Cast iron directly on concrete raises moisture and rust concerns — Layer in between?
If we are installing a Kohler alcove cast iron tub (for tub/shower combo) directly onto our concrete slab, is there any layer of anything we should put down first (e.g. like roofing felt paper, vapor barrier or similar)?
The underside of the tub is cast iron with NO enamel coating (whereas the utilized tub portion has white enameled protective coating). We have an excellent vapor barrier “under” our concrete slab on grade we will use as our finished floor; however, I know concrete is very porous. With moisture, cast iron can rust – as seen by supplier having just stored it outside in rain for a few days. I didn’t know if the amount of vapor/moisture that comes through slab floor would be enough to cause the cast iron underneath to build more rust over time. If so, is there something we can put down to protect cast iron from rusting or any protection needed for any other reactions that can be produced by concrete and cast iron coming into contact?
Thanks a bunch! 🙂
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Jamie, there should be no chemical reactions between cast iron and concrete. If your vapor barrier and water management strategy are good, your slab should be dry, once the construction moisture is out. But if the house is new, the slab will continue to evaporate moisture for up to a year or so. If you're in a cold climate and don't have good insulation details, you could get condensation at the slab, but probably not enough to bother the tub. At least not any more than will be present in the bathroom anyway.
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Thanks, Michael! It is new construction. We did not insulate under the entire slab floor as this is a shop we will be living in for a couple years while we build our house. What do you mean by "not any more than will be present in the bathroom anyway"?
What type of insulation details are you talking about - below or above the slab on grade finished floor?
Typically up here (borderline 5/6 zone), slab edge insulation is put on inside perimeter of footing walls (under slab) not outside (how I've seen buildingscience.com suggest): do you think inside edge insulation is best here as is typically done here?
Jamie,
Does the bottom of the tub actually touch the concrete? All the tubs I've installed were supported by wall-flanges and the front face. The drop-in ones that weren't held up that way had built-in plywood supports, or needed to be shimmed underneath.
Edit: Don't Kohler tubs come with four feet to sit on that you shim?
Yes, this cast iron tub has cast iron feet that sit on floor - our floor being concrete. Metal shims will be needed if the brand new concrete slab or tub feet are not level.
Jamie,
Cast iron tubs are quite thick -- there's enough iron there to overcome any worries that a little surface rust might compromise the tub structure.
That said, there is no harm in putting a layer of asphalt felt between the slab and the tub to assuage your worries.
Ah - I think I got it, Martin - thank you! Rust won't grow to level of holes, etc. unless there is a continuous steady water coming into contact with the cast iron right?
So, say on used/seen glazed portion if cracks/chips etc are not repaired with new enamel then water from daily showers/baths will continually rust out that spot more and more, deeper and deeper. So, underneath tub should never get this kind/level of regular moisture unless there is a water leak for example?
Being in a bathroom, the feet are going to rust irrespective of the floor the tub is on. You likely won't see it unless there is water there for the rust to bleed out onto the floor, but if that happens it may stain.
Thanks, Trevor! We will be leaving an access hole so we can keep an eye on it :)