Is radon mitigation necessary with well sealed foundation design for a passive or near passive house?
We are planning a near passive house in northern Vermont. Radon risk is small to moderate. Design includes 6″ subslab XPS and 2″ high density(60 psi) foam @ the footers + R 30 ICF’s.
Is punching a hole in the slab/ subslab 6″ foam for possible radon mitigation really necessary and worth the money?
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Per the EPA. Northern VT counties have average indoor radon levels between 2 and 4 pCi/L, which mean that you must provide a mitigation system. You could always vent it through an outside pipe vent connected to a socked drain tile loop around the interior of your foundation. If you install an interior vent, just make sure you seal the penetration well; passive and active vents release all radon before any leaks appear around the vent pipe.
Radon mitigation is only necessary if a radon test shows elevated radon levels inside your house.
If you are building a new house, it's a good idea to install sub-slab piping and a sub-slab layer of crushed stone so that you are able to install a radon mitigation system in the future if testing shows that it is necessary. The pipe will need to penetrate your slab, but there is no need to install a radon mitigation fan unless testing shows one to be necessary.
In the South and SW, the costs of passive radon mitigation are about $300 as you build the house, and about $2500-5000 after you build the house. Radon test kits bought at the lumberyard are very unreliable, and a good, professional ground testing cost at least $500. It makes no sense to skip installing at least a passive system.