Moisture is the single greatest source of construction
problems in the wall assembly of a home and its mitigation is a crucial component
of good building envelope design.
When left unattended, moisture within a wall assembly will
produce mold and mildew, decay and rot, thermal envelope breakdown and
structural damage.
There are three distinct ways that moisture can develop
within a wall system with the first being from the outside in as outdoor water
and moisture works its way into the wall assembly through imperfections and
breaks in the exterior skin.
The second way moisture develops is from the inside out as
warm, moisture-rich air from the inside of the house migrates into the wall
assembly and condenses when it meets a cold surface within. The third and, for
me, the least obvious way is the development of moisture from the assembly
itself as changing moisture levels in the framing material that make up the
assembly produce moisture problems within the wall.
Controlling external moisture from entering the wall
assembly has been greatly enhanced in recent years with the required
installation of rain-screen walls for new construction by the National Building
Code of Canada.
In rain-screen construction the outer layer of siding or
cladding of a wall assembly is separated from the inner wall by an air space.
This space is created with vertical furring strips or rain-screen drainage mats
and is vented to the outside to provide pressure equalization that allows any
moisture that makes it into the cavity to escape at the bottom. This new method
of construction ensures that if moisture makes its way through the outer
protective layer of the wall assembly it will naturally migrate out again. Lack
of rain-screen detailing was the primary culprit for the "leaky
condo" debacle that plagued homeowners some years back.
Moisture can move from the inside of a dwelling into the
wall assembly if moistureladen air from the interior spaces of a home is
allowed to make its way outwards. Wind loads, temperature gradients and
mechanical systems will create pressure differences between the inside and
outside of a building, forcing warm air outwards. As warm air migrates through
a wall assembly it will find a cool surface at that magic temperature and it
will condense. Think of your lawn after a clear night. The water drawn out of
the warm air will create moisture within the assembly and with it all sorts of problems
from rotting and structural deterioration of the assembly to fungal growth and
visual damage.
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Installing a continuous membrane on the warm side of the
wall insulation - typically a 4-6 millimetre polyethylene film - prevents warm,
moist air from the interior of the house from migrating into the wall assembly
and is the standard way to prevent this insideto-outside moisture problem from
occurring.
When constructing a wall assembly one must ensure that the
lumber used for framing is considered dry - The National Building Code of Canada deems this to be a
moisture level of 19% or less. Green lumber (wood that has been freshly cut)
will have a moisture level of 100%. As green wood dries it shrinks and
undergoes dimensional changes. It's crucial that the bulk of drying has already
happened before incorporating the lumber into the wall assembly. Not doing so
will create deformation issues as well as problems of rot and fungal growth as
excess moisture is trapped in the assembly itself.
It will come as no surprise to North Shore residents, who
understand the destructive effects of water and moisture more than most, that
unwanted moisture in a wall assembly will lead to a glut of construction
problems and needs to be prevented at all cost.
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