an interior vapour retarder and permeable sheathing, may not
provide satisfactory moisture performance in a cold climate. The same wall but with an
interior vapour retarder (vinyl wallpaper, latex paint, or kraft paper) showed acceptable
performance in a cold climate such as in Madison, but as expected, performed poorly in a
hot and humid climate, such as in Miami, Florida. The proposed wall assemblies with the
“variable-permeance-claddings wall” and “sandwich panel wall with low-permeability
insulation” both exhibited satisfactory moisture performance in both the cold and the hot
and humid climates, even with moderately severe indoor conditions. The proposed walls
also performed satisfactorily in a mixed climate, such as in Little Rock, Arkansas. Burch
et al. concluded that the proposed wall assemblies would provide satisfactory moisture
performance in all climatic regions of the United States.
Moisture Control: Coastal Regions
In coastal regions, deflecting or shedding rainwater off the façade is the first line of
defence against rainwater entry into building envelopes. As a second defence, draining
wet spells back to the outdoors reduces the length of time a material is exposed to water
and thus lessens potential damage to the wall. As a secondary barrier, a weather barrier is
used to block rainwater entry into the inner wall assembly while allowing for
breathability of the wall to facilitate drying.
Weather Barrier
Canada Mortgage and Housing Corporation (2004) summarized a study that examined
the effect of various factors on the performance of various weather barrier materials.
These factors included the type of substrate, outdoor weathering, various extractives and
surfactants, and fastener penetration. The study concluded that the weather barrier
performed well when the wall assembly included a drainage cavity between the weather
barrier and the cladding. The drainage cavity prevents water contact to the weather
barrier on both sides, because when this occurred water was observed to travel through
the weather barrier. [Comment: probably by the capillary action.] Aging, weathering, or
mechanical stretching did not affect the weather barrier’s pore size. The barrier did not
function under some combination of weathering in the presence of wood extracts and
other solutes as significant water was observed to pass through the weather barrier.
Drying the Wall Assembly
Joy et al. (1948) observed that ventilation of a wall cavity was effective in controlling
condensation in certain cases, particularly when air movement in the cavity was not
impeded. The use of vapour diffusion ports (VDPs) is a drying strategy that has been
used in the lower mainland of British Columbia (CMHC, 2002-1). The concept involves
creating one or two holes 75 mm (3 in.) diameter in the sheathing at the top and bottom
of stud spaces. These holes allow vapour in the stud space to dry to the outdoors.
PERD-079: Task 2 - <strong>Literature</strong> <strong>Review</strong> 10