Radon and Buying or Building a Home


Home buyers and builders in Montgomery County should be aware of the danger posed by radon gas.  If not properly mitigated, radon can cause lung cancer. Learn more about radon.




Buying a Home  (The New Radon Testing Law)

As of October 1, 2016, County law requires that “a single-family home located in the County must be tested for radon before completing a sale of the home.”  This requirement is contained in Chapter 40 of the County Code, as amended by  County Bill 31-15.



Where Does the Law Apply?

The radon testing requirement applies throughout the County, except in the Town of Barnesville, the City of Rockville, the Town of Kensington, and the Town of Poolesville.


What Homes Must Be Tested?

The law requires the testing of single-family homes, which includes detached single family homes and townhomes, except those units that are part of a condominium regime or a cooperative housing corporation.  In addition, the law does not apply if the sale of the home is:

  • Exempt from the transfer tax under Md. Tax-Property Code, §13-207, as amended;
  • By a lender or an affiliate or subsidiary of a lender that acquired the home by foreclosure or deed in lieu of foreclosure;
  • A sheriff’s sale, tax sale, or sale by foreclosure, partition, or by court appointed trustee;
  • A transfer by a fiduciary in the course of the administration of a decedent’s estate, guardianship, conservatorship, or trust; or
  • A transfer of a home to be converted by the buyer into a use other than residential or to be demolished.

The law applies to homes meeting the criteria above with a settlement date of October 1, 2016 or later.


Who Must Do the Radon Test?

The law requires the seller to perform the radon test, or permit the buyer to perform the radon test.  If the seller offers the buyer the opportunity to do the test, and the buyer chooses not to, the seller must perform the test to meet the statutory requirement that a test be performed.  Both the seller and the buyer must receive a copy of the results of the radon test.

The test may be done by the seller, buyer, or third party (for example a home inspector or radon testing professional) using an approved testing device.  


When Must the Test Be Performed?

In order to comply with the law, a radon test must be performed up to one year prior to the settlement date.  The test must be done with an approved testing device.


Which Testing Devices Can Be Used?

The radon test must be done using a testing device approved for use by the Montgomery County Department of Environmental Protection (DEP).  Each device has its own directions that must be followed to ensure accurate results.

Devices approved for use by DEP include:



Share County radon information as a downloadable, PDF (252KB). Building testing requirements on second page:

Radon flyer back

If distributing large quantities of radon information, share it as a brochure (requires folding):  PDF, 415KB


Building a New Home

Radon can enter your new house through cracks or openings in the foundation. The differences in air pressure between the inside of a building and the soil around it also play an important role in radon entry. If the air pressure of a house is greater than the soil beneath it, radon will remain outside. However, if the air pressure of a house is lower than the surrounding soil (which is usually the case), the house will act as a vacuum, sucking radon gas inside.


Building Code for New Homes

Since 1995, all homes constructed in Montgomery County must be built to resist radon entry and prepare the building for post-construction radon mitigation, if neccessary. These requirements are contained in Appendix F of the  International Residential Code (IRC).

In general, Appendix F establishes requirements for the construction of building foundations, the sealing of building envelope penetrations, and the installation of "passive" infrastructure to allow for the installation of an "active" radon mitigation system if a radon test indicates this is prudent.

For more information about the requirements of the building code, contact the Montgomery County Department of Permitting Services or call 311 (240-777-0311 outside of Montgomery County).


Talk to Your Builder

The following are common steps builders take to deter radon from entering your home.

  • Install a layer of clean gravel or aggregate beneath the slab or flooring system.
  • Include a gas-tight venting pipe from the gravel level through the building to the roof.
  • Seal and caulk the foundation thoroughly.

These construction techniques will be familiar to your builder. There is no need to hire a special contractor or architect. 


Understanding a Radon System

Radon-resistant construction techniques comprise a "passive" radon system. This system overcomes the vacuum effect experienced by most houses by creating a pressure barrier to radon entry. The system also includes a pipe to vent radon gas safely to the outdoors.

Sometimes a passive radon system isn't enough to prevent radon from entering a house. In this case, a fan can be installed to pull the radon gas from the underlying soil into the vent pipe where it can be exhausted outside the house. The addition of a fan and its associated wiring creates and "active" radon system.



Testing: The Final Word

The only way to know if your new home has a radon problem is to test. The EPA recommends that average annual indoor radon levels do not exceed 4.0 pCi/L. If your home is built with a passive radon system, you should test it immediately after moving in to make sure that radon levels are below the EPA guideline. Remember: If your radon level is 4.0 pCi/L or above, a fan can be installed easily to lower radon levels well below this guideline.

Even if you must install a fan, adding a radon control system to a house under construction is much less expensive than installing one after the house is built. The average cost for a radon control system in an existing house is comparable to other home repairs. Adding radon-resistant construction now will save you unnecessary expense and worry later.


For Architectural Drawings and Technical Information

Detailed model building standards, architectural drawings of radon systems, and fact sheets on alternative radon installations are available on the  EPA's Builder and Contractor Resources webportal.