Crawlspace treatment for radon mitigation?
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Crawlspaces have a radon collector under a radon barrier installed. The barrier is sealed with a proper sealant against clean concrete of walls and where needed individual pieces are overlapped and sealed with sealant. Duct tape is used only as a temporary fastener and is not the radon-seal that keeps the radon from entering the space.
The black plastic was in the past placed by others for moisture control (and is not effective for radon removal).
Before mitigation: After mitigation:



A furnace area is often found to be on a concrete pad. Sealing of the barier with caulk against the concrete is shown in this example:.
Furnace area before mitigation: Furnace area after mitigation:

Examples of taping overlapping barrier material with duct tape, each with a seal with appropriate caulk (not visible), which prevents radon entering.
The radon barier is depressurized which allows the contours of the collector system to be visible from above the barier
Treatment of a post (caulked radon-seal is again not visible)

In the center of this picture the collector system guides the radon into a white PVC pipe from where it is routed into the garage.