THE BACKFLOW PREVENTER
Backflow prevention valves are encouraged (and required) by many municipalities. The idea is that a relatively simple valve can help prevent sewage from backing-up into a building in the event of a mainline sewer surcharge.
Photo: Backflow Prevention Valve and Cover
We have recently completed projects where the backflow prevention valve has proven to be a significant connection to the sub-slab. The covers we have seen are not designed to be gas-tight (i.e, no gasket).
It is important that these valves remain accessible for inspection and maintenance, so any sealant should be readily removable and replaceable. If you have one of these valves in your home, and are concerned about its potential contribution to indoor radon levels, one option is to use a pliable sealant like Duct Seal. Duct Seal is like plasticine in consistency and stays flexible over time. It can be easily removed and replaced as needed.
Duct Seal can be found at most home improvement stores, often in the electrical aisles.