Here is the tiny house being built for my client right now by Tiny Green Cabins. The house is made with no wood whatsoever, including plywood and OSB! This is made for someone who cannot tolerate wood of any kind. She is extremely sensitive to offgassing and mold.
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Here are the specs:
Size is 8’ x 20’ x 12’-5 1⁄2” tall, with an approximate weight of 9k GVW. The trailer is a custom welded steel channel beam trailer.
There are options for the paint used on the trailer.
Steel Underbelly 2 x 4, 16 gauge joists. The cold-formed steel joists are bolted to the trailer frame.
Walls are framed with 2 x 3 18 gauge cold-formed steel studs @ 19.2 on center, fabricated with screws and welded connections.
Rain screen (furring) is made of metal (rain screen is important).
Roof structure is 18 gauge cold-formed stacking above joists, fabricated with screws and welded connections. No wood used!
For the bathroom there are different options – you can have RV hook-ups or a composting toilet and grey water system. (Nature’s Head is the best composting toilet. Others like Sunmar have some issues.)
There are a few options for windows. I prefer aluminum, but they cannot be sourced everywhere. There are other options in my window post that people tolerate well. Typar zero-VOC window flashing is being used.
Fabral smooth painted steel to wrap the exterior walls, and Fabral “w” metal roofing for the roof. Metal at walls is riveted and steel roof and trims screwed. Fabral should be tested first to make sure the paint finish is tolerable. Other metal finishes are available with different brands.
Ceiling is corrugated steel
Interior walls are Fabral steel attached with screws. The interior comes in different colors, or can be painted later with metal primer and paint. Caulking is used to prevent water vapor from entering the wall cavity.
Metal flooring is 2 layers of 18 gauge steel layered. Foam panels are used as a thermal break and insulation.
Doors are metal and glass.
Insulation options are XPS foam or foil-backed polyiso. Foam is being used as exterior sheathing/thermal break as well. Other materials could be considered for insulation but rigid foam was our top pick here. Tyvek tape is used to seal the sheathing if tolerated.
They also used caulk on the inside side for an extra barrier to water vapor entering the walls. (This is a double vapor barrier – note the risk. Please consult with an architect if you can only tolerate metal and foam).
Heating and cooling a 12-15,000 BTU inverter heat pump, Daikin LV series or Mitsubishi hyper-heat models installed by an HVAC company is recommended. Other heating options such as wall-mounted electric heaters or propane RV heater/AC combos (off-grid) could be used. Another propane option that is tolerable is a direct vent heater.
For a hot water heater, we are using propane on demand. This is the best way to go for water to keep the house to 50 amps or less.
Exhaust fans are very important in an all-metal house to keep the humidity as low as possible. In the bathroom, it should be exterior mounted as to not leak moist air into the ceiling. An ERV could be a good option if you have a composting toilet (this depends on your climate as well). In the kitchen, the exhaust fan needs to vent to the outside. A dehumidifier may also be needed if condensation forms on the walls or the mattress becomes damp (do not put a standard mattress straight on the ground).
For appliances, a propane fridge should be considered to reduce electricity needs (2-way or 3-way refrigerators can be good depending on your needs and if you are incorporating solar).
Flooring can be left as metal. Tiles can be considered although this would add considerable weight. Natural carpet or rugs can be considered as well to cover the metal.
Thanks to Luke Skaff, electrical engineer, for help on the technical aspects.
Always consult with an architect or engineer on moisture management in your building envelope.