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Insulating Existing Poured Concrete Crawlspace with Radon Mitigation Integration Design

trapdoor2038 | Posted in General Questions on

I am looking to insulate my poured (vented through bilco access) concrete crawl space from the 1980’s and tackle elevated radon simultaneously. It appears to have 1″ EPS on the outside of the wall that daylights at ground level. I am guessing it was just placed against the wall and backfilled, so I don’t know if it is much effective. It was poured with a 4mil poly sub-slab.

I have alredy sealed all cracks in crawlspace floors and walls with self leveling caulk. The first floor is a drop floor bolted into the concrete wall. I am thinking I will caulk the gap between the bottom of the wood ledger and the concrete wall. There are two existing 1″ floor drains that daylight only below the subslab. See Diagram 1. 

I plan to begin by bucking in an insulated door to the crawl space and then insulate the crawlspace with foam board on the floor and walls. I am thinking that I could use the two existing floor drains as radon draw points below the foam. Diagram 2. I could also use drainage mesh below the foam for more draw for the radon. 

Alternatively, I could do the layer of foam and then put a blanket of poly (20 mil for durability) over the foam at tie it either to the foam wall or above the top of the foam wall like a boat. I could then have the radon pull from between the poly and the foam, and even lay perforated pipe between these two layers for greater draw. Diagram 3. 

What approach is like best for insulating, radon, and cost purposes? It would be helpful to have some unbiased opinions of which is best. 

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  1. Jon_R | | #1

    Sub-insulation ventilation (with drainage mesh to allow air movement) can work. But so can an exhaust fan in the entire crawlspace (maintaining it at a negative pressure wrt the house).

    1. trapdoor2038 | | #2

      Would it make sense to insulate with the floors and walls first taping all seams? To see if that reduces radon levels before considering a passive pipe system or an exhaust fan. My only gripe with the exhaust fan is it will pull all air out of the conditioned space.

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