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New Slab Over Existing Slab in Basement to Control Moisture?

stolzberg | Posted in General Questions on

We’re planning to spray foam the rim joists and walls of basement in northwest Massachusetts with stone walls and old cement floor that seems solid but likely has no water barrier under it. Wondering how to handle water that runs down walls behind the spray foam and/or moisture coming up through the slab.  I just came across an article in Fine Homebuilding where Joe Lstiburek suggests putting down a polypropylene drainage mat tied to spray foam walls and connected to a sump pump and then pouring a new slab over the mat and old cement floor (  Has anyone tried this?  Would it make sense to put an impermeable water membrane over the drainage mat and under the new slab?  Could taped dimple mat (e.g., Delta FL or Platon) be used instead to cover the floor and tie into the spray foam walls?

p.s. we’re also working on grading and drainage around exterior of house to minimize water from that perspective.

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  1. kenmarcou | | #1

    Very similar situation with me! I’m in central MA near the Quabbin Reservoir. Built in a hill in 1840. Walk out basement. Brick basement wall above grade, Stone foundation below. Only half the basement has concrete slab. Half was wood on dirt for 120 years and now it has crushed stone and needs a slab so is still open and a source of moisture. In the hill, in the below grade parts along the stone foundation 2 decades ago my father dug an interior perimeter drain w/buried pvc pipe gravity fed to a sump pump at the lowest point. I get water trickling down that back stone wall. I need to redo the roof and gutters badly so I know that’s a huge reason why. Especially since it’s the back dark north side of the house. — But I think we’re supposed to have the perimeter drain, install dimple mat on the stone wall, spray foam the wall and up to the sill/rim joist up to where the first floor meets it. Then on the bottom/ground there should be the crushed stone, 2” EPS and then poly below the 4” slab which gets poured over to the perimeter drain area to cover it over again with concrete (if it’s still gravel like mine and needs concrete to cover it again) The under slab poly and the dimple mat get sealed together where they meet the the slab/wall joint (as to the eps and spray foam) so there is a continuous layer - because control layers always have to be continuous.

    Is this what you understand as well?? Can’t wait to hear from folks!

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