There is no single radon mitigation system. There are six primary system types, each designed for specific foundation conditions — and most homes with elevated radon require one primary method plus supplemental sealing. Knowing which system type applies to your home’s foundation eliminates confusion about what a contractor is proposing and whether the approach matches your situation.
1. Active Sub-Slab Depressurization (ASD)
Active Sub-Slab Depressurization is the most widely installed radon mitigation system in the United States. It is the standard approach for slab-on-grade homes and basement homes with concrete slab floors.
How ASD Works
A suction pipe penetrates the concrete slab, connecting to the aggregate or soil layer beneath. A continuously running electric fan draws air (and with it, radon) from beneath the slab, routing it through PVC pipe to discharge above the roofline. This creates negative pressure in the sub-slab zone relative to the home’s interior — preventing radon from finding pathways through cracks, joints, and penetrations into the living space.
ASD Applications
- Slab-on-grade homes (full footprint slab, no basement)
- Basement homes with concrete slab floors
- Homes with both a basement and upper-level slab additions
- Garage slabs connected to the main living area slab
ASD Governing Standard
AARST-ANSI SGM-SF (Standard of Practice for Mitigation of Radon in Schools and Large Buildings, adapted for single-family) governs ASD installation requirements including diagnostic testing, pipe sizing, fan placement, and performance verification.
2. Active Sub-Membrane Depressurization (ASMD)
Active Sub-Membrane Depressurization is the crawl space equivalent of ASD. Instead of drilling through concrete, the system creates negative pressure beneath a vapor barrier (membrane) installed over the crawl space soil.
How ASMD Works
A heavy-duty polyethylene vapor barrier (minimum 6-mil; professional installations use 10–20 mil) is installed across the entire crawl space floor, lapped up foundation walls, and sealed at all edges and penetrations. A suction pipe penetrates the barrier and connects to the soil or aggregate below via a perforated collection mat. The fan draws soil gas from beneath the barrier, routing it above the roofline through the same type of PVC pipe system used in ASD.
ASMD Requirements
- Foundation vents must be sealed — open vents allow outdoor air into the crawl space, defeating the sub-membrane vacuum
- Barrier seams must be lapped (minimum 12″ overlap) and taped
- Multiple suction points are often needed — crawl spaces typically require 2–4 collection points versus the 1–2 typical in ASD installations
- AARST-ANSI RMS-LB governs ASMD installation standards
3. Drain-Tile Depressurization
Many basement homes — particularly those built after 1980 — were constructed with a drain-tile system: a perforated pipe network running around the interior or exterior perimeter of the foundation, at or below the footing level, designed to channel groundwater to a sump pit. This drain tile can serve as a highly effective radon collection network.
How Drain-Tile Depressurization Works
When a sump pit is present and the drain tile is functional, the mitigator creates suction at the sump pit — either by sealing the pit with an airtight lid and connecting a fan, or by installing a dedicated suction pipe into the drain tile network. Because the drain tile runs around the full foundation perimeter, a single suction point at the sump can create negative pressure across a very large area — often the entire foundation footprint without any slab drilling.
Advantages Over Standard ASD
- No slab drilling required (the drain tile network is already in place)
- Often achieves better sub-foundation coverage than a single slab core hole
- Sump pit is already present — lid modification is the primary work
- Lower installation cost when drain tile is accessible
Limitations
- Requires a confirmed functional drain-tile system — older or poorly maintained tile may be silted or blocked
- Not present in all homes — many older homes and slab-on-grade construction have no drain tile
- May need to be supplemented with slab suction point(s) if tile coverage is incomplete
4. Block-Wall Depressurization
Concrete masonry unit (CMU) block foundation walls have hollow cores that communicate directly with the soil — a significant secondary radon entry pathway in older homes. Block-wall depressurization addresses this specifically.
How Block-Wall Depressurization Works
Small holes (2″–3″ diameter) are drilled through the interior face of the CMU block wall, typically just above the slab level, at 6–8 foot intervals around the affected perimeter. PVC pipe connects these holes, manifolding into the main ASD fan system or a dedicated fan. The fan draws radon from inside the block core cavities before it can migrate through mortar joints and wall cracks into the basement air.
When Block-Wall Depressurization Is Needed
- Post-mitigation testing still shows levels above 4.0 pCi/L after standard ASD is installed
- Visual inspection reveals significant efflorescence, spalling, or moisture infiltration through block walls (indicating active soil gas pathways)
- Home is pre-1975 CMU construction with no poured concrete wall facing
Block-wall depressurization is almost always an add-on to ASD, not a standalone system. Cost: $300–$600 in additional materials and labor when added to an existing ASD installation.
5. Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV)
HRV and ERV systems are whole-house mechanical ventilation systems that exchange stale indoor air with fresh outdoor air while recovering heat (HRV) or both heat and moisture (ERV). They are sometimes used as a radon reduction strategy — primarily in situations where other methods are impractical or as a supplemental approach.
How HRV/ERV Reduces Radon
By continuously introducing fresh outdoor air into the home, HRV/ERV dilutes indoor radon concentrations. They also reduce the negative pressure differential that draws radon into the home from the soil, because they balance indoor and outdoor pressure rather than allowing the home to depressurize relative to the soil.
Limitations as Radon Mitigation
- Less reliable reduction than ASD/ASMD — radon dilution depends on outdoor air exchange rate, and results vary significantly by climate and home tightness
- Higher operating cost — HRV/ERV units consume 100–400 watts versus 20–90 watts for a radon fan
- Does not address the root cause (radon entry from soil) — only dilutes after entry
- Not accepted as primary mitigation in all state radon programs
- Best suited as supplemental to ASD in homes where additional air quality improvement is also desired
EPA and AARST consider ASD/ASMD the preferred primary mitigation method. HRV/ERV may be appropriate as supplemental mitigation or in unusual foundation situations where ASD is genuinely impractical.
6. Natural Ventilation Enhancement
Natural ventilation — opening windows, operating exhaust fans, increasing air exchange — can temporarily reduce radon concentrations. It is not a mitigation system and is not recommended by EPA or AARST as a radon control strategy for several reasons:
- Effective only while windows are open — unpractical in most U.S. climates for the majority of the year
- Increases heating and cooling costs significantly
- Can create negative pressure that worsens radon entry
- Provides no permanent solution
Natural ventilation may be used as a short-term measure while a permanent system is being installed, but it is not a substitute for ASD, ASMD, or other mechanical systems.
Choosing the Right System: Decision Guide
| Foundation Type | Primary System | Common Add-On |
|---|---|---|
| Slab-on-grade | ASD | Sealing (cracks, joints) |
| Basement — poured concrete | ASD | Drain-tile depressurization if sump present |
| Basement — CMU block walls | ASD | Block-wall depressurization |
| Crawl space — vented | ASMD (with encapsulation) | Foundation vent sealing |
| Crawl space — encapsulated | ASMD | Additional suction points if needed |
| New construction (RRNC) | Passive pipe (fan-ready) | Fan activation if post-construction test elevated |
| Combination foundation | ASD + ASMD (separate systems or manifolded) | Sealing at transition zones |
Frequently Asked Questions
What is the most common type of radon mitigation system?
Active Sub-Slab Depressurization (ASD) is the most commonly installed radon mitigation system in the U.S. It applies to slab-on-grade and basement homes — the two most prevalent residential foundation types. For crawl space homes, Active Sub-Membrane Depressurization (ASMD) is the standard.
Can one system work for multiple foundation types in the same home?
Yes, but it typically requires separate or manifolded systems. A home with a basement and a slab-on-grade addition, for example, may need ASD suction points in both zones, connected to a single fan via manifold pipe — or two separate fans if the zones are not contiguous. An experienced mitigator will design for the full footprint, not just the primary foundation type.
Does the type of radon system affect the cost?
Yes, significantly. A standard single-point ASD in a poured concrete basement is the least expensive ($800–$1,500). Adding drain-tile depressurization at the sump typically adds $100–$300. Block-wall depressurization adds $300–$600. ASMD with full crawl space encapsulation can run $2,500–$5,000+ depending on crawl space size and membrane quality.
What type of radon system works in a home with no basement and no crawl space?
Slab-on-grade homes use ASD — a suction pipe drilled through the concrete slab connects to the aggregate beneath. Interior routing typically runs through a garage wall or utility closet to the attic. Exterior routing is an alternative when interior access is limited. The challenge in slab homes is pipe routing to above the roofline without a basement or crawl space to work through — but it is fully achievable in almost all cases.
What is the difference between ASD and ASMD?
Both use a fan to create negative pressure below the home’s floor system. ASD drills through a concrete slab and draws suction from the sub-slab aggregate or soil. ASMD installs a vapor barrier over the crawl space soil and draws suction from beneath the barrier — no concrete is present to drill through. The fan, pipe, and discharge components are identical; only the suction connection method differs.
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