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:

  1. With the air currents;
  2. By diffusion through materials; and
  3. 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 a 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.