The walls of my tiny house are made of Magnesium Oxide board which works in place of drywall. It does have a problem with cracking along the seams, at least in a tiny house.
MgO board is a cementitious wall board that claims it is non-toxic and VOC-free, and very impervious to mould. I have some doubts about the VOC claims because a person I know sent in a sample for testing and it showed formaldehyde offgassing. And, many people react to it. I reacted strongly to the dust and I also found the board itself retained its scent for 4 years and counting. Brands are Dragonboard (US, and coming to Canada soon), Magnum Board (available in Canada) and Jetboard. Jetboard is the only brand I know of that is made in the US and it may be more reliable in its claims of being non-toxic, though the binder is proprietary and it contains fiberglass.
Cementitious boards that can be used around the shower are Durock (zero-VOC) and Hardibacker (GreenGuard Gold).
You can plaster, paint or tile over MgO. Here are some tips on using plaster and what didn't stick to MgO.
A joint compound/drywall mud that works with MgO is One Pass.
If painting, the MgO boards would need to be primed. See finishes for a list of suitable primers and paints. I used Mythic primer and paint which was tolerable (only available in the UK now).
Tile Backing Systems
While a cement board can be used with the Kerdi shower system, there are also waterproof boards that are tile backer boards and waterproofing in one: Kerdi Board and Wedi Panels are recommended to produce the most mould-proof shower or bath assembly. Both are XPS foam with a backing, I would test those against your sensitivities. Here are some videos of the Kerdi shower system, the Kerdi Board and the Wedi Panels.
Non-Toxic Shower Stall Options
- Ceramic are good if the glaze does not contain lead or radioactivity.
- Porcelain - is an inert suitable material.
- All Imported glazed tiles should be tested for lead and radioactivity. A client just tested American made tiles that stated they were lead-free but when tested they showed high levels of lead. So it might be wise to test any glazed tile. And be extra careful when removing them as the lead dust is particularly harmful. Tile over if possible instead of removing.
- You may also want to avoid tiles with Microban fungicide added to them.
- Slate and Marble - have a sealer on them already, and marble has e a resin (which is applied to most marble before it hits stores). I have not been able to pick up a smell from it but make sure it is tolerable for you.
- Glass tiles are inert and totally safe for the chemically sensitive homeowner.
- Concrete tiles - my preference because of the beautiful designs. Look for "eco" brands or ask which additives are used. Needs to be sealed with AFM if not sealed already.
I had to make a custom stainless pan for my tiny house because we did not plan the bathroom size around the ready made ones. Shower pans typically come in acrylic which would be tolerable for most people. They also come int fiberglass (it does offgas and scratches easier), cultured stone (should be tolerable), stone (sometimes with PVC under), solid surface such as Corian (minimal offgassing) and cast iron (very expensive but tolerable).
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 their work can be seen at OUR Ecovillage.
The downside 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. I have seen mould grow it in when it was not touched up.
For a how-to get ahold of this book: Tadelakt.
Metal Shower Stall
I used an aluminum shower stall in my tiny house and was really happy with it. I have a whole post devoted to my shower stall. Stainless steel could also be used.
The galvanized buckets being used in tiny houses are not proving to be durable and leak proof in the long-run.
Fiberglass and Acrylic and Solid Surface
Fiberglass and acrylic may be an option for those not super sensitive if you installed it yourself with a non-toxic adhesive such as AFM Almighty Adhesive. Sensitive people find that fiberglass takes a few weeks to a few years to offgas. Fiberglass is low end (not that durable, higher maintenance and not that attractive).
Acrylic surrounds are a step up. There are affordable options as well as high-end option such as those by Kohler. They offgas a little but not as much as fiberglass.
Corian and Swanstone surrounds have almost no offgassing themselves, however the epoxies approved by the companies do, and I'm not sure what the alternative is.
Tubs can be installed with mortar and not glue. Porcelanosa and acrylic can be installed with mortar.
Some people like a tub with claws so that no leaks can go undiscovered behind the tub.
For a non-toxic shower curtain, I use this EVA one.
For non-toxic bath mats there are two options, natural rubber, which still has a rubbery smell, or TPE plastic, which is non-toxic and odour free but almost always contains Microban which you may want to avoid.
Chemical-Free Mortar Grout and Caulk
In order to prevent mould in the grout I would use a AFM Grout Sealer , and be sure to stay on top of maintenance and re-seal it every few years. The grout should be replaced when the mould is deeper than just surface.
Don't lay tiles over toxic particle board, or mould prone green board. Always use a cementitious board behind tiles or the Kerdi and Wedi boards.
Seal well between the sink and the wall, the bath tub and floor, around the toilet (if using a toilet with water) with caulk. I have a post about finding a tolerable caulk. My top pick is Ecobond.
A fan is absolutely vital to reducing moisture and therefore mould. I would splurge on this item. If your fan is within a ceiling cavity like mine is, use an external mount fan like this one. Otherwise the fan can leak moist air into the ceiling. If you have a composting toilet and don't want to overrun the fan with your bath fan it may be advisable to use an HRV such as this one. It's always good to have a window too, to air things out.
To keep VOCs out of the bathroom and prevent mould, avoid laminates, particle board cabinets, green board, PVC shower curtains, melamine, OSB, conventional wood stains and sealers, conventional tile sealers, grouts with toxic additives, conventional caulk, and conventional paints and primers.
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For natural beauty products I use and recommend see this post.
I recommend these organic towels from Amazon (vetted by my most extremely sensitive friend), or Coyuchi towels which are made for the chemically sensitive.
I have a composting toilet in the bathroom as it is a great sustainable option and makes it easier to be semi-off-grid in a tiny house. This post discusses some issues with composting toilets for the chemically sensitive. The one I recommend is Nature's Head.