RMS Building Science & Radon Entry 2 — Questions and Answers
Question 1: Which foundation type typically presents the greatest challenge for radon mitigation due to the large soil contact area?
- Basement with poured concrete walls
- Slab-on-grade construction (Correct answer)
- Crawlspace with bare soil floor
- Pier-and-beam foundation
Correct answer: Slab-on-grade construction
Slab-on-grade foundations have extensive soil contact across the entire floor footprint, making it difficult to create a sub-slab depressurization field that covers the whole area.
Question 2: The 'stack effect' in buildings causes radon levels to be highest in which location?
- Upper floors and attic
- Middle floors
- Lower floors and basement (Correct answer)
- All floors equally
Correct answer: Lower floors and basement
The stack effect creates negative pressure at the lower levels of a building relative to outdoor air, drawing soil gases including radon upward through foundation openings.
Question 3: What term describes the process by which radon-laden soil gas is pulled into a building due to indoor-outdoor pressure differences?
- Diffusion
- Advection (Correct answer)
- Emanation
- Adsorption
Correct answer: Advection
Advection is the bulk flow of soil gas driven by pressure gradients, and it is the dominant radon entry mechanism in most buildings when indoor pressure is lower than soil pressure.
Question 4: Which soil characteristic most strongly promotes radon transport toward a building foundation?
- High clay content
- High moisture content
- High permeability (coarse gravel or sand) (Correct answer)
- Low porosity
Correct answer: High permeability (coarse gravel or sand)
Highly permeable soils such as coarse gravel and sand allow soil gas to move freely, greatly increasing the rate of radon transport to the building foundation.
Question 5: A sump pit without a sealed lid is a significant radon entry pathway primarily because it:
- Contains standing water that emits radon
- Provides a direct opening to permeable gravel fill below the slab (Correct answer)
- Increases indoor humidity, enhancing radon solubility
- Creates positive pressure under the slab
Correct answer: Provides a direct opening to permeable gravel fill below the slab
Sump pits typically penetrate through the slab into the permeable drainage aggregate below, creating a direct, low-resistance pathway for soil gas to enter the living space.
Question 6: During winter in cold climates, radon levels in basements often increase because:
- Radioactive decay rates increase at lower temperatures
- Windows and doors are closed, reducing dilution ventilation (Correct answer)
- Soil becomes more permeable when frozen
- Radium concentration in soil increases in cold weather
Correct answer: Windows and doors are closed, reducing dilution ventilation
In winter, buildings are tightly closed to conserve heat, which reduces natural ventilation and allows radon to accumulate to higher concentrations.
Question 7: Floor-wall joints in poured concrete basements are common radon entry points because they:
- Are made of a more porous concrete mix
- Often have shrinkage gaps where the floor slab meets the foundation wall (Correct answer)
- Contain no reinforcing steel to block gas flow
- Are exposed directly to outdoor soil pressure
Correct answer: Often have shrinkage gaps where the floor slab meets the foundation wall
As concrete cures and settles, a small gap commonly forms at the joint between the floor slab and the foundation wall, providing a continuous pathway for soil gas entry.
RMS Building Science & Radon Entry 2
Which foundation type typically presents the greatest challenge for radon mitigation due to the large soil contact area?