How do you fix radon and water infiltration in a dry stone foundation?


How do you fix radon and water infiltration in a dry stone foundation?

john flannery July 14, 2020Last updated: Sept. 25, 2020

Gosh, do I need help w this one. It's a 1820s house with a largely stone foundation and backed against a small mountain. The "root cellar" with mechanicals and a dirt floor is under the living room, the rest is crawl space with dirt floor or slab. The 10x14' cellar has heavy springtime water ingress where the dry stacked stone meets bedrock.

The previous homeowner reported ~12" of standing water during a tropical storm and the sump losing power. The exterior water ingress can only be managed so much by gutters/french drains, given grade against mountain. There's no appetite for the cost of a full excavation to "footings" - if that's even possible w this type of foundation.

There's also a 3 month reading of radon at 4.0. So... I'm thinking a layer of washed gravel in the cellar is necessary to allow the water to flow to the sump. Then a radon-rated vapor barrier on ground/walls, taping best able around mechanicals and piers - and adherred to the skim of mortar under the rim.

Questions: 1) do you think an interior french drain to the sump is necessary? 2)  spray foam insulation on walls necessary? 3) would you pursue a radon system drawing under the vapor barrier or an HRV w radostat?  I don't want to unnecessarily over-engineer this... Oh, and there's mice galor! So thinking of caulking rim/joists and filling w rock insulation btwn. Heard they hate that stuff. Many thanks in advance for any advice and/or validation.

Responses (5)

john flannery
john flannery July 15, 2020, 1:40 p.m.

Wow, thanks for the guidance! Foam insulation risks on stone walls... Also drying? There's no capillary break under the sill (plastic unknown in 1820!). So would moisture/rot be a concern besides freezing? I'm in Zone 5a  by the way; low temps -20F to-15F or -28 to -26C in winter. 

Mike Reynolds July 15, 2020, 5:28 p.m.

If when you say "also drying', is that asking if you should be  concerned about foam trapping moisture? If so, yes that can be a risk in some situations. Rockwool (mineral wool) is sort of a failsafe choice in that regard, it isn't harmed at all by moisture, and the moisture will also move right through it. And if you mean about putting it on the inside instead of foam, it will still insulate, so the risk of freezing is still there.  Here is our page on choosing rigid insulation boards, you can see some options there and the characteristics about how each type handles moisture, including mineral wool. 

A lot of times with those old stone houses and foundations we sort of steer people towards super efficient heat sources rather than trying to insulate if there is any risk at all of structural damage, meaning things like Geothermal or just an air source heat pump. Again, this is all theory, so best is to have someone qualified look at it and see the condition of the mortar. Your climate zone does put you at risk, -20F is hardly balmy :) 

john flannery
john flannery July 15, 2020, 7:24 p.m.

Lol, even thinking about -20 makes me want to move to Costa Rica. Thanks again...deeply appreciated.

Mike Reynolds July 16, 2020, 9:58 a.m.

Happy to help John,  good luck!