So let's dig in.
The walls of my tiny house are made of Magnesium Oxide board which works in place of drywall. Even in a cob house, I would use MgO board for the bathroom.
MgO board is a cementitious wall board ideal for those with chemical sensitivities. It is non-toxic, VOC-free, and a very impervious to mould. If you have a wood-framed wall it will be easy to attach the board to that. Good brands are Dragonboard (US, and coming to Canada soon), Mag Board (US) and Magnum Board (available in Canada!)
Ceramic and glass tiles are inert and totally safe for the chemically sensitive homeowner.
So you don't want ceramic or glass tiles, you say. OK, well then it gets more complicated:
- Stone should be tested for radon and sealed with a non-toxic sealer
- Slate and Marble - make sure they don't have a chemical sealer on them already, and with marble you don't want it to have a resin on it either (which is applied to most marble before it hits stores). Seal with a natural sealer as listed in Finishes. (Marble may also need a specfic grout, talk to your contractor).
- Concrete tiles - my preference because of the beautiful designs (see left). Needs to be sealed with a non-toxic finish.
- Imported glazed tiles should be tested for lead and radioactivity.
- It's preferable to have something glazed/sealed in the factory but make sure if it's something other than ceramic tile that it is fully cured and won't be off-gassing.
- Porcelain - from what I know it should be an inert suitable material, but it may not be able to be laid in Portland Cement as grout. Look into it.
Finishing MgO Board
What about concrete?
There are other concrete sealers that in theory should seal (unfinished) concrete, so that may be an option, and actually it is very much in style right now. I could not find one that was non-toxic and waterproof.
It is a labour intensive finish made from lime plaster and olive oil soap which together produce a chemical reaction that provides waterproofing.
There are builders and artisans who are experts in this technique in BC and their work can be seen at OUR Ecovillage.
The down side of this finish is that you may need to polish it with olive oil soap as often as every month, and like any plaster finish, cracks will likely form, needing touch ups. So, you have to be willing to do a little upkeep with this one.
For a how-to get ahold of this book: Tadelakt.
2. Reducing Chemicals and Mould
Grout and Caulk
In order to prevent mould in the grout (I have yet to see grout not get mouldy with time) I would use a AFM grout sealer (see Finishes), and be sure to stay on top of maintentece and re-seal it every few years.
Don't lay tiles over toxic particle board, or mould prone green board.
Seal well between the sink and the wall, the bathtub and floor, around the toilet (if using a toilet with water) with Silicone. I have a post about finding a tolerable silicone caulk. Be totally vigilant with sealing anywhere water could creep in and promote mould growth.
A fan is absolutely vital to reducing moisture and therefore mould. I would splurge on this item. It's always good to have a window too, to air things out. Another useful tip is to squeegee the walls (and floors if you have an open drain) after showering, this can make a big difference.
To keep VOCs out of the bathroom - avoid laminates, particle board cabinets, plywood, greenboard, drywall, PVC shower curtains, melamine, conventional wood stains and sealers, conventional tile sealers, grouts with latex additives, caulk (other than aquarium grade silicone), and conventional paints and primers.
3. A Chemical-Free Shower Stall
This was the most difficult aspects of the entire build to figure out and I have a whole post devoted to the shower stall.
For a non-toxic shower curtain I use this EVA one.
4. Cleaning Products
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For natural, cheap, non-toxic cleaning products see: Cleaning Products.
I have a composting toilet in the bathroom as it is a great sustainable option and makes it easier to be semi-off-grid. This post discusses my issues with some of the smelly additives some composting toilets require.
Sources: Re|shelter and Prescriptions for a Healthy House, 3rd Edition: A Practical Guide for Architects, Builders & Homeowners