Moisture Intrusion
Moisture intrusion is the uncontrolled movement of moisture
into a building where it is unwanted and undesirable.
What does a home inspector look for?
Moisture intrusion can cause major structural damage and can
threaten the safety of its occupants. One of the main responsibilities of a
home inspector is to find and report on parts of the building that may be
suitable to moisture intrusion. Many defects these can be found, confirmed,
diagnosed, explained and documented during a home inspection.
Moisture related problems include rain penetration,
structural decay, mold growth, high indoor humidity, condensation, wet
foundation, ice dams, and several other reasons. We are all familiar with
these, and these issues are preventable and manageable.
Effective moisture management involves a degree of
uncertainty. Best practices for moisture management should not be
considered as absolute fix to any given situation. During the inspection, it is
possible to find equally effective or better solutions. The absence of building
material may not necessarily mean that a moisture problem exists.
Understanding how moisture moves.
H20, moisture, or water vapor moves in and out of
a building in three ways:
- With
the air currents
- By
diffusion through materials; and
- By
heat transfer
Out of the three of these vehicles, air movement (or lack
of) accounts for more than 98% of all water vapor movement in building
cavities. Air moves from a high-pressure to a low-pressure area by the easiest
path available. This means, that any hole or crack I the building envelope can
allow vapor to freely move about.
When vapor is transfers through air currents, it happens
quickly; so, to control this, a building needs to be sealed so that air only
comes in through when we have doors/windows or conditioned through an HRV +
HVAC.
The other two forces are diffusion and heat transfer which
are much slower. The building materials we use helps slows down moisture
diffusion to a degree, but it doesn’t stop it completely. Insulation helps to
reduce heat transfer.
Psychrometrics… what?
Like all things in our world, the Laws of Physics govern how
moist air reacts in various temperature. A Psychrometric chart used to
determine at what temperature and moisture concentration that water vapor will
begin to condense – this is also known as the dew point.
Relative humidity, or RH for short, is the amount of
moisture in a cube of air compared to the maximum moisture it could
hold.
For example, air at 20°C with 14.8g H20/kg has
100% RH. However, the same air at 15°C reaches 100% RH with only 10.7g H20/kg.
Colder air holds about 28% less moisture than warm air, the moisture that can
not be carried with the air is visible as condensation on cold surfaces – the
dew point.
During construction, vapour diffusion-retarders are used to
reduce the amount of moisture transfer. Except in ventilated spaces such as
attics, insulation, and vapor diffusion-retarders work together to reduce the
risk of condensation to form on a building’s ceilings, walls and floors.
