How Moisture Moves Around in Your Home

Water’s intrusion into a building can determine how long that building will survive. Moisture enters and moves through buildings as liquid water and as water vapor. The four categories of water movement are:

Liquid flow. Driven by gravity or wind, water flows into and through holes and cracks in houses. Leaks in roofs and plumbing can deposit large amounts of water in a home.
Seepage. Liquid water creates its own suction as it moves through tiny spaces within and between building materials. This suction draws in ground water and also redistributes water from leaks, spills, and condensation.
Air movement. Air movement carries water, in the form of vapor, into and out of the home and its cavities.
Vapor diffusion. Responding to differences in the amount of water vapor between indoor and outdoor air, water vapor moves slowly through apparently solid materials like masonry and wood.

Liquid flow is the most serious water threat since it moves large amounts of water rapidly. Seepage can also move liquid water rapidly into a home through damp soil and porous skirting materials.

Water vapor movement by air leakage occurs mainly when heating or cooling systems are operating. Winter air leakage tends to carry moist indoor air outdoors—drying the indoor air. However, winter air leakage going through the home’s shell can deposit moisture in building materials through condensation. Summer air leakage tends to bring moist, hotter air from outside into the home—increasing the humidity of indoor air and again possibly causing condensation within the home’s wall, floor, and ceiling cavities.

Both Residential Energy: Cost Savings and Comfort for Existing Buildings and Saturn Energy Auditor Field Guide contain more information about solving moisture problems.