Sealing heating duct openings through floor to reduce radon?

Anyone have any good ideas on what to use to seal the openings that was created for the heating registers installed on the first floor. There is about a 3/4'' gap around the register looking up at them from the basement.Can I just foan it up with latex expanding foam. I wasn't sure how the thermal expansion of the heat duct would effect this or if it is against code. I suppose I could just tape the gap with hvac foil tape, but I prefer a more permanent solution. I'm trying to control radon migration thru the basement ceailing by sealing all holes thru the OSB subfloor that were made by the various contrators.
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If the house is new, the contractors were required to seal the holes with fireproof caulk. Great Stuff http://www.dow.com/greatstuff/ is a pretty permanent solution. Apply it slowly because it expands a lot and you want to see how much is really needed.

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Firestop is required, and foam isnt that. You will need a tube or two of the firestop seal.
Now...if you have a radon problem, you need to concentrate more on the removal of the radon from the basement, since you wont stop it from making its way into the home.
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