The Consequences of Excess Moisture on Building Durability

While providing a method for passive ventilation under a raised home theoretically solves the moisture problem, the method itself can fail to perform. We will discuss the reasons for that shortly. But the consequences of moisture buildup under a home are alarming.

When moisture builds up in a crawl space, these damages to structure can happen:

• White wood rot fungus
• Mold
• Black mold
• Infiltration of insects, snakes, reptiles, and frogs
• Standing water
• Mushrooms
• Wet and moldy fiberglass (Insulation may be installed correctly, but when it gets wet, it gets heavy, pulls away, and grows stalactites.)
• Condensation on ducts
• Stalactites
• Wet walls

Consequences to Occupant Health from Crawl-Space Moisture Buildup

It’s not just the building that suffers when the crawl-space ventilation method fails. Occupant heath also suffers. While a miasma of troubles grows under the floor, the consequences trickle up to the home. They create or exacerbate:

• Poor air quality
• Allergies
• Asthma

Code Requirements

It’s clear that getting the air to move under the raised foundation to draw out the moisture in the crawl space is critically necessary, and the building codes reflect this by what is required in the codes. Some designers and builders call for mechanically moving the air in and out of the crawl space, or even conditioning the space, which are two more expensive options available for a house built on a crawl space.

For other designers and builders that rely on natural, passive ventilation, these are the requirements according to the International Building Code (IBC):

IBC 1202.4: Under-Floor Ventilation

Under-floor ventilation. The space between the bottom of the floor joists and the earth under any building except spaces occupied by a basement or cellar shall be provided with ventilation openings through foundation walls or exterior walls. Such openings shall be placed so as to provide cross ventilation of the under-floor space.

Openings for under-floor ventilation. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 square meters for each 100 square meters) of crawl-space area. Ventilation openings shall be covered for their height and width with any of the following materials, provided that the least dimension of the covering shall not exceed ¼ inch (6 millimeters):

1. Perforated sheet metal plates not less than 0.070 inch (1.8 millimeters) thick.
2. Expanded sheet metal plates not less than 0.047 inch (1.2 millimeters) thick.
3. Cast-iron grilles or gratings.
4. Extruded load-bearing vents.
5. Hardware cloth of 0.035 inch (0.89 millimeters) wire or heavier.
6. Corrosion-resistant wire mesh, with the least dimension not exceeding ¹⁄₈ inch (3.2 millimeters).
7. Operable louvres, where ventilation is provided in accordance with Section 1202.4.1.2.

The continuous perimeter vent system has a net free ventilation area (NFVA) of 4.81 square inches per foot and as stated before is a two-part system that uses a covering (flashing) to cover the vent. This equates to 15 feet of the continuous perimeter vent creating ¹⁄₂ square foot of NFVA, or roughly the same as a typical box vent. The holes in the covering are ¹⁄₈ inch, in the least dimension, as stated above. Besides being better for bug control, it is also acceptable in the Wildland-Urban Interface (WUI) Fire Area throughout California.

From the 2018 IBC:

1. 1202.4.1.1 Ventilation area for crawl spaces with open earth floors. The net area of ventilation openings for crawl spaces with uncovered earth floors shall be not less than 1 square foot for each 150 square feet (0.67 m2 for each 100 m2) of crawl space area.
2. 1202.4.1.2 Ventilation area for crawl spaces with covered floors. The net area of ventilation openings for crawl spaces with the ground surface covered with a Class I vapor retarder shall be not less than 1 square foot for each 1,500 square feet (0.67 m2 for each 1000 m2) of crawl space area.
3. 1202.4.2 Ventilation in cold climates. In extremely cold climates, where a ventilation opening will cause a detrimental loss of energy, ventilation openings to the interior of the structure shall be provided.
4. 1202.4.3 Mechanical ventilation. Mechanical ventilation shall be provided to crawl spaces where the ground surface is covered with a Class I vapor retarder. Ventilation shall be in accordance with Section 1202.4.3.1 or 1202.4.3.2.
5. 1202.4.4 Flood hazard areas. For buildings in flood hazard areas as established in Section 1612.3, the openings for under-floor ventilation shall be deemed as meeting flood opening requirements of ASCE 24 provided that the ventilation openings are designed and installed in accordance with ASCE 24.