How Sub-Slab Ductwork Could Undermine Your Radon Mitigation Efforts
What is Sub-Slab Ductwork and Why Should You Care?
Sub-slab ductwork might sound technical, but if your Calgary home has it, you could be dealing with more than just HVAC airflow—it could be a highway for radon gas. This type of duct system places your home’s ventilation ducts beneath the concrete slab foundation. Builders often used this method in homes constructed between the 1960s and 1990s to save space and improve energy efficiency. But while it might have made sense back then, sub-slab ductwork can complicate modern radon mitigation efforts.
Radon is an odorless, colorless gas that seeps from the ground into homes, posing a serious health risk. Prolonged exposure to high radon levels is the second-leading cause of lung cancer, according to Health Canada. If your home has sub-slab ductwork, the very system designed to circulate clean air could actually be pulling radon-laden air from beneath the slab and distributing it throughout your home.
How Sub-Slab Ductwork Increases Your Radon Risk
Gaps Around Ductwork Let Radon Gas Sneak In
Sub-slab duct systems often develop gaps and cracks over time, especially as the home settles. These gaps allow radon gas from the soil to enter the ductwork and, in turn, your living spaces. I’ve seen homes where the ducts looked perfectly fine on the surface, but closer inspection revealed small cracks around duct joint - enough to let radon in.
Imagine this scenario: Your HVAC system kicks on, circulating what you assume is fresh air throughout your house. But instead, it’s drawing radon gas from below the slab and spreading it room to room. This can happen without you knowing unless you’ve conducted radon testing recently.
Negative Air Pressure Draws Radon Indoors
Another problem with sub-slab ductwork is that it can create negative air pressure. If the ducts aren’t perfectly sealed, the air inside the ducts can escape into the ground below the slab, causing the surrounding soil to fill the void by pulling more radon gas toward your home.
This “suction effect” is like trying to drink a milkshake with a leaky straw—you pull harder to get the drink, and in this case, your HVAC system pulls more radon from the soil.
Why Sub-Slab Ductwork Complicates Radon Mitigation
Standard Mitigation Systems Might Not Be Enough
Homes without sub-slab ducts typically rely on an Active Soil Depressurization (ASD) system, which creates negative pressure under the slab to keep radon out. However, when ducts run beneath the slab, the airflow dynamics change. If the ducts are leaking air, they disrupt the pressure field the ASD system relies on, making it less effective at keeping radon at bay.
That’s why homes with sub-slab ducts often require customized mitigation systems that take both the HVAC and radon risks into account.
Sealing Gaps Can Be Tricky
If your ducts have leaks or the joints aren’t properly sealed, you’re not just dealing with a radon problem—you’re also dealing with air leaks that affect your HVAC efficiency. Sealing these gaps is essential for effective radon mitigation, but it can be tricky since sub-slab ducts aren’t always easy to access. Using high-quality sealants, such as Titebond radon sealant, is critical for a long-term solution.
In one project we handled, a homeowner had sealed cracks around their ducts themselves but missed some key spots. When we tested the home again, radon levels were still high because just one small gap was letting gas seep in.
Routine Inspections and Maintenance Are Essential
Homes with sub-slab ductwork also require more frequent inspections to keep radon levels in check. Over time, seals can deteriorate, new cracks can develop, and HVAC systems can shift, all of which could allow radon to enter. Regular maintenance ensures that your home remains safe and that your mitigation system continues to perform as intended.
Effective Strategies for Mitigating Radon in Homes with Sub-Slab Ductwork
Step 1: Conduct a Radon Test
If you’ve never tested your home for radon, the first step is to conduct a long-term radon test. Here's what to look for when purchasing a radon testing device. I recommend using devices which measure radon levels over several months, giving you an accurate picture of any fluctuations. Since sub-slab ducts increase radon risk, testing is especially important in homes with these systems.
Step 2: Seal Duct Leaks and Cracks
Proper sealing is critical. A professional will inspect your ducts and use specialized sealants designed for radon mitigation. Don’t use just any caulk—radon sealants are more durable and ensure a better seal against soil gases. This step alone can significantly reduce radon levels, especially when combined with a mitigation system.
Step 3: Install a Customized ASD System
Since standard ASD systems might not work optimally with sub-slab ductwork, your radon professional will design a customized system that complements your HVAC setup. They may need to install additional vents or use different fan types, such as electronically commutated (EC) motor radon fans, to ensure optimal performance.
For homes with complex sub-slab layouts, we often recommend fans with variable speeds, which allow us to fine-tune the pressure under the slab.
Step 4: Monitor Radon Levels Regularly
Even after sealing and installing a mitigation system, it’s important to retest your home. Radon levels can change over time, especially as your HVAC system ages or shifts. Using continuous radon monitors provides real-time data, ensuring your home stays safe year-round.
When to Call in a Radon Expert
Know the Signs of Radon Intrusion
Homes with sub-slab ductwork might not show obvious signs of a radon problem, but certain red flags can help you spot an issue early.
Inconsistent airflow from your HVAC system.
Moisture buildup or strange smells near your ducts.
Sudden spikes in radon levels after HVAC use.
If you notice any of these, it’s time to schedule a professional inspection. Here are 5 signs you may need a radon mitigation system.
Hire a C-NRPP Certified Professional
Mitigating radon in homes with sub-slab ductwork isn’t a DIY project. Make sure to hire a C-NRPP certified radon mitigation professional. These experts have the training and experience to design mitigation systems that work effectively, even in challenging setups.
At Spectra Radon, we specialize in tackling complex radon issues, including homes with sub-slab ductwork. Our team ensures every crack is sealed and every system is optimized for maximum safety.
Protect Your Home with the Right Mitigation Plan
Radon exposure is a serious concern, and if your home has sub-slab ductwork, it’s crucial to address the additional risks it presents. Proper sealing, professional testing, and customized mitigation systems are the best ways to keep your home safe from radon gas.
Don’t wait until it’s too late—schedule a radon test today to protect your family’s health. If your home needs a customized solution, Spectra Radon has you covered with expert services tailored to meet Health Canada guidelines.
Take the first step towards a safer home. Contact Spectra Radon for a risk free consultation, and we’ll help you design the perfect radon mitigation plan for your home, even if sub-slab ductwork is part of the picture.