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Almost certainly for drainage. Carrying any water leaking in from the walls to the sump. My understanding of radon mitigation would be that this gap and the sump pit need to be sealed from the top and power vented to the exterior. Any underslab drainage on the "basement" side should be connected to the vent system as well. If you already had a radon ...


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I would insulate the outside walls with the rigid foam board insulation like you described first, but do not do the floor at this point in time. Then see what kind of improvement that provides. I'm not sure what I would do about the existing vapor barrier that's on the walls. If you leave it in place and put the foam board on top of it, then it's possible ...


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In most cases, you would have both a supply and return in the crawl. If the crawl is properly air sealed/encapsulated, then the air in the crawl should be the "same" as what is circulated in the home, so no issue with a return in there. If it's not properly encapsulated, then I would take the steps necessary to do so before adding a return as you may ...


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Sounds a lot like my son's new house. Since you already have an HVAC duct that runs through the crawl space just add a register to that duct. My guess is that unless you're actually going to seal the crawl space and make it air tight relative to the rest of the basement, you'll have enough leakage around the access door so that no additional return is ...


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