by Peter Beaupre
Radon is an odorless and colorless radioactive gas that
forms during the decay of radium 226, an element found in
varying concentrations in some soils and bedrock. It's a health
hazard, because prolonged exposure to it can lead to an
increased risk of lung cancer. No one really knows how many
people get cancer from this each year, but the EPA estimates
that somewhere between 5,000 and 20,000 lung cancer deaths in
the U.S. can be attributed to radon exposure annually.
Fortunately, most home radon problems can be corrected
fairly easily. In the seven years I've been performing radon
mitigations in New Hampshire and Vermont, I've never
encountered a problem I couldn't fix. In most cases, dealing
with the problem costs only $1,000 or so — a cheap price,
all things considered.
Where Does Radon Come
In outdoor locations, radon leaks out in small amounts and is
harmlessly diluted by the air. But if you build a house on a
site that contains radon in the soil or bedrock, trouble can
start. First, digging a foundation can open pathways that make
it easier for the radon to reach the surface; blasting ledge,
in particular, can open fissures that let out a lot of radon.
Second, when you build a house on such a site, the radon tends
to become concentrated indoors, rather than being diluted by
the outdoor air. This is worsened by the slight negative
pressure that exists in many homes as a result of fuel-burning
appliances, ventilation fans, and the natural stack effect.
This tends to draw radon from the ground through floor drains,
pipe penetrations, and small cracks in the walls or slab.
Radon levels are measured in picocuries per liter,
abbreviated pCi/L. The recommended action level is an annual
average of 4 pCi/L or more. Some homeowners perform a radon
test on their new or existing homes, but 90% of my business is
associated with homes that are changing hands. That's because
real estate agents in the two states where I operate are
required to have buyers sign a waiver if they elect not to have
a radon test done. Practically all buyers choose to have the
test done (the cost is typically less than $100 and ordinarily
comes out of the seller's pocket). If the numbers come back
high, I'm usually called in.
Sealing cracks and other entry points may reduce radon levels
somewhat, but it's virtually impossible to solve the problem by
sealing alone. The most successful approach is usually to embed
one or more vent pipes beneath the slab and connect them to a
vertical pipe that vents the radon to the outdoors.
cases, a sub-slab ventilation system can be completely passive,
like a plumbing vent stack. If the slab was poured over a
uniform 4- to 6-inch layer of crushed stone or some other
permeable material, and the vent stack is located inside, in a
heated space, the natural stack effect often produces enough
vacuum to vent the sub-slab adequately.
Active systems. If natural
ventilation isn't sufficient, it becomes necessary to add an
in-line fan, typically in the attic, to increase the suction.
Depending on the situation, in-line fans can have a capacity of
anywhere from 90 to 150 cfm. Because a radon reduction fan must
operate 24 hours a day, 7 days a week, only a high-quality fan
designed for the purpose should be used. Systems that make use
of a vent stack outside the heated envelope of the house
— usually because there's no good place to conceal it
inside — always require a fan.
Radon ready. We encourage
anyone who is building a new home to install a central vent
stack in an interior partition wall and cap it in the attic.
That costs next to nothing if it's done at the concrete work
and framing stage, but it saves a lot of trouble if the house
later turns out to have high radon levels. It's easy to
activate the system by cutting an opening in the roof,
extending the pipe, and sealing the penetration with a neoprene
boot. (If that doesn't provide adequate venting, adding an
in-line fan in the attic will.) In practice, though, no one
takes the trouble to do this, which means that I almost always
have to find a way to retrofit one.
A Case in Point
The project photographed here involved a beautifully finished
4,000-square-foot house on a steeply sloping site with dramatic
views of nearby mountains. Preparing the site had involved
considerable blasting to clear a driveway and level the
foundation, which may have contributed to an initial radon
level of 17 pCi/L. That number wasn't especially high, but I
was concerned about the fill beneath the slab, which the
builder told us consisted of a compacted layer of sand
overlying broken shale from the blasting.
The problem is that compacted sand — especially if
it's damp — is so dense that it's difficult to draw much
air through it, even with a very powerful fan. My job would be
much easier if foundation contractors would pour slabs over a
4- to 6-inch layer of 3/4- to 1 1/2-inch crushed stone.
Although the stone costs more than sand, it's self-compacting,
which saves the labor needed to compact 2-inch lifts of sand.
Radon infiltration is also reduced if the concrete guys leave
the poly vapor barrier under the slab intact, rather than
slashing holes in it to reduce the amount of bleed water and
To make things even more interesting in this case, the
partial basement was finished as a recreation area, and radiant
heat coils embedded in the slab limited the number of places we
could safely drill through the concrete. Finally, the open-plan
living space above left nowhere to hide an indoor ventilation
Installing the Pipe
Our first step was to cement a special check valve into the
floor drain and seal visible cracks in the floor slab (see
Figure 1). Although that won't solve the radon problem in
itself, it's necessary to prevent the vent pipe embedded in the
slab from sucking air through such openings, effectively
short-circuiting the whole system.
1. A special check valve is used to prevent the
sub-slab ventilation system from sucking indoor air
through the floor drain. It opens for drainage when
weighted down by accumulated water, but when the drain
is dry, a spring-loaded ball provides a seal that
maintains negative pressure beneath the slab (left).
Once the valve has been cemented in place (center), any
visible cracks in the slab or foundation walls are
repaired with a durable sealant (right).
Creating a vent sump.
Fortunately, the builder had provided us with photos of the
sub-slab taken just before the pour. The photographic coverage
wasn't complete, but we could see that one out-of-the-way
corner of the utility room was free of rebar, radiant pipe, and
other obstructions. Using a rotary hammer, we punched a 6-inch
opening in the slab before cutting through the extruded
polystyrene insulation beneath (Figure 2).
2. Once a vent opening through the slab has been
made with a rotary hammer ( top left), the fill beneath
is dug out to create a sump (top right). The finished
sump is about a foot deep and as wide as a worker can
reach by hand (above).
That made it possible to dig a cavity in the sand beneath
— partly by hand and partly with the aid of a shop vac
— to provide a sump for the bottom of the vent pipe.
After cementing the 3-inch PVC vent pipe in place with
hydraulic cement, we were ready to cut a hole through the
adjoining partition wall and extend the vent horizontally along
the corner of the ceiling and out through the exterior band
joist (Figure 3).
3. A length of 3-inch PVC — the lower end
of which will be cemented into the slab — turns
to pass through a partition wall (left). The sanitary
tee will accept a second vent pipe leading to the
garage. Below, the main vent line continues through a
finished portion of the basement (later enclosed in a
pipe chase) before exiting through an opening in the
band joist. Like plumbing vent pipe, radon venting must
be pitched enough to allow condensate and rainwater to