Is Crawlspace Ventilation Required?

Those Screened Vents May Not Be Necessary

Crawlspace Under House - 157191299
Crawlspace Under House. Getty / slobo

Even if you don't know what crawlspace ventilation is, you've probably seen those rectangular, screened vents along the side of foundation walls and wondered what they were about.

No, they aren't picture windows for the rodents crawling under your house. These are crawlspace vents, long required by building code as a way to prevent moisture build-up in the ground from rotting away joists, beams, and flooring.

The International Residential Code (IRC), which "brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences," according to EnergyStar, has made changes regarding crawlspace ventilation in R408.3 "Unvented crawl space."

Motivation For Un-Venting a Crawlspace

In the past, crawlspaces were treated like spaces that did not belong to the house envelope. They were a not-interior, not-exterior space.

Many homeowners and builders prefer unvented crawlspaces. The reason is because those vents--which are basically like open windows--make it that much harder to heat and cool the house. Indeed, when you want to heat up a chilly bathroom or kitchen ceramic/porcelain floor you begin at floor level--with radiant heating.

In addition, the more entry points in crawlspaces, the more chances for vermin, bugs, and moisture to come in. Screens don't always filter out every unwanted pest, and screens often come loose or develop holes.

Unvented Crawlspace Requirements:  Mechanical and Sealing Up

But with IRC changes, crawlspace can be unvented; that is, in the traditional way of venting from the exterior. Now, you have the option of "unventing" a crawlspace by combining these two major points:

  1. Mechanical:  Mechanically circulating air between the upper conditioned area and crawlspace.  The device must move at least 1 cubic foot of air per 50 square feet of crawlspace area; and
  1. Sealing Up:  Completely sealing up the crawlspace floor with vapor retarding material. This means lapping the edges of the vapor retarder up against the inner foundation walls, overlapping separate sheets with each other by at least six inches, and sealing up those seams.

Also, crawlspace walls must be insulated to appropriate R-values.