Schools, daycares, apartment buildings, clinics, and offices sit on the same fractured bedrock as every house in Duluth — but the building is bigger, the pressure fields are more complicated, and the fix starts with a diagnostic phase rather than a fan.
Commercial radon work is not a scaled-up house. A 900-square-foot basement slab behaves as one connected space. A 40,000-square-foot slab under a school, an apartment building, or a converted warehouse behaves as several separate spaces that happen to share a roof, and treating it like one is the most common and most expensive mistake in this field.
Three things change the moment a building gets large. First, slab area. Soil gas has to travel under the concrete to reach a suction point, and the distance one fan can influence is limited by what is under the slab — clean gravel moves air a long way, compacted clay barely moves it at all. Second, interior structure. Footings under demising walls, corridor walls, and load-bearing partitions run down through the slab and act as dams, cutting the sub-slab area into compartments that do not communicate with each other. Third, mechanical systems. A commercial building has air handlers, exhaust fans, makeup air units, and kitchen or lab exhaust that create pressure fields of their own, all day, every day.
That last item is why occupancy patterns matter. A school at 7 a.m. with the ventilation ramping up is a different building, pressure-wise, than the same school at 9 p.m. with everything off and the stack effect running. Radon follows pressure, so the measurement you take has to reflect how the building actually operates.
Start with data, not a proposal. Any commercial radon project should begin with multi-point measurement and a look at the mechanical drawings. Anyone quoting a system for a commercial building sight-unseen is guessing. Call (218) 520-9679 to set up an assessment.
In a house, the pressure story is simple — warm air escapes upstairs, replacement air gets pulled from the soil, radon comes with it. In a commercial building, the HVAC system often dominates that story.
If a building exhausts more air than it supplies — common when restroom and kitchen exhaust runs continuously while the outside air damper is closed down to save on heating — the whole building sits at negative pressure relative to the ground. It is actively pulling soil gas through every slab crack, utility penetration, and elevator pit it has. No amount of sealing will out-run a building that is depressurizing itself.
That means the correct first move in some commercial buildings is not a fan on a pipe. It is HVAC balancing: increasing outside air, adjusting the supply-to-exhaust ratio, and bringing ground-contact zones to neutral or slightly positive pressure. This approach is recognized in commercial radon practice as building pressurization, and where the mechanical system has the capacity for it, it can solve a moderate problem without a single core hole.
It is not a universal answer. Pressurization costs conditioned air, which in a Duluth heating season is real money, and it only works while the system is running and balanced. In practice the two approaches are frequently combined: active sub-slab depressurization handles the zones with the highest soil gas load, and targeted HVAC adjustments handle the rest and keep the building from fighting the system. Deciding which mix is right requires knowing both the radon numbers and the mechanical capacity.
Residential testing puts one device in the lowest lived-in level. That protocol does not transfer. Commercial and multifamily measurement means testing every ground-contact room across the footprint, or a defined sample of them, because one wing can read 1.2 pCi/L while the wing on the other side of a footing reads 11.
In multifamily, ground-floor units and below-grade common areas are the priority. Upper-floor units are a much lower concern because they have no soil contact, though shafts, stairwells, and chases can carry soil gas upward in a building with strong stack pressure.
Schools and daycares. The highest sensitivity category. Children breathe more air relative to body weight, they occupy the same rooms for long hours across many years, and classrooms are frequently on slab-on-grade or in below-grade wings. Minnesota has actively emphasized radon testing in schools and child care settings, and a documented testing program is increasingly something parents and licensing bodies expect to see.
Apartment and condo buildings. Testing needs to cover ground-floor units and shared below-grade space. Mitigation has to be designed around occupied units, tenant notification, and piping routes that do not run through private space.
Offices and clinics. Usually good HVAC capacity to work with, which makes a combined pressurization-plus-depressurization approach practical. Clinics with below-grade imaging or records storage often show the highest readings in the building.
Warehouses and light industrial with occupied office areas. The warehouse floor itself may be lightly occupied, but the attached office block is where people spend eight hours a day, and it usually sits on the same slab.
Churches and community buildings. Often older, often with fellowship halls, nurseries, and classrooms in a basement level, and often with minimal mechanical ventilation to help.
| Single-family home | Commercial / multifamily | |
|---|---|---|
| Typical suction points | 1–2 | 4 to 20+, depending on slab area and barriers |
| Testing devices | 1–2 in the lowest lived-in level | Dozens — every ground-contact room or a defined sample |
| Design / diagnostic phase | Usually a walkthrough | Formal: pressure field extension testing, HVAC review, drawings |
| System cost | $1,500–$3,000 | Quote-only; driven by slab area and diagnostic results |
| Timeline | One day | Weeks to months, phased around occupancy |
The single question that determines a commercial system's design and its budget is: how far does one suction point actually reach?
Pressure field extension testing answers it empirically. A test hole is cored, a vacuum is applied, and small test holes are drilled at increasing distances across the slab. A micromanometer measures whether the applied suction produces a measurable pressure drop at each one. If you can still read negative pressure 50 feet away, one point covers a large area. If it dies at 12 feet, you need many more points and the project gets substantially larger.
This is also how footing barriers get mapped. A test hole 20 feet away on the same side of a corridor wall may read fine, while a hole 8 feet away on the other side reads nothing — which tells you that wall is a wall underground too, and each side needs its own suction. Paying for a diagnostic phase is normal and worth it. It converts a guess into a design, and it is far cheaper than installing an undersized system and discovering the problem during post-mitigation testing.
Downtown Duluth and Lincoln Park have a large inventory of buildings from the early and mid twentieth century — masonry structures with basements, sub-basements, and slabs poured directly over the thin rocky soil that sits on the Duluth Complex. Many have been converted from their original use, which means mechanical systems that were added later and rarely balanced with soil gas in mind.
The hillside adds a second factor. Buildings cut into the slope frequently have a substantial below-grade footprint on the uphill side, with occupied rooms buried against rock and fill. That is a lot of soil contact per square foot of building, and it is exactly the geometry that produces one wing testing high and another testing clean.
Then there is the heating season. Duluth keeps buildings under stack pressure for most of the year, and a tall commercial building generates a stronger stack column than a house does. The engineering principles are the same ones described on our residential radon mitigation page — reverse the pressure difference so soil gas goes up the pipe instead of into the building — but the scale changes the execution completely.
Commercial radon mitigation is quote-only, and any number given before diagnostics is fiction. Cost is driven by slab area, the number of separate sub-slab zones, what the pressure field extension test shows about soil permeability, fan count and staging, pipe routing distance, roof penetrations, electrical work, and whether the job has to run after hours.
Expect a two-stage engagement: a diagnostic phase with its own fee, then a fixed proposal built from what the diagnostics found. That structure protects the owner. It replaces a padded contingency — which is what you get when a contractor prices blind — with a real design.
The practical case for testing a commercial building is documentation. A building owner who has tested, has records, and has acted on elevated results is in a defensible position. One who has never measured has no answer if a tenant, a parent, an employee, or a buyer asks.
Commercial real estate transactions increasingly include radon in environmental due diligence, and a documented, commissioned system with test results is an asset in that process. For multifamily owners, tenant inquiries are becoming more common, and having a clear, factual answer is worth far more than hoping the question does not come up. None of this requires alarm — it is the same ordinary risk management as fire systems, water testing, and elevator inspections.
MDH licensing applies to commercial work exactly as it does to homes. Anyone performing radon measurement or mitigation in a building they do not own or lease must be licensed by the Minnesota Department of Health. Industry certification alone is not sufficient in Minnesota. Ask for the license number before signing anything — on the measurement contract as well as the mitigation contract.
A house is tested with one or two devices in the lowest lived-in level. A commercial or multifamily building is tested at many points across the entire ground-contact footprint, because interior footings divide the sub-slab area into separate zones that do not share air. One classroom wing can read under 2 pCi/L while another reads over 10. Testing typically uses continuous monitors that log hourly, so you can see how levels change when the ventilation system cycles between occupied and unoccupied modes.
Sometimes, yes. If a building is running at negative pressure because it exhausts more air than it supplies, it is actively drawing soil gas in through the slab. Rebalancing the supply-to-exhaust ratio and increasing outside air can bring ground-contact zones to neutral or slightly positive pressure and reduce radon substantially. The limits are real: it consumes conditioned air, which is expensive during a Duluth heating season, and it only works while the system runs as balanced. Most projects combine HVAC adjustment with active sub-slab depressurization in the worst zones.
There is no honest flat number. Cost scales with slab area, the number of separate sub-slab zones, how far a single suction point reaches, fan count, pipe routing, roof penetrations, electrical work, and whether the installation has to happen after hours. A small office block with good sub-slab gravel and one zone is a fundamentally different project than a 60,000-square-foot school on clay with a dozen isolated zones. Expect a paid diagnostic phase followed by a fixed proposal based on what the diagnostics actually measured.
No. The priority is every ground-floor unit with soil contact and all below-grade common areas — laundry rooms, storage, mechanical rooms, community rooms, and parking levels. Upper-floor units are a much lower concern because they have no direct connection to the soil. That said, elevator shafts, stairwells, and utility chases can move soil gas upward in a building under strong stack pressure, so a few upper-level checks are worth including to confirm the assumption holds in your building.
Children breathe more air relative to their body weight than adults, they spend long hours in the same rooms, and they do it over many consecutive years, so cumulative exposure adds up faster. Classrooms are also commonly on slab-on-grade or in below-grade wings with direct soil contact. Minnesota has emphasized radon testing in schools and child care settings for these reasons. A documented testing program is straightforward to run and increasingly expected by parents and licensing authorities.
It measures how far the suction from one point actually travels under a slab. A test hole is cored, vacuum applied, and a micromanometer checks for a measurable pressure drop at test holes drilled at increasing distances. The result tells you whether one suction point covers 50 feet or 12, and where footings block airflow entirely. That single measurement determines suction point count, fan sizing, and therefore the entire budget. Skipping it means guessing, and an undersized commercial system is far more expensive to correct than to design correctly.
It is managed, not eliminated. Core drilling is loud and dusty, so that work gets scheduled for evenings, weekends, school breaks, or wing-by-wing so only a small part of the building is affected at a time. Piping is routed through mechanical spaces and chases rather than occupied rooms wherever possible. In multifamily buildings, tenant notification and access scheduling are planned before work begins. A realistic phasing plan should be part of the proposal, not an afterthought.
Multi-point measurement and a mechanical review will tell you whether you have a problem, where it is, and what solving it involves — before anyone proposes a system.