RMS Ventilation & Airflow Management 3 — Questions and Answers
Question 1: What is the recommended minimum exhaust fan airflow rate for a typical residential sub-slab depressurization system?
- 10-20 CFM
- 30-70 CFM (Correct answer)
- 100-150 CFM
- 200-250 CFM
Correct answer: 30-70 CFM
Most residential SSD systems operate effectively in the 30-70 CFM range, though actual requirements depend on sub-slab permeability and building size.
Question 2: During a post-mitigation test, radon levels remain elevated despite an operating SSD system. What should the contractor investigate first?
- Replace the radon test kit with a different brand
- Check for unsealed cracks, openings, or bypass pathways (Correct answer)
- Install a second fan in series
- Increase building temperature
Correct answer: Check for unsealed cracks, openings, or bypass pathways
Persistent elevated radon after SSD installation often indicates unsealed penetrations or cracks that allow radon to bypass the pressure field.
Question 3: Heat Recovery Ventilators (HRVs) are sometimes used as part of a radon mitigation strategy primarily to:
- Generate negative pressure under the slab
- Dilute indoor radon while recovering heating/cooling energy (Correct answer)
- Filter radon from supply air
- Pressurize the sub-slab area
Correct answer: Dilute indoor radon while recovering heating/cooling energy
HRVs dilute indoor radon concentrations by introducing fresh outdoor air while recovering thermal energy from exhaust air to improve efficiency.
Question 4: When a radon mitigation pipe must pass through a conditioned living space, which installation practice is required?
- Pipe must be painted to match wall color
- All pipe joints must be permanently sealed gas-tight (Correct answer)
- Pipe diameter must be reduced to 2 inches
- Pipe must be insulated with fiberglass wrap
Correct answer: All pipe joints must be permanently sealed gas-tight
When mitigation pipes run through conditioned space, all joints must be permanently gas-tight sealed to prevent radon from escaping into the living area.
Question 5: What does 'stack effect' refer to in the context of radon entry into buildings?
- The method of stacking radon test canisters for sampling
- Buoyancy-driven airflow that draws soil gases upward into a building (Correct answer)
- Pipe configuration using vertical stacks
- The accumulation of radon in basement corners
Correct answer: Buoyancy-driven airflow that draws soil gases upward into a building
Stack effect describes buoyancy-driven air movement where warmer indoor air rises and exits at upper levels, drawing cooler soil gas (including radon) in at lower levels.
Question 6: A sub-membrane depressurization (SMD) system differs from SSD primarily because:
- SMD uses higher-powered fans
- SMD is installed in crawl spaces under a vapor barrier (Correct answer)
- SMD discharges exhaust indoors
- SMD only works in new construction
Correct answer: SMD is installed in crawl spaces under a vapor barrier
SMD systems are designed for crawl spaces where a membrane (vapor barrier) is laid over the soil and the suction pipe draws from beneath that membrane.
Question 7: Which condition most severely limits the effectiveness of a single-suction-point SSD system?
- High indoor radon concentrations above 10 pCi/L
- Low sub-slab permeability with isolated aggregate pockets (Correct answer)
- Exterior temperatures below freezing
- Building height greater than two stories
Correct answer: Low sub-slab permeability with isolated aggregate pockets
When sub-slab material has low permeability or is divided into isolated pockets, a single suction point cannot extend its pressure field throughout the entire area.
RMS Ventilation & Airflow Management 3
What is the recommended minimum exhaust fan airflow rate for a typical residential sub-slab depressurization system?