Building out a shower over foil faced polyiso interior sheathing
My husband and I are renovating a 1900 farmhouse. The exterior walls have old aluminum siding over older wood siding. There is cellulose insulation blown into the 2 x 4 wall cavities. The walls had 1″ wood ship lap interior sheathing. We have sheathed all the interior walls with 1″ foil faced polyiso rigid insulation. We are drywalling directly over the polyiso since the ship lap behind it gives us a good substrate to which to screw the drywall (with 2 1/2″ screws), but I am uncertain about what to do for the shower and tiled bathroom walls.
Can we install cement backer board directly over the poly iso? Foil is on both sides. I have read some info about concrete not being in contact with aluminum, but does any of that apply to stable backer board?
Should we/can we use a waterproof membrane between the polyiso and the backer board in the shower? Use a Kerdi type membrane over the backer board? I am not so worried about the tiled bathroom walls, but the shower walls are my major concern–both for stability of the system (foil facing not being corroded) and avoiding moisture problems in the wall.
Any thoughts?
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Replies
C. Foley,
I've never heard of any problems arising from installing cement backerboard directly against foil-faced polyiso. However, it's conceivable that the backerboard is alkaline enough to eventually damage the foil facing.
If you choose to use cement backerboard, you'll need to install a waterproofing layer on the interior side of the backerboard -- for example, a liquid-applied membrane such as Laticrete Hydro Ban or Mapei Mapelastic AquaDefense. Since this type of wall assembly has multiple interior vapor barriers already -- both the foil-faced polyiso and the liquid-applied membrane are vapor barriers -- there is no harm in adding an additional vapor barrier -- namely a layer of polyethylene between the foil-faced polyiso and the cement backerboard. The polyethylene should eliminate worries about the alkalinity of the cement backerboard affecting the aluminum facing.
For more information on tile backerboard, see Tile Backerboard Options.