HVAC - Floor Plenum - 2/21/2001 - House Systems HVAC Heating Air Conditioning

Floor Plenum

Question:

We plan to build a 1200 square foot home, and are interested in a high level of energy efficiency. Several other homes in this Sierra foothill region have used a floor plenum system with apparent success. Should we use this system?

Answer:

The use of the crawl space under the home as a plenum for the distribution of heated air was developed and investigated under funding from several wood technology organizations in the 1970s. The system was called the Plen-Wood system.

The idea of a floor plenum is that a crawlspace, with specific detailing including insulation and air sealing, can be used for air distribution. Heated air is delivered at one point, typically from a downflow furnace, and room air registers simply cut through the floor where needed. This system has the potential to reduce construction costs and increase comfort, but also has several potential liabilities that must be considered before deciding to incorporate it in construction of a home.

1. Codes. Many jurisdictions require fully-ducted heating supply runs, and will not permit use of an open crawlspace plenum.
2. Increased air leakage. Forced air from the furnace will pressurize the entire crawlspace plenum to a slight positive pressure. For any given level of air sealing, the result is likely to be higher air leakage and resultant heat loss.
3. Increased heat loss area. Even if the crawlspace is detailed for excellent air tightness, the heating of the entire floor plenum area to a temperature over 100ºF when the furnace operates will generally lead to greater overall heat loss.
4. Possible increased radon exposure. In high radon areas, any defect in the polyethylene film (plastic sheeting) placed on the crawlspace floor may lead to radon accumulation in the crawlspace, from which it will readily move to the occupied portions of the home.
5. Danger of condensation under cooling conditions. The delivery of cool air to the crawl space could lead to condensation of moisture from hot, humid outdoor air if it comes into contact with poorly insulated or air sealed portions of the crawlspace.

The designer or builder of a home using the floor plenum system should be aware of these potential shortcomings, get early code approval, and design carefully to avoid negative consequences.

Another related construction method that builders using crawlspace foundations might consider is the sealed, insulated crawlspace. Increasingly, researchers and energy conscious builders are adopting crawlspace construction methods that treat the crawlspace much like a basement, eliminating vents and adding airtightness details, and insulating the walls and floor. The heating (or cooling) energy normally lost through the floor from the living area and from ducts in the crawlspace is kept inside the overall building rather than being wasted through ventilation, and the risk of problematic condensation and of frozen pipes is reduced. Check with your code officials before specifying a sealed crawlspace, as this type of construction may not be permitted in many locations.