How air, moisture and heat move through a house

In plain English

Chapter 4 asks remediators to understand basic building physics, because a fix that ignores it can create the next problem. The core rules: heat moves toward cold, moisture moves toward dry, and air moves from high pressure to low. Every cubic foot of air pushed out of a house is replaced by a cubic foot coming in from somewhere, which may be a crawlspace, an attic or a garage. Wind, the stack effect, exhaust fans, dryers and leaky ducts all drive that movement.

Moisture enters and moves through buildings in four ways: bulk liquid water (roof leaks, floods, broken pipes), air carrying humidity through gaps, vapor diffusion through materials, and capillary wicking in concrete, wood and gypsum. Bulk water brings the most water fastest. Heat moves by conduction, convection and radiation, and cold surfaces are where condensation forms.

R520 warns that pulling large volumes of air out of a home can make gas appliances backdraft. It also points out that occupants change the moisture load through cooking, bathing, laundry and lots of houseplants, and that crowding a space raises its humidity. For a homeowner, the practical lesson is that a mold problem is often a moisture-and-airflow problem first. Before blaming a single leak, look at exhaust fans that vent nowhere, ducts running through a hot attic, and a crawlspace or garage feeding air into the living space.

Key points

• Air out equals air in; know where the replacement air comes from.
• Four moisture paths: bulk water, air leakage, vapor diffusion, capillary wicking.
• Bulk water leaks move the most water in the least time.
• Strong exhaust can backdraft water heaters and furnaces.
• Daily living adds a real moisture load indoors.

Official text

This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.

Citations

• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 4, pp. 33–36. Best practice
• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 4, p. 49. Best practice

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