RMS Building Science & Radon Entry 3 — Questions and Answers
Question 1: Which HVAC condition is most likely to increase radon entry into a home?
- A balanced ventilation system with equal supply and exhaust
- A central return air system that depressurizes the lower level (Correct answer)
- A supply-only ventilation system that pressurizes the house
- A heat recovery ventilator (HRV) operating at rated capacity
Correct answer: A central return air system that depressurizes the lower level
Central return air systems, especially those drawing air from lower floors without balanced supply, can depressurize the basement and increase the pressure differential that draws radon in from the soil.
Question 2: What percentage of indoor radon typically enters through the building foundation and sub-slab soil in most US homes?
- Less than 20%
- Around 35%
- Up to 50%
- Greater than 80% (Correct answer)
Correct answer: Greater than 80%
Studies show that in the vast majority of homes, more than 80% of indoor radon enters through foundation openings and cracks from soil beneath and around the structure.
Question 3: Hollow concrete block foundation walls are especially problematic for radon because they:
- Contain radium deposits that emit radon directly
- Create internal voids that act as conduits for soil gas flow (Correct answer)
- Absorb moisture, increasing radon emanation
- Block sub-slab depressurization systems from working
Correct answer: Create internal voids that act as conduits for soil gas flow
The hollow cores of concrete block walls provide interconnected channels through which soil gas can travel laterally along the wall and enter at any opening, including mortar cracks.
Question 4: Radon entry through building materials (e.g., concrete, granite countertops) is classified as which type of entry mechanism?
- Advection from outdoor air
- Diffusion from the material itself (Correct answer)
- Pressure-driven infiltration
- Convective transport
Correct answer: Diffusion from the material itself
When radon emanates from radioactive decay within building materials and migrates through pore spaces into indoor air, the mechanism is diffusion driven by concentration gradients.
Question 5: Which of the following best describes the relationship between indoor-outdoor pressure differential and radon entry rate?
- Radon entry rate is independent of pressure differential
- Greater negative indoor pressure increases radon entry rate (Correct answer)
- Positive indoor pressure increases radon entry rate
- Pressure differential only affects radon in crawlspaces
Correct answer: Greater negative indoor pressure increases radon entry rate
A greater negative indoor pressure relative to the soil creates a stronger driving force that pulls more soil gas, including radon, through foundation openings per unit time.
Question 6: A home built on a crawlspace with a polyethylene ground cover but open vents would be expected to have radon levels that are:
- Higher than an unventilated crawlspace with no ground cover
- Lower than an unventilated crawlspace with no ground cover (Correct answer)
- Identical to a fully conditioned crawlspace
- Unaffected by the ground cover or venting
Correct answer: Lower than an unventilated crawlspace with no ground cover
Ground cover reduces radon emanation from exposed soil and venting dilutes radon in the crawlspace, both of which lower the concentration driving force into the living space above.
Question 7: Why does soil gas entry typically exceed outdoor air infiltration as a radon source, even though both enter through similar openings?
- Outdoor air contains no radon at all
- Soil gas has radon concentrations hundreds to thousands of times higher than outdoor air (Correct answer)
- Soil gas enters under positive pressure while outdoor air enters under negative pressure
- Outdoor radon decays before reaching the foundation
Correct answer: Soil gas has radon concentrations hundreds to thousands of times higher than outdoor air
Soil gas radon concentrations commonly range from 10,000 to 100,000 pCi/L, while outdoor air averages about 0.4 pCi/L, so even small volumes of soil gas entry significantly elevate indoor levels.
RMS Building Science & Radon Entry 3
Which HVAC condition is most likely to increase radon entry into a home?