Venting Subfloors: The Whole Enchilada

All pluses have their minuses.
- Murphy's Laws

Good solutions to problems should be as simple as possible, but not any simpler. Leave out a a piece of the puzzle and you may end up with a new problem that is every bit as big as the one you were trying to solve.

Consider structural wood-decked basement floors. Late last November, a systems-engineered design concept for avoiding mold problems under structural basement floors was introduced during a seminar at the Denver HBA. Joe Lstiburek presented the application details. He described a conditioned crawlspace system used previously to fix a toxic mold problem that existed beneath a structural subfloor. While it's not the only method that can work, it already has a small track record and plenty of thought behind it.

"We have to come to terms with these unique environments," said Lstiburek. "We need to change practices."

The good news is that a number of contractors embraced Lstiburek's conditioned crawlspace design concept. The bad news: Word has it that a few contractors have left out or misapplied at least one critical piece of the puzzle.

This article lays out the essentials and cautions contractors to include all described elements. To meet the various design requirements, better products can and will be applied over time. But for the concept to work properly, all the fundamental design elements must be present.

Drainage prerequisites

Ventilation systems can't solve problems caused by standing water in crawlspaces. The drainage system for the site must carry water away from the building. A new, well-illustrated paper by Lstiburek ("Water Management Guide" at www.eeba.org) contains the details described in this section.

Site grading is step one. While an 8.33 percent slope is the minimum (10 percent if clay soils), the steeper the grade, the better. Overhangs and gutters help carry bulk water on rooftops away from the foundation. Sidewalks and driveways must also have sufficient slope to drain away bulk water.

The cap layer of backfill adjacent to the building should be impermeable clay soil to facilitate drainage above the over-excavation for the I l-foot-plus deep excavation zone. Beneath the cap layer should be free-draining backfill (e.g., sand) that facilitates the drainage of any water that gets past the cap layer, reducing the movement of that water laterally. A perforated drain pipe-located below the base of the void boxes, surrounded by coarse gravel that is covered with a filter fabric-carries water either to the sump or to daylight.

There are workable variations to this last element (e.g., the CFI's Drainbuster system). But drainage is the key. Without drainage of any accumulated bulk water, the presence of standing water within the structural subfloor environment will render the rest of the system described below ineffective

Seven-part ventilation package

Cover the .soil. Crawlspace research conducted during the early 1990s showed that even an imperfect ground cover helps fans do their job by drastically reducing crawlspace relative humidity levels. Pick a cover that is opaque (e.g., black, not clear, polyethylene). Before laying down the ground cover, pick up any scrap wood or other organic material from the ground.

Seal the ground cover. Fasten the ground cover to the foundation and any piers. Tape all seams. View it as a sealed diaper; you wouldn't want the elastic on a diaper to sag and leak, would you? Provide enough clearance below the beam and subfloor to allow for free ventilation of the crawlspace.

Select proper exhaust fan. Pick a highquality ventilation fan. It should be rated to run 10,00() hours. It should be quiet-1.5 cones or less. It should be sized properly; figure about 80 cfm (more on that below).

Exhaust air from the house, 2-for-1 style. When you exhaust air from the crawlspace to the outdoors, draw air down from the house above, not in from the outdoors. To do this, install two small floor grilles, located far away from the fan in areas that will not eventually become isolated rooms in the basement. The four major benefits of this approach:

• it exhausts air from the crawlspace, which will please code officials;
• by using house air, the crawlspace stays warmer (and thus will have a lower relative humidity); and
• by using house air, the fan constantly draws indoors a small amount of outdoor air, thus improving indoor air quality for the home's occupants.
• provides cross ventilation as required by code.

Suitable pressure differential. Lstiburek recommends that the crawlspace fan setup should be able to maintain a slight negative pressure-between 1 and 2 pascals-within that space compared to the basement above. (Technical note: A Pascal is a very small unit of air pressure. For example, the heated air rising up a water heater flue often operates between 5 and 10 pascals of positive pressure.)

Select proper combustion appliances! All combustion appliances should be able to resist the negative pressures placed on the basement by the crawlspace fan. Because leaky ductwork often creates as much as 5 pascals of negative pressure between the basement and main floors, it can cause combustion gases to be drawn down the water heater flue and into the basement. To avoid this problem, buy either power-vented or sealed-combustion water heaters (this is critical!), fireplaces and furnaces for a home with this particular approach to subfloor ventilation.

Controller runs Jim 24/365. Because the fan provides the home with fresh air, it should operate constantly. This increases the importance of not over-sizing the fan, to prevent over-ventilating the house under wintertime conditions. Lstiburek's sizing formula for the fan: 7.5 cfm per person, plus 0.01 cm/ft2 of floor area. Example: for a 2,000-square-foot, four-bedroom home above a I,000-square-foot basement, the fan should be 7.5 cfm X 5 (people), plus 3,000 X 0.01; that's 37.5 cfm plus 30 cfm = 70 cfm. If the minimum size fan available to you is too large, then add one or two 90-degree elbows to decrease the effective flow.

The HVAC link

As mentioned above, the fan venting the subfloor space will need to operate under slight negative pressure (-1 to -2 pascals), with respect to the basement spaces above. Unfortunately, the negative pressure caused by leaky ductwork within the basement reaches about 5 pascals (or more) of negative pressure in roughly one-third of new homes built today. This sets up an undesirable tug of war with the subtloor exhaust fan.

Solution: seal the ductwork. Sealed ductwork will provide homeowners with improved comfort year-round, re duce comfort callbacks and reduce the potential for bleeding siding (sometimes exacerbated by leaking ductwork within closedfloor systems). Once ductwork is sealed, you can downsize the heating and cooling equipment. However, the airflows and pressures within sealed ductwork will be different than those within leaky ductwork, so your contractor actually needs to carefully redesign (size) the entire duct system to assure proper airflows.

If you don't seal the ductwork, then you should test either the pressure difference created between the basement and subfloor environments, or measure the flow out the exhaust fan, to make sure the ventilation system is operating properly.

Incomplete solutions

Word on the street is that some contractors in some applications have left out key pieces. Examples include the following:

• Case 1-Contractors connected the subfloor to the basement area with floor grilles and an exhaust fan but then didn't select sealed-combustion appliances. This scenario could lead directly to backdrafting of combustion gases down the water heater flue and into the home. Once reburned, those combustion gases can generate dangerous levels of carbon monoxide within the home. Don't do this.
• Case 2-One builder was apparently installing the standard type and size of subfloor exhaust fans-large and noisy. When a 200 cfm fan draws its exhaust air from the house 24/365, it will be overventilating the house, drawing in more air than needed, which will unnecessarily increase utility bills and potentially lead to comfort problems. Don't do this.
• Case 3-One builder combined cases I and 2. Don't even think about doing this.

Whole enchilada

If you use this approach, you need all the pieces. It gets complicated. But the complexity here pales beside the potential problems caused by failure of some current-practice designs. Give this approach some serious thought. Then tune up the details to fit your applications. But please: don't overlook any of the elements.

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