November 30, 2007

Engineering studies have determined that the days of venting crawlspaces with outdoor air must come to an end. The old rule of 1 square foot of free area for every 150 square feet of crawlspace (1:150 rule) has scant scientific backing and should not be applied.

First, we must understand where the 1:150 rule originated. The rule was first published in a January 1942 FHA document called "Property Standards and Minimum Construction Requirements for Dwellings." The rule required a minimum ventilation area of 1:150 of the footprint of the house. While the anonymous FHA author recognized that the best way to control moisture is to put a vapor barrier on the ground, a suitable product was not available (polyethylene was not invented until the early 1950s). Thus, generous ventilation was recommended without conducting proper studies to verify the efficacy of this approach.

The problem with venting crawlspaces is that too much humidity in summer months (or all year, depending on location) creates conditions conducive to mold growth on surfaces within the crawlspace. Specifically, the relative humidity becomes higher within the crawlspace because the air temperature decreases. For example, outdoor air at 80 degF and 65% RH entering a crawlspace might cool to 70 degF, resulting in 90% RH. And mold can grow simply from RH over ~80%; bulk water or condensation on surfaces is not necessary.

The latest and greatest recommendations by research groups such as the Energy & Environmental Building Association suggest the following:

Assure proper grading, extended downspouts, etc. to manage precipitation and surface water.

Completely seal the crawlspace from the outdoors.

Apply continuous 6-mil polyethylene sheeting to the floor, fastened 12” up the foundation wall, and with all joints taped. Poly is required even if the floor of the crawlspace is concrete (however, poly may have been installed under the concrete floor – ask your builder).

Make the crawlspace part of the conditioned space, just like a basement with a very low ceiling.

If necessary, install a dedicated dehumidifier for the space and set the controls to limit the RH to <60%.

Thermohygrometers that have a the ability to record RH over time can be purchased from stores such as Radio Shack® for <$50.

Consulting with an engineering/building science expert is warranted if a person decides to veer from the newly beaten path outlined above due to a crawlspace being inaccessible or where these recommendations are not cost-effective.

Buildings that have had improper crawlspace design may have supported significant mold growth or may have housed rodents and other animals. To determine if a crawlspace requires only redesign or if surface cleaning is also warranted, contact an experienced firm with both engineering and microbiology/air quality credentials.

For more information regarding crawlspace ventilation or other indoor air quality concerns, please contact Cassidy Kuchenbecker at (608) 785-1900 or [email protected].