Image Credit: www.newhudsonvalley.com If a foundation includes a continuous footing that divides the subslab gravel into two sections, then it's important to insert one or two plastic pipes through the central footing to connect the crushed stone sections on either side.
Image Credit: Environmental Protection Agency If a passive radon mitigation system doesn't succeed in lowering radon levels to a safe range, it's possible to install a radon exhaust fan in the vent pipe. This turns the passive mitigation system into an active one. The usual location for a radon exhaust fan is in the attic. [Photo credit: Advanced Radon Services — http://radonkansascity.com]
Image Credit: Advanced Radon Services Some active radon mitigation systems include a vent pipe that depressurizes the basement sump.
Image Credit: Environmental Protection Agency To install a radon mitigation system in a dirt-floored crawl space, a length of horizontal perforated pipe is installed on the dirt floor. Then a layer of sealed polyethylene is installed above the dirt floor and above the perforated pipe.
Image Credit: Environmental Protection Agency If a contractor wants to retrofit an active radon mitigation system in an older house, the first step is to drill a 4-inch-diameter hole in the concrete slab.
Image Credit: Brian Baler This architectural drawing shows one approach to installing a passive radon mitigation system.
Image Credit: Environmental Protection Agency This architectural drawing shows one approach to installing an active radon mitigation system.
Image Credit: Environmental Protection Agency
Several colorless, odorless gases can injure your health. For example, carbon monoxide can kill you in minutes. Radon takes longer — usually decades — to kill you, and (fortunately) death is less certain.
People who have lived for many years in a house with elevated levels of radon gas have a higher than average chance of getting lung cancer. Because of this risk, the U.S. Environmental Protection Agency (EPA) advises homeowners to test the air in their homes for the presence of radon. If testing reveals radon at levels above 4 picocuries per liter, you should probably arrange for a contractor to install a radon mitigation system in your house.
How does radon get into a house?
Radon is a naturally occurring radioactive gas that is present in outdoor and indoor air. Radon is produced by the natural decay of uranium, an element that is present in nearly all soils. Because of the stack effect, the lower layer of indoor air in most homes is at a negative pressure with respect to the outdoors, especially during the winter. This negative pressure draws soil gases into the home through cracks in the foundation: for example, through cracks between a basement slab and the foundation walls, through control joints in the slab, through cracks around water pipes that penetrate the foundation, and through sump pits. About 1 in 15 U.S. homes has a high level of indoor radon.
It’s difficult to predict the radon levels in an untested home. High radon levels are possible in new homes as well as old homes, and in tightly sealed homes as well as leaky homes. According to GBA technical director Peter Yost, the following factors affect a home’s radon levels: the radon concentration in the soil under the house; the moisture content of the soil (dry soil allows radon to move more freely than damp soil); the pressure difference between the air in the soil and the indoor air; the size of the holes and cracks in the home’s foundation; and the air exchange rate of the house.
Up until a few years ago, some radon experts suggested that soil maps might be a useful guide for homeowners curious about indoor radon levels. These days, however, the standard advice has changed; homeowners are now told to ignore the maps. It’s possible to have a high indoor radon level in any state in the country, so the only reliable way to determine the radon level in your home is to test the indoor air.
Radon is sometimes present in well water, but rarely in surface water drawn from springs, lakes, or streams. According to a web site maintained by the American Cancer Society, “For the most part, water does not contribute much to overall exposure to radon.”
Is radon really dangerous?
According to most authorities, radon is the second leading cause of lung cancer in the United States.
Your chance of getting lung cancer from exposure to radon depends most strongly on whether or not you smoke cigarettes. Radon and tobacco smoke reinforce each other, so if you smoke cigarettes and live in a house with a high radon level, you get a double whammy. On the other hand, if you are a nonsmoker, high radon levels in your home are much less likely to cause health problems.
Other factors affecting your chances of getting lung cancer from radon are:
- The radon level in your home’s indoor air;
- The amount of time you spend in your home; and
- Whether you spend your indoor hours in the basement or on an upper floor, where radon levels are much lower than in a basement.
If you smoke cigarettes, living in a home with a radon level of 8 picocuries per liter — a level that is twice the “action level” — is dangerous. About 12% of cigarette smokers who spend their entire lives living in a home with a radon level of 8 picocuries per liter will get lung cancer. However, if you live in a home with a radon level of 8 picocuries per liter and you don’t smoke cigarettes, your chance of getting lung cancer drops from 12% (the rate for smokers) to only 1.5%.
Whether or not the presence of radon in homes is mostly a problem for smokers depends on your perspective. Some analysts feel that the health data aren’t strong enough to justify much concern for nonsmokers; for example, one skeptic, Michael Shaw, advises, “If you’re worried about lung cancer, don’t smoke.”
As we await more data on the effects of high radon levels on the health of nonsmokers, it isn’t worth rolling the dice with the health of your family. If testing shows that your home has radon levels above 4 picocuries per liter, the prudent course is to take measures to lower your home’s radon levels.
A variety of do-it-yourself test kits are available to test radon levels in your home. These test kits must be left in your home for a period of days or weeks, and are then mailed to a lab for analysis. Most experts divide these test kits into two categories: short-term tests that take 90 days or less, and long-term tests that take more than 90 days.
Most experts advise homeowners to test the lowest occupied floor of your house. In some homes, the level of radon in the basement is above 4 picocuries per liter (the action level), but radon levels on the first and second floor are in the safe range. There is no need to test the air in your basement if no one lives down there. So if your basement is unfinished, you should test the air on your first floor.
Radon levels can fluctuate from day to day, so long-term tests tend to be more accurate than short-term tests. Most homeowners start with a short-term test. If the results are near or above 4 picocuries per liter, it’s worth following up with a long-term test.
If your levels are below 4 picocuries per liter, relax. If they are above 4 picocuries per liter, you’ll probably want to hire a radon mitigation contractor.
New construction details
Before describing a typical radon mitigation job in an older home, it’s worth stepping back to discuss recommendations for builders of new homes.
In some states, specifications for radon-safe homes are bundled together and labeled as “radon-resistant new construction” (RRNC) recommendations. One such program is promoted by the New York State Department of Health.
Since it’s impossible to test the air in a new home before the home is completed, and since radon levels in a new home might be low, it makes no sense to install an active radon mitigation system in every new home. Instead, the best approach is to install only a few essential components: the components of a passive mitigation system.
If you’re taking this approach, you should:
- Install a 4-inch-deep layer of clean 3/4-inch crushed stone (without fines) under any basement slab or a slab on grade.
- Install at least one horizontal length of 4-inch perforated PVC pipe in this layer of crushed stone; one end of the perforated pipe should be connected to an elbow or tee that connects to a non-perforated 4-inch PVC riser (a vertical pipe that penetrates the home’s roof).
- Install a layer of 6-mil polyethylene above the crushed stone layer, under the concrete slab.
- Caulk or permanently seal all cracks in the slab as well as all penetrations through the slab and the slab perimeter.
- If the basement has a sump, install an airtight sump lid.
- Wire an electrical cable to an electrical box in the attic near the vertical vent pipe (in case it turns out that an exhaust fan is needed in the future).
The vertical vent pipe should be routed through warm spaces in the center of the house in order to maximize the stack effect. The vent pipe should have as few horizontal sections and elbows as possible.
The vent pipe should terminate 12 inches above the roofing. Passive air flow through the vent pipe will be stronger if there is no cap or elbow on the top of the vent pipe. Exposed sections of the vent pipe should be clearly labeled with the words “radon vent system.”
Alert readers will realize that many components of this passive radon mitigation system will serve multiple purposes — for example, they help ensure that the foundation will stay dry. (For more information on keeping foundations dry, see Fixing a Wet Basement.) These components are well worth installing in all new homes.
This passive radon mitigation system described here allows soil gases to flow from the subslab region through the vent pipe to the outdoors. The driving force is the stack effect. In most homes, a passive radon mitigation system is adequate to keep radon levels below 4 picocuries per liter.
What if I need a fan?
If testing shows that radon levels are still high, then an exhaust fan can be installed in the vent pipe. The usual location for a radon exhaust fan is the home’s attic. Radon mitigation fans need to operate continuously.
Radon exhaust fans should not be located within a home’s conditioned envelope; all pressurized lengths of pipe need to be outside the home’s conditioned space. (Here’s why: if the vent pipe ever develops a leak, you don’t want the fan to send radon-rich air into the home.) If it isn’t possible to install a fan in the attic, consider installing it in the garage or on the outside of the building. It’s even possible (although not ideal) to install the fan above the roof.
The ratings of radon exhaust fans range from 60 cfm to 200 cfm; these fans generally draw between 20 to 100 watts. Low-flow fans work well for homes with a porous layer of crushed stone under the slab; high-flow fans are required when the soil under the slab is dense and damp.
Retrofitting a radon mitigation system in an existing house
Older homes usually lack a porous layer of crushed stone under the basement slab, so passive radon mitigation systems are unlikely to work. If you have an older home with high radon levels, you’ll need to hire a certified radon mitigation contractor to install an active subslab depressurization system.
Although this type of system isn’t as complicated as it sounds, the work will still probably cost you between $1,000 and $2,000.
If your basement or crawl space has a concrete slab, your contractor will drill a 4-inch-diameter hole in your slab. Once the drill has broken through the concrete, some of the soil is excavated from the hole. It’s unlikely that the contractor will discover a layer of crushed stone under the slab, so it usually makes sense to remove enough soil to make a void under the slab, equal in volume to a basketball or a wastebasket. Then the 3-inch or 4-inch riser pipe is installed; the pipe originates just below the bottom of the concrete slab, not at the bottom of the hole.
Where the vent pipe penetrates the slab, the gap around the pipe is sealed with hydraulic cement. The contractor will also seal all visible cracks in the basement slab, and if necessary will install a new sump with an airtight lid. If the basement walls are made of concrete blocks (CMUs), or if the concrete walls are cracked, it may be necessary to parge or seal the basement walls.
If your existing slab is very leaky, it may be impossible to seal the slab well enough for a subslab depressurization system to work; at that point, your contractor may recommend the installation of a new concrete slab.
The rest of the radon mitigation system resembles the system described for new construction, except that a fan is almost always required.
In some cases, the vent pipe is run through the basement rim joist and extends up to the roof along the exterior wall. While this type of installation is unsightly — especially because of the need to install the fan outdoors — routing the vent pipe on the exterior of a house is sometimes the best way to go. It’s certainly less expensive than running the pipe through the home’s interior.
If your basement or crawl space has a dirt floor, a length of horizontal perforated pipe is installed on the floor, and polyethylene sheeting is installed above the pipe. One end of the perforated pipe is connected to a riser that penetrates the polyethylene through a carefully sealed hole, and the rest of the system is similar to systems installed in homes with concrete slabs.
What about granite countertops?
On July 24, 2008, a New York Times article reported that some granite countertops emitted high levels of radon. The article quoted Stanley Liebert, the quality assurance director at CMT Laboratories in Clifton Park, New York, who said that he had assessed a few granite countertops “that might heat up your Cheerios a little.” Needless to say, the article caused many homeowners to worry.
Since the article was written, researchers have looked into the question of whether granite countertops are dangerous. Most experts have concluded that there is no need to worry about the granite in your kitchen.
According to Kansas State University’s radon program, “At this time, EPA does not believe sufficient data exist to conclude that the types of granite commonly used in countertops are significantly increasing indoor radon levels. … While any granite in a home may contribute some very small percentage of indoor radon, the U.S. EPA recommends testing the total indoor radon level, and if necessary acting to reduce the amount of soil-produced radon gas as the primary means for indoor radon gas reduction. … While a radon testing professional can test your home for radon, to EPA’s knowledge, there is no agreed-upon method specifically for measuring radon or radiation from granite countertops. Direct measurements in a building of the gamma radiation or radon emanation from a material, such as granite, is not a reliable indicator of radon concen-trations that will be in the air you breathe. Attempts to use such measurements for estimating risk are subject to large errors.”
Information provided by the New York State Department of Health is also reassuring: “The bottom line: No action needs to be taken to remove granite countertops in existing homes.”
A beneficial side effect
The main drawback to an active radon mitigation system is the cost of the electricity required to run the fan. If you pay 12.5¢ per kWh for electricity, the cost to operate a radon fan will range from about $22 to $110 per year.
If you install an active radon mitigation in your home, your indoor radon levels will drop. In the case of an older house without any poly under the slab, an active radon mitigation system often provides a side benefit: lowering basement humidity levels. If your house falls into that category, you may be able to eliminate use of your dehumidifier — in which case the cost to run the radon fan may be more than offset by dehumidifier savings.
Martin Holladay’s previous blog: “A Backyard Test of Peel-and-Stick Flashings.”