FLOOR PLENUM


GOAL:
To become aware of energy conscious design and construction that will save energy, cost less to operate, and have less of a negative impact on the natural environment.

OBJECTIVES:
The student will better understand underground thermal properties and how it can be used to provide a more energy efficient home by designing an underfloor plenum for a residence.

GLOSSARY:
Underfloor Plenum - The space between the floor and the ground (the crawl space) used to supply or return air from the mechanical system.
Thermal Lag - The time delay in the temperature rise of the fluid leaving the collector during startup because part of the insulation goes to heat the collector itself. In a building, the time delay in the transmission of heat from an external to an internal surface.
Plenum - A cavity through which air is moved. In some passive solar designs a plenum may be used to evenly distribute heat which otherwise would collect at a single point. The cavity can be used for supply air, return air, or can be switched to use for either.
Chain Wall - A short wall or knee wall.
Perimeter - The outer boundary of the house.
Berm - A man-made mound or small hill of earth.

LESSON/INFORMATION:
Underground temperatures can be very beneficial in balancing the thermal comfort of the home. Normally we think more about the above ground temperatures and other climatic elements in designing a home for thermal comfort. The better design use and understanding of these elements produce a naturally comfortable home. An energy efficient design means that the mechanical system, which should be used for the extreme swings in climate, can be kept to a minimum size and should not have to be used as often as in a conventional house.

If you take the average yearly temperature (see Table 1) of an area, that will approximately give the constant ground temperature at about 30 feet below the surface of the ground in an area which is exposed to the sky. Sky exposure will assure that there is heat gain during the day and heat loss at night.

Table 1
City Average Temperature
Alexandria 65.0°F
Baton Rouge 67.4°F
Lafayette 68.0°F
Lake Charles 68.3°F
New Orleans 68.3°F
Shreveport 65.9°F

These constant temperatures of 65°F to 68°F can be fairly comfortable in both summer and winter. What is even more interesting is that the temperature at 15 feet below ground is even better. Because of the thermal lag due to the mass of the earth, the temperature at 15 feet is cooler during the summer months and warmer during the winter months (See Figure 1).
dr-h60
Figure 1 Underground Temperature of 30° N. Latitude

When a building is put on a site, the ground can no longer receive heat during the day and lose heat at night. This tends to raise these underground temperatures closer to the surface of the ground and closer to the building.

To take advantage of these comfortable temperatures is easily accomplished with an underfloor plenum. A plenum, whether underfloor or above the ceiling, is used as a large air supply or air return space. Duct work is sized according to CFM (cubic feet of air per minute) needed in a room. A plenum system is based on air pressure (supply) or vacuum (return).

The underfloor plenum requires a continuous chain wall around the perimeter of the house. This chain wall is air tight, insulated, and water-proofed on the exterior. Berming dirt around the chain wall helps to thermally ground the house to get closer to those more comfortable temperatures, and provides for good drainage away from the house to keep the plenum dry. Visqueen is laid on the ground with an 8 inch overlap at the joints to prevent moisture vapor migration.

Figure 2
dr-h61a
The most efficient use of the plenum is for air supply in the winter (See Figure 2) and air return during the summer (See Figure 3). To switch from one to the other is accomplished with a damper at the HVAC unit.
dr-h61b
Figure 3

It is possible to be comfortable during much of year by just moving unconditioned air through the plenum. This is done by turning just the fan from the HVAC unit on which moves the comfortable temperatures from the plenum to the rest of the house.

ACTIVITY:
Indicate whether the following statements are TRUE or FALSE.

___1. Underground temperatures can be very beneficial in balancing the thermal comfort of the home.

___2. An energy efficient design means having a large mechanical system.

___3. Underground temperatures become constant at about 30 feet below the surface of the ground.

___4. The constant underground temperature is the same as the average yearly temperature of an area.

___5. A plenum is used to supply or return air.

___6. The underfloor plenum is a crawlspace open to the outside.

___7. Visqueen on the ground in the plenum prevents moisture migration from the earth.

___8. The natural temperature of the plenum is comfortable most of the year.

___9. The comfortable temperature in the plenum can be moved to the rest of the house with a simple fan.

___10.The temperature of the plenum remains fairly constant throughout the year.

TEACHER'S NOTES:
1. Encourage students to investigate the use of different materials.

2. Pin up the different solutions so all students can see them, and have a discussion on the positive and negative aspects of them.

3. Send for the Plen-Wood System brochure. American Plywood Association, Post Office Box 11700, Tacoma, WA 98411

ANSWERS TO TRUE/FALSE:
1. True
2. False
3. True
4. True
5. True
6. False
7. True
8. True
9. True
10. True

RECOMMENDED READING:
Plen-Wood Systems brochure

Natural Louisiana Architecture. Cazayoux, Hebert and Winn. Baton Rouge, LA: Louisiana Department of Natural Resources, 1991.

REFERENCES:
"The Comfort Zone" .....E.J. Cazayoux, USL Monthly Press Release.

Comments or questions to: TechAsmt@LA.GOV

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