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Radon Mitigation in New Construction: What to Build In

Radon vent beneath a new foundation

The cheapest time to deal with radon is while your house is still a hole in the ground. Radon-resistant new construction (RRNC) adds a gravel layer, plastic sheeting, a sealed vent pipe and careful caulking during the build. That gives you a passive system that vents soil gas above the roof and can be switched to an active, fan-driven system later if a test says you need one. You still have to test after you move in, because nothing about the passive features proves your actual indoor level.

This guide covers what goes into the ground and the walls, how passive and active systems differ, what to put in your builder contract, and what happens if the test comes back high.

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TL;DR

  • A radon-resistant build uses five features: clean gravel, 6 mil poly sheeting, a vent pipe through the roof, sealed foundation cracks and joints, and an attic junction box for a future fan.
  • A passive system is a baseline, not a guarantee. An active system adds a fan and is far more effective at lowering levels.
  • Building these features in is much cheaper than retrofitting a finished house later.
  • Test after construction. The EPA’s action level is 4 pCi/L, and the only way to know your number is to measure it.
  • Put the radon details in your contract and plan drawings before the foundation is poured.

Table of Contents

Why radon is a design problem, not just a testing problem

Radon is a radioactive gas that forms when uranium in soil and rock breaks down. It enters a house through cracks and openings in the foundation, pulled in by pressure differences between the soil and the building. The EPA explains this entry mechanism in its guidance for people building new homes: when the air pressure inside is lower than in the soil around it, the house acts like a vacuum on the ground.

The World Health Organization notes that well-tested, durable and cost-efficient methods exist for preventing radon in new buildings, and that prevention should be considered when new structures are built. That is the logic of RRNC. You are not trying to fix a problem after the fact. You are shaping the foundation so soil gas has an easy path out and a hard path in.

Two things make this more than a box-ticking exercise:

  • Radon varies house to house. Levels can differ substantially even between adjacent buildings. According to EPA analysis of radon reduction strategies, radon maps help with planning but cannot promise what a finished home will measure.
  • The system is invisible. A passive vent pipe has no moving parts and no indicator light. Only a test tells you whether it works.

The five features of radon-resistant construction

The EPA’s radon-resistant construction basics describe a set of techniques that use common materials. Here is what each one does and what to look for on site.

1. A gas-permeable layer under the slab

A 4-inch layer of clean, coarse gravel goes below the foundation slab. Its job is to give soil gas a loose, open space to travel through, so it can reach the vent pipe instead of pushing up through the slab. “Clean” matters: fine material packs down and blocks the air movement you are paying for.

2. Plastic sheeting over the gravel

Heavy-duty polyethylene sheeting, 6 mil, goes over the gravel before concrete is placed. It works as a vapor retarder and as a barrier to soil gas. Ask to see it before the pour. Tears, gaps at the footing and unsealed overlaps are common and easy to fix at that stage, and impossible to fix afterward.

3. A vent pipe from the gravel to the roof

A 3- or 4-inch PVC pipe runs from the gravel layer, up through the house, and out through the roof. In a passive system, warm air rising in the pipe creates a gentle draw. The pipe should be sealed at every joint and labeled so a future owner or remodeler does not mistake it for plumbing.

4. Sealing and caulking

Cracks, control joints, plumbing penetrations, sump pits and the joint where the slab meets the wall all get sealed. The Minnesota Department of Health stresses that how well a passive system performs depends on this sealing. Sealing alone will not stop radon, but it makes any vent system work better.

5. An electrical junction box in the attic

A junction box near the vent pipe, usually in the attic, lets an electrician connect a fan later without opening finished walls or ceilings. The EPA’s technical bulletin on activating passive systems recommends this small, inexpensive step because it saves real electrical work down the line. It costs very little at framing and a lot more once drywall and insulation are in.

The EPA says more than 3 million homes have been built with these techniques since 1990, so none of this is exotic. A competent builder has done it before.

Passive vs. active radon systems

The two systems share the same piping. The difference is the fan.

Passive system Active system
How it works Stack effect (warm air rising) draws soil gas up the pipe An inline fan pulls soil gas up the pipe
Moving parts None Fan, plus an indicator or manometer
Typical reduction Roughly 50% Up to 99%
Electricity None Small continuous draw
When used Installed at construction as a baseline Installed at construction, or added when testing shows a high level

The reduction figures come from ASTM’s practice for radon control in new low-rise residential buildings as summarized in the research behind this article. They are general figures, not a promise for any one house.

The practical point is that a passive system is built to be upgraded. If your post-construction test comes back at or above the action level, an electrician connects a fan at the junction box. The EPA notes that passive-to-active conversions can often use smaller, more efficient fans than retrofits in older homes, because new homes tend to have better sealing and a permeable layer already in place.

Should you just install the fan from the start?

Some homeowners choose to. If you are building in an area with elevated radon potential, or you plan to finish a basement for bedrooms or a basement ADU, ask your builder to price an active system at construction. A fan adds some cost and a small electric load, but it removes the wait-and-see step. Whether it is worth it for your site is a conversation to have before the slab is poured, not after.

Testing is not optional

The EPA recommends action when radon is 4 pCi/L (150 Bq/m3) or higher. Even homes built with radon-resistant features need post-construction testing to confirm levels are below that line.

A few practical points:

  • Test after the house is closed up and the HVAC is running normally. A test in a half-finished house tells you little.
  • Retest if you finish a basement, add a room or change the HVAC. Any of these can change air pressure and soil-gas behavior.
  • Four is not a safety threshold. The WHO recommends a lower reference level of 100 Bq/m3 (about 2.7 pCi/L), and says it should not exceed 300 Bq/m3 (about 8.1 pCi/L) where that is not feasible. If your result lands between the two numbers, talk with a certified radon professional about whether to lower it further.

If you or anyone in your home smokes or has had lung health concerns, mention that to your doctor along with your radon result. Radon is a recognized lung cancer risk, and smoking multiplies the danger.

The cost-benefit case is strong. An EPA analysis of programs that build homes with radon-reducing features and activate systems when levels exceed 4 pCi/L estimates a return of $25.99 to $53.80 per dollar spent.

Testing is not optional — overview diagram

What this means in Southwest Utah

Utah has its own guidance. The Utah Department of Environmental Quality publishes radon techniques for new home construction and explains that when sealing is not enough, an active soil depressurization system can extract soil gas and vent it outside. Check with your local building department about what your jurisdiction currently requires, since adopted codes and amendments can differ by city and county.

If you are building in Washington or Iron County, treat radon as a standard line item on every new home, basement and addition that touches the slab. Soil, slope and bedrock all vary across the area, which is why a neighbor’s test result tells you little about yours.

Radon and the rest of your foundation plan

Radon control is not a separate trade bolted on at the end. It shares space with other foundation decisions.

  • Soil and site. Soil conditions affect drainage, slab design and the gravel layer. Our guide on soil tests for new homes covers what to require before design is locked, and new home site prep steps covers the order of work so you do not pay twice.
  • Moisture. Gaps that let in water often let in soil gas too. If you are weighing options for sealing below-grade concrete, basement waterproofing methods show how leak diagnosis and joint sealing are handled. Our own page on basement moisture control covers the site, foundation and humidity order of operations.
  • Basement finishing. If you plan to finish a basement later, the radon pipe, slab sealing and sump details need to be drawn in now. Our Utah basement finishing cost guide includes slab and radon costs, and the basement permits guide lists what inspectors want to see.

A vent pipe routed through a bedroom wall or a future closet is much easier to plan on paper than to relocate in a finished space.

What to put in your builder contract

Radon features should appear in writing, not as a verbal assurance. Before you sign, ask for:

  1. A line in the specifications naming the five RRNC features, including pipe diameter and sheeting thickness.
  2. A plan view of the pipe route from the sub-slab gravel to the roof, shown on the drawings.
  3. Photos or an inspection window before the slab pour showing gravel, sheeting and pipe stub.
  4. A junction box in the attic at the pipe location, wired to a switch location or left ready for an electrician.
  5. Pipe labeling along the run, so later trades know what it is.
  6. A post-construction test by a certified professional or a certified kit, with the result handed to you at closeout.
  7. A stated plan if the test is high: who activates the fan, at whose cost, and within what timeline.

Item 7 is the one most contracts skip. If you are buying a spec home rather than building your own, ask the same questions. A homebuyer in a common online thread describes finding high radon in a new build and negotiating with the seller afterward. That is the situation a written plan keeps you out of.

What if the test comes back high?

First, do not panic about a single short-term result. Radon fluctuates with weather, season and how the house is operated. Retest, or run a longer test, to confirm.

If the confirmed number is at or above 4 pCi/L:

  1. Have an electrician connect a fan at the attic junction box, or have a certified radon mitigation professional do it.
  2. Retest after the fan has run for a few days.
  3. Check the sealing at slab joints, sump lids and penetrations, because better sealing improves fan performance.
  4. Retest periodically, and after any major renovation.

Because the pipe, gravel and junction box are already in place, this is usually a modest job compared with drilling through a finished slab. The EPA notes that building these features in is significantly cheaper than retrofitting a home later.

If the radon features were skipped, an exterior or retrofit system may be needed. That means drilling through the slab, running a pipe on the outside of the house and installing a fan, with finished flooring and landscaping in the way.

Exterior radon pipe beside finished landscaping

Our perspective on radon in custom homes

We build and remodel homes across Washington and Iron Counties, and the radon conversation always goes better early. Here is how we think about it.

Treat it as part of the foundation, not an add-on. The gravel, sheeting, pipe and junction box are cheap while the site is open. Our process is built around a single point of contact for design and construction, so the person drawing your plans is the person who knows what the crew will do on pour day. That keeps small details like a pipe route from getting lost between trades.

Show it, don’t just say it. We use 3D renderings and virtual consultations in design. For radon, that means you can see where the vent pipe runs through the house before framing starts, and decide whether it belongs in a utility chase rather than a future bedroom.

Be honest about what passive means. A passive system is a good baseline. It is not a clean bill of health. We tell clients to plan for a test at closeout and to treat a fan as a normal, expected possibility rather than a failure.

Think about the next project. Many of our clients later finish a basement, add a room or build an ADU. A radon system that was planned with those futures in mind saves rework.

— Jake

Building a new home in Southwest Utah

If you are planning a custom home in Washington or Iron County, we can walk you through foundation details, including radon-resistant features, as part of the design phase. See our custom home construction services or the overview of custom new home construction in Utah, then contact us through our site to set up a virtual consultation.

FAQ

Are new homes built with radon mitigation systems?

Not automatically. Radon-resistant features are standard practice in some places and optional in others, depending on the local code and the builder. Ask for the features in writing, and confirm with your local building department what applies where you are building.

Is a radon test needed for a new home?

Yes. Passive features reduce the chance of high radon, but they do not prove a result. The EPA says homes built with radon-resistant features still need post-construction testing to confirm levels are under 4 pCi/L.

What is the difference between a passive and an active radon system?

Both use the same vent pipe from under the slab to above the roof. A passive system relies on warm air rising to draw soil gas up, while an active system adds a fan. Active systems are consistently more effective, and a passive system can be converted by connecting a fan at the attic junction box.

Is it cheaper to install radon protection during construction?

Yes, in general. The materials are inexpensive and the work happens while the slab and framing are open. Retrofitting means cutting through finished floors and running a pipe through or around a completed house, so you should expect to pay more.

Sources

Images in this article are editorial illustrations, not Prestigious project photography.

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