The EPA and the U.S. Geological
Survey have rated every county in the United States as Zone 1 to 3 for radon risk. Links to state maps with county by-
county risk levels can be found at www.epa.gov/
The EPA recommends that all homes
in Zone 1 counties be built with radon-resistant features,
which can be easily upgraded to a radon remediation system
Since homes in Zones 2 and 3 can also have
high levels, it is best to check with your state radon office
to see if they are aware of any local “hot spots.”
The techniques for radon-resistant building vary for
different foundation types and site conditions, but all contain
the six basic elements described below.
these steps creates a passive soil depressurization system, which sufficiently lowers radon levels in about 50% of
homes requiring mitigation.
If radon levels need to be lowered
further, the system can be easily converted to an active
system by adding an inline fan, which can meet the
target levels in nearly all cases (see Figure at above left, showing a typical radon mitigation system installation).
of radon remediation is to lower the average indoor radon gas level to less
than 4 pCi/L, and preferably 2 pCi/L.
Radon Gas permeable layer. This is usually a 4-inch layer of
clean, course gravel installed beneath the slab for
drainage, but which also allows the gas to move freely
beneath the house. In areas where native soils are
sufficiently permeable to build on, a loop of perforated
pipe inside the footings is an option, and may also
serve as a drain tile.
The perforated pipe should be
about 12 inches in from the foundation wall and 1 inch
below the slab, with a minimum diameter of 3 inches
for slabs under 2,000 sq ft and 4 inches for slabs up to
4,000 sq ft. Where subgrade soils are compacted or
frozen, another option is to use geotextile drainage
mats to create a gas-permeable layer on top of the
subgrade but beneath the slab.
Plastic sheeting to Stop Radon Gas Penetration. Lay minimum 6-mil polyethylene
sheeting (or 3-mil cross-laminated sheeting) on top of
the gas permeable layer. This helps keep soil gases
from entering the home and also keeps concrete from
clogging the gravel layer. Overlap seams by at least
12 inches, and repair any punctures or tears with tape
or a patch of sheeting material.
Radon Gas Vent Pipe Details. Run a 3- or 4-inch PVC pipe from the gas-
permeable layer up through the house to the roof to
vent soil gases above the house. Where better suction
is needed, connect the subslab tee to a minimum 10-foot
length of perforated, corrugated pipe run horizontally
in the gravel layer.
The vertical pipe should be as
straight as possible and should be located inside the
insulated shell of the building to keep it warm,
inducing the stack effect.
Field data has indicated that 4-inch vent pipes work
better than 3-inch vent pipes for passive systems. Some
builders cap the stub just above the basement slab and
connect the riser to the roof only if the house tests high
for radon. If so, clearly label the capped pipe so no one
mistakes it for a plumbing drain in the future.
Our photo (above left) shows an air pressure gauge or manometer that is usually installed on a professionally-installed radon vent pipe. This device simply measures the pressure difference between the room interior where the radon suction point is installed through the floor slab and the pressure in the vent pipe interior.
As long as the room air pressure is slightly higher than the slab or suction pipe air pressure radon gases will be more inclined to pass up the vent pipe than enter the room - that is, the room is at "positive pressure" with regard to the air pressure under the floor (the most likely entry path and source of radon gas).
Sealing and caulking to Stop Radon Gas Entry. Seal all cracks, perimeter
joints, control joints, and other openings in the foundation
floor with long-lasting materials to reduce soil
gas entry. Seal large openings with expanding foam or
nonshrink mortar or grout.
Seal smaller holes with a
high-grade elastomeric sealant conforming to ASTM
C920-87. If the home has a sump, it should have an
airtight cover and, if needed, can have a floor drain
with a trap (filled with oil so it will not evaporate).
the sump is not connected to the drain tile loop, it can
be vented into the radon system with a 3-inch pipe
connected to a special sump cover available from
suppliers of radon mitigation products. Also seal and
caulk the rest of building envelop to reduce the stack
effect in the home.
The tighter the home, the less the
building will draw radon out of the soil. Also tightly
seal any return air ducts that pass through basements
Seal ducts and air-handling units Where Radon Gas is Present. Placing any
return-air ductwork under the concrete slab is not
recommended, since this will tend to draw radon
into the ductwork and distribute it around the house.
If supply ductwork must pass through a subslab space,
it should be seamless or sealed airtight with durable
aluminum tape or duct mastic.
Also tightly seal any air-handling units or ductwork passing through basements, crawlspaces, or any areas in contact with the slab. In addition to saving energy, this will prevent the HVAC system from drawing radon out of the soil.
SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs
Junction box to power radon mitigation system fan. Install an un-switched junction box in
the attic or attached garage within about 6 feet of the
vent pipe. A dedicated circuit is not needed. In the
event that the passive system is not enough to keep
radon levels below 6 Pci/L, then an inline fan will
need to be added and run continuously.
should be located so that all positively pressurized
sections of the system (from the fan to roof outlet)
are located outside of habitable space. An active vent
system should also have a visible or audible alarm to
alert the occupants in the event of a loss of pressure
or airflow in the vent pipe.
A post mitigation radon test of 2 to 7 days should be
done within 30 days of system installation. For an accurate
reading, all windows and doors must be closed
12 hours before and during the test, except for normal use
for entry and exit.
Watch out: home ventilation systems, particularly powerful exhaust fans, can subvert a typical sub-slab suction type radion mitigation system by creating negative air pressures within the building. The radon system needs to be able to handle these pressure variations in the home.
See VENTILATION, BALANCED for an optimum approach to bringing in fresh outdoor air without increasing heating or cooling costs and without risking subverting the radon mitigation system.
Also see BACKDRAFTING HEATING EQUIPMENT for warnings about potentially fatal carbon monoxide hazards if exhaust fans cause back-drafting of heating appliance exhaust into the building.
For a Thorough Background in Radon Hazards, Radon Mitigation, & the History of Radon Concerns in the U.S. also see these articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
"Radon's Threat Can Be Subdued", part 4 - [PDF] radon prevention advice for new construction, radon in well water - how it is removed, sources of information about radon gas and radon contamination.
"Defeating Radon" part 1 - [PDF] Terry Brennan, Bill Turner, Solar Age Magazine - How does radon get into buildings, how do I know if a building has a radon gas problem, how can I solve radon problems in existing homes, and what can I do to prevent radon from entering new homes. Part 1: where Radon comes from, how to diagnose radon
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
(Mar 30, 2012) Frank said:
Sealing basement air returns, I think that's what's being said here, may help reduce levels but may damage your HVAC system.
Better to have you a/c coil freeze than have high levels but check for alternatives. The only alternative, if you need an air return, may be a mitigation system.
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Questions & answers or comments about the best methods for reducing indoor radon gas hazard levels.
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"Radon Basics", Q&A article, Solar Age, April 1984, includes advice for radon-resistant construction for an underground house built of concrete
"Radon Basics-PDF", Q&A article, Solar Age, April 1984, includes advice for radon-resistant construction for an underground house built of concrete
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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