Construction Design-Radon

Radon is a naturally occurring soil gas that is present in many locations throughout North America.  The gas can’t be seen, smelled, or tasted but it is known to cause lung cancer.  Radon control systems have been in the International Residential Code Book for a few cycles now.  In Appendix F, there are several options for how these systems can be constructed when building new.  There are also strategies for mitigating elevated levels of radon in existing buildings.  (A helpful resource: Soil Gas Mitigation Standards for Existing Homes.)

How does radon enter a home?

Radon comes from radioactive decay in rocks which moves though the soil and into the atmosphere.  If there is an enclosed structure between the ground and the atmosphere, such as a home, radon gas can accumulate inside the structure.  Radon usually enters a home through cracks or penetrations (there are many) in whatever floor system a home has that is in direct contact with the soil.  This floor system can be a basement slab, the concrete floor of a slab on grade home, (or a concreteless slab on grade system).  Another source can be from a dirt floor in a crawl space.  There are also mentions of radon from granite used in building materials, like granite countertops, though most experts agree that the amount of radon released from building materials is very small.

Radon mitigation in new construction

Many of the systems being designed for new homes are passive, meaning any soil gas present beneath the foundation is naturally vented by way of a pressure differential to the outside. A vent stack used in new construction must be a pipe with a minimum of three inches in diameter and extended from beneath the slab or floor system in direct contact with the soil, through the building and above the roofline.  If an active system is needed, such as when radon levels are found to be above the 4 pCi/l level, a fan is installed in the pipe to help evacuate the soil gasses.  The fan creates a mechanically induced negative pressure under the floor system that is in contact with the soil, essentially sucking the radon out.

The IRC has three options on how to build a radon system for new construction. The first is by installing 4 inches of clean 1/4- to 2-inch gravel which is placed under the entire floor system that is in contact with the ground.  A vent pipe is then installed with a tee into the bed of gravel and is extended to the outside of the home, through the roof.  The IRC requires a soil gas retarder, which usually consists of a 6-mil (or thicker) polyethylene sheet. The polyethylene needs to be continuous, and any seams must lap at least 12 inches. Any penetrations through the poly need to fit tightly with any rips or tears in the material sealed or covered with additional poly. (The nice thing about the soil gas retarder is that it can also act as a sub-slab vapor retarder.)  Another option is to use closed cell spray foam, this method can meet the requirements of the radon soil gas retarder, sub-slab vapor retarder and required sub-slab insulation.  (More info about using sub-slab closed cell spray foam will be coming in a future article.)

The second option is 4 inches of sand with an imbedded geotextile drainage mat that will allow soil gases to evacuate from under the slab. There are several commercially made geotextile mats that are designed for radon control, one such product is called VaporMat.  As with the gravel method, a pipe will need to be extended from the soil to outside the building.  Follow the installation requirement from the manufacturer of the geotextile drainage mat.  These systems also require a soil gas retarder, usually a 6-mil or thicker polyethylene sheeting is used.

The third option is to have an engineer design a system. Best advice: Talk with a local building official, as local codes may have different requirements.

It’s important that any potential penetration between the sub-slab space and the home be sealed. Plumbing and conduit openings must be sealed where they penetrate the floor. Any joints in the floor must also be sealed. Sump pits are required to have a gasketed cover. Basically, anyplace where radon gas can move through the sub-slab floor system that is in contact with the ground needs to be sealed.

Additionally, provisions must be in place to add a powered vent fan to the system should an elevated level of radon (above the 4 pCi/l level) be detected after the home is built. The code wants to see the fan, if needed, placed outside the habitable space.  Most are installed in the attic. A way to power the fan must be installed close to any fan location.

What about existing homes?

There are methods to install a radon control system in an existing home.  Nearly all these methods are active and include a fan.  Many juristictions require mitigation professionals receive training and be licensed before installing a radon system in an existing home.  There are several steps involved in the mitigation process, first, the home needs to be tested to confirm the level of radon.  The home may need further testing to determine if there is a pressure issue in the home that is causing the radon to enter.  An example, exhaust only ventilation strategies may induce a large enough negative pressure on the home which could cause an elevated radon level.  Joe Lstiburek is known for saying “if radon were valuable, this is how we would mine it.”  Simply modifying these ventilation systems, or installing a balanced mechanical ventilation system, may take care of the problem.  Any radon mitigation system will need to be designed and installed based on the home’s foundation.  After the system is installed, more testing is needed to confirm proper operation and radon reduction.  This work is best left to a trained professional with proper monitoring and testing equipment.

Things to watch out for.

There is little risk of a problem with a passive radon system installed in a home, though I have seen a few.  I recently blower door tested a new home that failed the test.  Part of the reason it failed was we found air leaking around penetrations in the concrete foundation.  It seems the negative pressure test was pulling air down the radon vent pipe located on the roof and into the rock layer of the radon system.  There were several areas of poorly sealed pipe penetrations throughout the concrete slab which caused the air to move into the home.  I have run across this condition in a handful of homes I’ve tested over the past couple years.  It is now something I look for during all blower door testing.

There can be issues when the radon system is required to be active.  The fan operating in an active radon mitigation system would be designed to create a slightly negative pressure under the slab or floor system in contact with the soil.  If there is some connection between the soil and home through cracks or holes, this negative pressure can extend to inside the home.  The biggest concern is if a home contains an atmospherically vented fossil fuel appliance, spillage or back drafting of the appliance may occur.  This leads to carbon monoxide accumulations inside the home, a much more immediate problem than the long term affects of radon exposure.  This can be remedied by installing only sealed combustion appliances.  Building durability and occupant comfort can also be affected.

Gary Nelson from the Energy Conservatory brought up the topic of radon during a conversation I had with him a few months ago. He has an active system in his house because of elevated radon levels. His system consists of a few inches of sub-slab gravel and drain tile tied to a sealed sump. During remodeling, there were several areas of the concrete slab that could not be effectively sealed. After the remodel was complete, Gary found that the house pressure had become slightly negative—about -3 Pascals. He was able to trace the negative pressure to the radon sump, which was -11 Pa. Gary surmised that the pressure in the sump should have been around -2 to -3 Pa. He wasn’t concerned with how the negative pressure was affecting the home, but he was concerned about the effect it was having on comfort. Gary did some calculations and found operating the radon fan at the exhaust rate it was designed for would result in a continuous 80 CFM of exhaust-only ventilation. This, along with the natural air leakage of the home—his home tested at roughly 40 CFM of natural air leakage, along with the operation of a balanced ERV ventilation system in a cold climate, would have resulted in a lower than desired humidity level during the winter months. Gary slowed the exhaust rate of the radon fan and found that operating the ERV on a low setting, he was able to maintain 35% humidity during the winter.

Another illustrative situation: I was recently on an energy audit where a portion of a basement contained an active radon mitigation system; this system was installed in an existing home. The walls and ceiling in the radon mitigation area were completely sealed from the rest of the home using closed-cell spray foam, and the door was air sealed to separate the from radon area from the rest of the house. The floor was completely covered with a heavy sheet of plastic. A fan, located outside and directly adjacent to the area, was connected to a pipe that fed to the underside of plastic sheeting. A pressure test showed the room to be at a -7 Pascals with respect to the outside.  I did not test under the plastic sheet but would imagine the pressure under to be much higher. Luckily, the negative pressure inside the sealed room was not communicating with the rest of the home, someone did a good job air sealing between the two spaces.

What if that -7 Pa had been present throughout the house? First, any atmospherically drafting fossil-fueled heating equipment would have been in danger of spillage or back drafting. Negative pressures can back draft combustion equipment with as little as a -3 Pa, causing carbon monoxide to spill back into the house.

Also, any holes or penetrations in the air control layer can cause outside air to infiltrate the house. In cold climates, that air can meet with a warm a surface, such as the backside of the drywall, which can cause condensation in the wall assembly. In a hot-humid climate, warm, moist air infiltrates to an air-conditioned interior, meeting cool drywall—also causing the potential for condensation.

The aforementioned Soil Gas Mitigation Standard or Existing Home guide has a section covering house pressures and fossil-fueled heating equipment:

“Altering pressure in the building, directly or indirectly, may cause flue gas spillage from combustion appliances. Clients and impacted residents shall be advised of any significant flue gas spillage that is observed. If flue gas spillage is observed to result from the mitigation system operation, the system shall be deactivated until the condition has been evaluated and corrected. In such event, the client or impacted resident shall be advised to contact an HVAC contractor or other qualified person to evaluate and correct any significant flue gas spillage conditions as well as to verify proper appliance installation and performance.”

There is no requirement listed in that manual on how to determine if there is spillage occurring. In my opinion, combustion appliance zone (CAZ) depressurization testing should be completed after the installation of any active radon system in a home that contains a naturally vented fossil fuel heating system, new or existing. We don’t want radon present in our homes, but the presence of carbon monoxide is a much more immediate problem.

Radon is a long-term concern for occupants of a building.  We have figured out ways to reduce the risks, when building new, follow the codes that are currently in place, and in existing homes, action would only be needed if the radon level is above the 4 pCi/l level.  The only way to know if it’s present in your home is to test.

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