Data recorders were put into crawl spaces of several homes. Two homes are side by side. A third data recorder logs outdoor conditions at these two homes. Both homes have insulated floors and vapor barriers on the crawl space earth. It is winter and foundation vents are closed in both homes. One home has a “vapor pressure” controlled mechanical ventilation device (a fan). The other does not. Peaks are periods of rain. There are periods of intermittent rain, warm dry air, and cool dry air.

The vapor pressure controlled device turns on its fan when outdoor vapor pressure is lower than crawl space vapor pressure. Thus, a high ventilation rate occurs only when outside air has a drying effect.

1. The most important fact is how quickly crawl space relative humidity rises and falls
   with outdoor vapor pressures. When a weather front moves in or out, and outdoor
   vapor pressure rises or falls, humidity in both crawl spaces rises or falls, immediately.
2. If moisture had to move through the ground and vapor barriers, these relative
   humidity changes could not possibly occur immediately.
3. The average relative humidity in the home without the vapor pressure controlled
   device is 70%, whereas the home with the device averaged 52%. Using a fan only at
   the right time becomes highly beneficial.
4. This is winter. In summer, without a ventilator, even with all the foundation vents
   open, the relative humidity will be 80 to 90%. See Reference : Samuelson “Relative
   humidity exceeded 90% for 44 weeks”.

Smartvent, Inc.
3255 Cimarron Drive
Conway, AR 72032

Web: http://www.smartvent.net
Phone: 501-329-4915
E-mail: sales@smartvent.net