Comparing Indoor Air Pressure to Soil-Gas Pressure

Keeping air pressure in the lowest rooms slightly higher than the pressure of gasses in the soil can provide the following benefits:

Minimize entry of radon through pores, cracks, and holes in the floor. Radon causes an estimated 20,000 cases of lung cancer annually in the U.S.
Minimize entry of water vapor from the soil through pores and cracks in concrete floors. NOTE: Indoor water vapor raises air conditioning costs. Indoor humidity above 50% encourages growth and reproduction of dust mites. Indoor humidity above 70% encourages the growth of molds (fungi) on organic materials such as wood, paper, etc..
Assist in removal of combustion gasses through natural draft (non-powered) chimneys used with heating equipment in the lowest room.
Minimize entry of other gasses from the soil through pores, cracks and holes in the floor.
Minimize entrance of moisture through the walls and roof. Reference: ANSI/ASHRAE Standard 62.2-2004 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.

NOTE: A negative air pressure in the lowest room of some buildings can pull in fumes from lawn insecticides and septic tank drainage fields through cracks, holes and pores in concrete floors and foundation walls.

Unsaturated soil is porous, so the soil-gas pressure under buildings is usually equal, or nearly equal, to outdoor air pressure.

The United States EPA recommends sealing air leaks in the shells of buildings to reduce energy costs, allow for improved environmental control, and minimize the amount of outdoor air needed to maintain a slight positive indoor air pressure. “Measurements in existing schools show that a slight positive air pressure equal to the pressure of as little as 0.001 inches of water column (.25 Pascals) relative to sub-slab and outdoor air pressure, reduces indoor radon levels by preventing radon entry.” NOTE: In buildings that continue to have too much air leakage area in the building shell, it may be impractical to consistently keep indoor air pressure higher than the pressure of gasses in the soil. The next best option is to prevent indoor air pressure from becoming significantly lower than the pressure of gasses in the soil.

In the following example, the Pressure-Detective^TM is used to determine if the air pressure in the lowest room is higher than the soil-gas pressure under a concrete floor.

This floor-fitting for used to access the soil-gas pressure under a concrete slab that sits on top of soil or gravel.

Parts include:

Nipple, 1/4 inch x 3 inch RB
Hose Barb, 1/4" Hose Barb x 1/2" Female Pipe Threads
Grommet
Electrical tape

The floor-fitting is provided with each unit at no additional cost.

For your convenience, a six-foot length of 5/8 inch O. D. clear plastic hose is
included with each unit. Please let us know if you need a different length.

Notes:

1. The grommet is used to prevent the assembly from falling through the hole in the floor.
2.The lower tape-wrap must have an outer diameter that will fit snugly in the hole in the floor. Avoid stretching the tape as it is wound around the nipple. Otherwise, the wound tape is likely to slip up, or down, as it shrinks.

Drill a 3/4 inch diameter hole through the floor and at least three inches into the soil.
CAUTION: Drill far enough away any foundation wall to avoid drilling into foundation footings.
Insert the floor fitting part way into the hole in the floor
Use a caulking gun to inject silicone, or equivalent, sealant beneath the grommet and above the tape-wrap to prevent air leakage.
Press the assembly down so that the grommet rests on the floor.
Mount the Pressure-Detective vertically or horizontally.
Connect one end of the 1/2" I.D. hose to the floor fitting.
To enlarge the picture, click on it.
Connect the other end of the clear plastic hose to the outlet port of the pressure sensor. Leave the inlet port open and exposed to the room air pressure.
The balloon will expand and the reflection of the colored disk will disappear whenever the the indoor air pressure is sufficiently greater than the soil-gas pressure by at least:

0.5 Pascals (.002" water column) when the unit is mounted on a wall like a clock.

2.0 Pascals (.012" water column) when the unit sits on a horizontal surface facing upwards.

When the indoor air pressure becomes less than, or equal to, soil-gas pressure, the balloon will collapse and the reflection of the colored disk will reappear.

NOTE: Model PD7E provides an electrical signal whenever the balloon expands.