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Radon in Wood Foundation Homes: A Canadian Buyer's Guide
Wood foundations sound like something you would find on a century-old farmhouse, but they are more common in newer Canadian homes than most buyers realize. A preserved wood foundation, often called a PWF, is built from pressure-treated lumber instead of concrete. PWFs are widespread across the Prairies because they perform well in cold climates and are quicker to install.
Here is what most buyers do not know: a home with a wood foundation can still have high radon levels. The foundation material does not change how soil gas moves into a building, and in some cases, the air leakage paths inside a wood foundation make radon entry easier.
This guide walks through a real inspection where a home with a preserved wood foundation tested above the Health Canada radon guideline of 200 Bq/m³. You will learn what the inspection found, why wood foundations behave differently for radon, and what to ask about during the purchase process.
Watch the Inspection
https://www.youtube.com/watch?v=mki-1v6ROD0
Key Issues Found
- The home tested above 200 Bq/m³ on a short-term radon monitor (in the 240 Bq/m³ range)
- The home was built on a preserved wood foundation (PWF) rather than a concrete foundation
- Soil gas was entering through penetrations in the polyethylene vapour barrier and around plumbing lines
- A long-term follow-up test was recommended to confirm seasonal radon levels
- Active soil depressurization (ASD) mitigation was recommended, with design adjustments for the wood floor system
What Is a Preserved Wood Foundation?
A preserved wood foundation, or PWF, is a foundation system built from pressure-treated wood instead of poured concrete or concrete block. PWFs typically include a wood-framed floor system over a sub-slab gravel layer, with a heavy polyethylene sheet acting as both a vapour barrier and a soil-gas barrier.
PWFs are common in newer Canadian homes, particularly across Alberta, Saskatchewan, and Manitoba. They are faster to build than concrete and easier to insulate, but they require careful sealing to keep moisture, pests, and soil gases out. The Canadian Wood Council maintains the design standards for PWFs in Canada.
Why Wood Foundation Homes Can Still Have Radon Problems
Radon is a soil gas. It comes from the ground, not from the foundation material. As long as the home sits on uranium-containing soil, which describes most of Alberta, radon will be present in the soil gas beneath the building.
The foundation's job is to keep that soil gas from entering the living space, but wood foundations have different vulnerabilities than concrete:
- The poly vapour barrier can be punctured by plumbing, electrical, and mechanical penetrations
- Seams in the poly sheet must be properly lapped and sealed, and often are not
- The wood floor system can have gaps that let sub-floor air leak into the basement
- Settling and movement over time can pull seals apart
When any of these issues exist, soil gas, including radon, has a clear path into the home.
What the Radon Test Showed
The home in this inspection tested at approximately 240 Bq/m³, above the Health Canada action level of 200 Bq/m³. That is the level at which Health Canada recommends taking action to reduce radon.
The reading was taken on a digital radon monitor on-site during the inspection. Short-term results give a quick picture but can be affected by weather, ventilation, and how the home is being used. A long-term test, typically 90 days, gives a more accurate picture of year-round exposure.
In this case, the level was high enough that mitigation planning could begin immediately, with the long-term test running in parallel to confirm seasonal patterns.
How Mitigation Works on a Wood Foundation
Active soil depressurization, the standard radon mitigation method in Canada, can absolutely be installed in a home with a preserved wood foundation. The design needs to be adapted:
- The suction point must be placed where it can pull evenly under the wood floor system
- The poly sub-floor barrier must be sealed at all penetrations
- Air-tightness of the floor system becomes important, since the system creates negative pressure beneath it
- Two suction points may be required if the under-floor area is divided by support beams or partition walls
A contractor without PWF experience may install a system that works fine in a concrete-foundation home but underperforms in a wood-foundation one. This is why building diagnostics, sometimes called communication testing, are non-negotiable for wood foundation homes. If you want a deeper explanation of how active soil depressurization works, read Radon Mitigation Explained for Canadian Homebuyers.
What This Means for Homebuyers
If you are buying a home with a preserved wood foundation, do not assume the foundation material protects you from radon. It does not.
Ask for a recent radon test. If there is not one, plan to do a long-term test after closing. If radon levels are elevated, work with a C-NRPP certified contractor who has specific experience with wood foundation homes. Mitigation is highly effective on PWFs as long as the system is properly designed.
The cost of mitigation may be slightly higher on a PWF than on a concrete foundation, because of the extra sealing work involved. It is still a one-time investment that protects your family's long-term health.
Radon is one of several hidden hazards buyers should watch for in Canadian homes. Older homes can also conceal materials like vermiculite insulation in the attic, which carries its own asbestos-related risks. Both are reasons to never skip the right testing.
What to Look For
Use this checklist when assessing a wood foundation home:
- Identify whether the home has a wood, concrete, or hybrid foundation
- Check for visible penetrations through the poly vapour barrier (plumbing, electrical, HVAC)
- Look at the seams of the poly sheet where you can see them. Are they overlapped and sealed?
- Look for a sump pit, and confirm whether it is sealed and capped
- Ask your inspector whether the basement smells musty or earthy, which can signal soil gas infiltration
- Confirm whether a radon test has been performed, and ask to see the results
Frequently Asked Questions
Are wood foundation homes safe?
Yes, when properly built and maintained, preserved wood foundations are a code-approved foundation system in Canada. The pressure-treated wood is rated for ground contact, and a PWF can last as long as the home. The key issues for buyers are moisture management and air sealing, both of which affect radon entry.
Can a wood foundation have higher radon levels than a concrete one?
Sometimes. Radon levels depend on the soil under the home and how well the foundation is sealed, not on the foundation material itself. A poorly sealed PWF can have higher radon than a well-sealed concrete foundation, and the reverse is also true.
Do I need a special contractor for radon mitigation in a wood foundation home?
Yes. Look for a C-NRPP certified contractor with specific experience designing mitigation systems for preserved wood foundations. The system design and sealing requirements are different from a concrete home.
What is the Health Canada action level for radon?
Health Canada's current action level is 200 becquerels per cubic metre (Bq/m³). At or above this level, Health Canada recommends taking action to reduce radon in the home.
Can I just seal the wood foundation to fix the radon problem?
Sealing alone almost never solves a radon problem. It can help, but the only reliable way to bring levels safely below the guideline is an active soil depressurization system. Sealing supports the mitigation system. It does not replace it.
How long does a radon test take?
A short-term test runs from 48 hours to 7 days and is useful for quick screening. A long-term test runs at least 90 days and gives a much more accurate picture of average exposure. Health Canada recommends a long-term test for any final decision about mitigation.
Is radon worse in winter?
Radon levels are often higher in winter in Canadian homes. Homes are closed up tighter, furnaces depressurize basements, and frozen ground above the soil pushes more soil gas down and into the building. Testing in winter is a worst-case snapshot.
My realtor said radon is not a big deal in newer homes. Is that true?
No. Newer homes are often tighter, which means radon that gets in tends to stay in. Radon levels in new construction are frequently as high or higher than in older homes. Every home in Canada, new or old, should be tested.
Final Thoughts
A wood foundation home is not automatically a radon problem, but it is not automatically radon-safe either. The only way to know is to test. If levels come back high, mitigation works as well on a PWF as on a concrete foundation, provided the system is properly designed.
If you are buying a home in southern Alberta, including Medicine Hat and Brooks, factor radon testing into your purchase planning. It is one of the lowest-cost, highest-impact things you can do for your family's long-term health.
I work for you, not the deal. When I inspect a home, I tell you what I find, and I help you understand what it means. For more on the kind of issues that get missed during a typical walk-through, read Home Inspection Today: Learn What Others Miss.