Details for mechanically vented crawl spaces

"It ain't over til it's over."
- Yogi Berra

Based on the number of questions that builders and code officials bring to Colorado about E-Star crawl spaces, somebody could probably publish a magazine Crawl Spaces R Us and have it fully subscribed. Most recently calls came in from Denver, Montrose and Pagosa Springs on just how to mechanically vent a crawl space.

A refined version of the material below is being written for Pacific Northwest National Lab. the folks who intervene on building codes for the U.S. Department of Energy. Expect this to be one of 15 to 20 two-pagers published as an official packet this fall. The individual pieces will deal succinctly with perceived conflicts between best practices recommended by building scientists and the International Residential Code.

If you think the item below is a dry, dull read, filled with code citations of the kind that you could use for direct discussions with your local building officials. well, that's the idea.

Note: while this is written to address at-grade crawl spaces. the concept applies to crawl spaces beneath basement floors as well. Note too that some details were developed in Colorado and may still be modified before final publication. Check E-Star's Web site (www.e-star.com) for the full document dealing with crawl spaces beneath basement floors.

Problem: While codes now allow conditioned crawl spaces with no passive vents directly connected to the outdoors (see code citations below), they are still sometimes ignored. Additionally, code officials and builders are uncertain as to the allowable design details and construction practices.

Background: The theory behind traditional crawl design is that passive foundation-wall vents reduce moisture problems within those spaces while allowing means for gases, moisture and other contaminants rising from the soil to exit the space without entering the structure above. Unfortunately, field research shows that venting does not assure these benefits and may in fact exacerbate potential problems.

Historical research by William B. Rose (University of Illinois Urbana-Champaign) showed that FHA requirements for crawl space ventilation were established in 1942 prior to completion of any extensive field research (194748). In 1994, Rose found that current code guidelines for crawl space construction strongly reflect the work of the earlier era. Rose concluded, "There is no technical basis in the literature for current or past crawl space ventilation requirements."

Expanded field evaluation since the late 1980s proved that many crawl spaces vented according to code were often experiencing severe moisture and mold problems. In colder climates, a vented crawl space allows entry of cold air during the heating season. Entry of cold air can drive up the relative humidity within the crawl space where insulation is installed in the floor above (per code option). When the higher RH air reaches sufficiently cold surfaces, such as rim joists, it condenses.

For this reason. building scientists recommend eliminating crawl space vents in favor of conditioning those spaces.

Description: Through Exception 4 in Section R408 (see code citations below), the IRC specifically allows crawl space designs in which an exhaust ventilation system replaces fixed ventilation openings through foundation walls. To comply with Exception 4. a mechanically vented crawl space design must include a continuously sealed ground cover, should not have fixed ventilation openings to the outdoors, and must be supplied with a continuously operating exhaust fan. In the absence of specifics listed in the 2000/2003 IRC, the following recommendations provide guidance. (Several of the guidelines were developed during 2002-03 in Colorado as a local amendment to the 2003 IRC.)

Sizing the fan. There are three generic design options:

• Size to code minimum: Section R408.2. Exception 4. allows elimination of foundation vents when continuously operated mechanical ventilation is provided at a rate of 1.0 cfm for each 50 square feet of under-floor space (e.g., 20 cfm for a I ,000-square-foot crawl space).
• Size to crawl space volume: size for one air-change per hour -- the assumed ventilation rate for crawl spaces with passive vents in foundation walls. For a 1,000-square-foot crawl space with 3.5 feet between -round and the floor deck, a fan would need to exhaust 58 cfm.
• Size to ASHRAE 62.2: an updated residential ventilation requirement, passed in May 2003. It specifies whole-house ventilation at the rate of ([number of bedrooms + I I times 7.5 cfm plus 10.01 cfm/ft' of conditioned space]) cfm. A 2,000-square-foot, two-story, three-bedroom home would require 50 cfm of ventilation -- more than twice the code minimum for crawl-only ventilation. Sizing the exhaust fan to this level and then providing intentional supply openings between the house and crawl space means the fan will provide double duty, venting both the crawl space and the house above.

Type of fan: Code does not specify details about fans used in crawl spaces. The Colorado amendments to the IRC specify a fan rated for 44,000 hours (5 years) of continuous operation with flex connections or other installation detailing to reduce vibration and noise associated with the fan. The fan must he connected to a trouble light or an alarm to signal occupants. when the fan fails, of the need for maintenance.

Supply air for the exhaust fan: transfer-air openings, one per 250 square feet of crawl space floor area, are installed in the floor deck between the crawl space and conditioned rooms above. The continuously operating exhaust fan pulls house air down through these openings. Airflow through the transfer openings is restricted through a capped "duct hoot" that has holes drilled in the cap: see table below).

Pressure balance: the crawl space exhaust fan, typically 30-60 cfm, will exert slight negative pressure on the house above. The resulting negative pressure will roughly equal that from a properly vented bathroom fan, but considerably less than that generated by dryers and kitchen exhaust fans. The recommended design spec: when all exhaust appliances and any forced-air HVAC system are all turned on simultaneously, any zone with an atmospherically vented gas appliance should experience less than 3 pascals of negative pressure. (Colorado guidelines point builders toward sealed HVAC ductwork and elimination of atmospherically vented water heaters, in order to avoid pressure problems altogether.)

Plan review/inspection guidance:

Removal of debris: the under-floor grade is clean of all construction materials, vegetation and (non-soil) organic material (R408.4).

Insulation: installed in accordance with the R-value and method specified in Section Nl 102.1.7 (e.g., R-20 in Denver, using the prescriptive compliance method), or as required to meet another method of compliance selected by builder.

Ground cover: -"continuous vapor retarder" would be sheet material with less than 1.0 permeance (e.g., polyethylene film) that is sealed at joints as well as to the foundation wall plus to all piers and other penetrations; or a concrete slab.

Fan, when using Exception 4: sized to provide a minimum of I cfm per 50 square feet of under-floor space.

Supply-air pathways: Look for pathways allowing house air to be drawn. under slight negative pressure. into the crawl space (e.g., one transfer-air grille per 250 square feet of crawl space floor area). Calculations should specify transfer air details.

Code citations:

IRC 2000/2003, Section R408 Under Floor Space

R408.1 Ventilation. The under floor space ... shall be provided with ventilation openings through foundation walls or exterior walls.

Exceptions:

4. Ventilation openings are not required where continuously operated mechanical ventilation is provided at a rate of l.0 fin n (1.02 Us). for each 50 square feet (10m3) of under-floor space floor area and ground surface is covered with an approved vapor retarder material.

IMC 2000/2003, Section 403: Mechanical Ventilation

403.1 Ventilation system. ...The system shall not he prohibited ,from producing negative or positive pressure.

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007