Continuous Perimeter Foundation Vents

The next innovation in crawl-space ventilation
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Sponsored by Joto-Vent System USA, Inc.
By Kathy Price-Robinson

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 1203.3: 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 18 inch (3.2 millimeters).

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 12 square foot of NFVA, or roughly the same as a typical box vent. The holes in the covering are 18 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.

IBC 1203.3.2 Exceptions

The following are exceptions to Sections 1203.3 and 1203.3.1:

  1. Where warranted by climatic conditions, ventilation openings to the outdoors are not required if ventilation openings to the interior are provided.
  2. The total area of ventilation openings is permitted to be reduced to 1/1,500 of the under-floor area where the ground surface is treated with an approved vapor-retarder material and the required openings are placed so as to provide cross ventilation of the space. The installation of operable louvers shall not be prohibited.
  3. Ventilation openings are not required where continuously operated mechanical ventilation is provided at a rate of 1.0 cubic foot per minute (cfm) for each 50 square feet (1.02 L/s for each 10 square meters) of crawl-space 2015 IBC 249 floor area and the ground surface is covered with an approved vapor retarder.
  4. Ventilation openings are not required when the ground surface is covered with an approved vapor retarder, the perimeter walls are insulated, and the space is conditioned in accordance with the International Energy Conservation Code (IECC).
  5. For buildings in flood-hazard areas as established in Section 1612.3, the openings for under-floor ventilation shall be deemed as meeting the flood-opening requirements of ASCE 24, provided that the ventilation openings are designed and installed in accordance with ASCE 24.

This 11,500 exception will come into use occasionally when using the continuous perimeter vent system. That’s because the NFVA of the vent is 4.81 square inch per foot, and when a large house with a large crawl space has a garage along one side of the house foundation, the continuous perimeter vent is not enough linear footage to reach the 1150 ratio as required by code. In these instances, the exception can be used with installation of an approved Class 1 vapor retarder in the crawl space.


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Originally published in Architectural Record