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My question is about the separation of the bottom of the 2" concrete slab I'm installing as subfloor to my second floor bathroom, and protection of untreated wood beams and joists in 1876 house: I"ll put a moisture barrier between tiles and conrete atop the slab, but what's the best material to isolate the concrete subfloor (which will retain some moisture across its life) from the surrounding wood so essential to supporting my house!?! Thanks. John

  • Is the subfloor being replaced with modern plywood or is it original? What is the subfloor material? Any gaps? – Freiheit Jul 27 '16 at 12:56
  • Tar paper under a mortar board is common. But I've wondered about using tyvek once or twice before. – Ben Welborn Jul 27 '16 at 13:10
  • you call the concrete a subfloor, what will you be pouring the concrete on. 2" that is gonna be one hellofa transition from the bathroom to the adjoining room. – Alaska Man Jul 27 '16 at 21:23
  • Replaced subfloor (by opening ceiling below and sistering 2x4s 2 inches from top of antique joists and filling gaps with 5/8" plyw. Then put down tar paper atop plywood, laid hose for the hotwater heating floor heating system. Atop that put concrete with a shower membrane below to double the isolation of the wood. Concrete flush with hall. rise in height will = tiles + thinset. Pretty minor. Comments ? Nervous about the tubing BETWEEn tar paper n concrete filled membrane. But didn't want concrete touch thick and tighly grained old growth beams for the life of the bathroom! Thanks. John – John C Jul 28 '16 at 14:35

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