Backyard Sauna build

Ever since I spent some time in Finland I wanted to have my own sauna retreat in the back yard. It's such a relaxing feeling after a hard day to sit in the heat and then cool off afterwards.

I thought I would share some pictures taken during the construction of my sauna, it's interesting because it includes some construction details which are useful to anyone considering their own sauna build.

Having a smallish, irregular shaped house block there were a lot of factors to consider during the planning stage: Seclusion of away from the house vs convenience and availability of services such as electric connection or plumbing; The cosy-ness of woodfired vs simplicity of electric heating.
The cost of electricity is talked about a lot these days, and a sauna heater uses a LOT of power, but I consider that unless you have your own private forest nearby and the time to cut firewood it's still going to work out a lot cheaper than buying wood at, say, $20 per bag at a service station, plus no need to carry, stack, restock and store a stockpile if bought in bulk. Nevertheless the cost of hiring an electrician to wire up the sauna has to be factored in.

I settled on a 2.2x1.8m all-timber cabin built adjacent to but completely separate from my house, with access through an outdoor patio area. About 2m of herb garden was sacrificed for this luxury:

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I decided not to use a concrete slab due to cost.
The floor framing was built in a 5-sided shape to reduce the feeling of enclosure in the existing patio area, and access to the back yard. The cabin would have a door on the corner:
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Once the wall framing was up, it started to take shape:

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Addition of the ceiling vapour barrier/sarking and colorbond roof occurred just in time before a rain shower. A small skillion roof with its own gutter will be used to collect rainwater for use in the sauna. Once in place, the ceiling lining could be installed. The interior lining is tongue-and-groove spruce. Sauna interiors must be softwood and the spruce has a light appearance, piney-smell and is more durable than western red cedar, which tends to dry out and turn brittle when exposed to the heat and dry-moist cycling of a sauna.

A small window was included in the side wall to provide a glimpse outside from the sauna, in Finland this would be double-glazed for insulation but in the Australian climate a single pane of glass is sufficient.

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In building the back wall, allowance was made for a ventilation duct which is essential for good airflow. The finished sauna would have a sliding hatch over the entry to this duct, which can be opened or closed as required.
You can see this in the following picture:

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The interior walls were lined with aluminium-backed vapor barrier on the inside, and rockwool insulation was used on the outer side:

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The outside weatherboards of western red cedar (rough side out) give the cabin a rustic but refined appearance. I coated the boards immediately with Intergrain Natural stain, a hybrid water-based finish which was recommended to me as being durable but also low in VOC. I am hoping for a better result than some other water-based finished I have seen which peel after around a year or so of exposure:

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I had the wiring for the heater and interior lighting installed by a licensed electrician. The heater chosen is 6.8kW and can be connected via single- or three-phase power to its own direct circuit at the switchboard.
Three-phase power for these heaters would be ideal, and allows for smaller diameter (therefore cheaper) wiring, however as we only had single phase the circuit required a 6mm2 electric supply and 32A breaker. The cost of having extra cables for three phase connected to the house from the street and extra meters would have been much higher.

The heater manufacturer specifies the use of non-PVC insulated cable due to heat embrittlement on this type of insulation. I did have some trouble sourcing a 6mm2 diameter cable in Australia with silicon insulation, but eventually did find some cable rated to 180C in a 4-core version (only needed 3 of these conductors) from a specialist supplier. The cabling was run under the floor so not really exposed but better safe than sorry!

The manufacturer also warns that the heater should not be connected through an RCD, as moisture absorbed into the heater element can cause earth leakage and continually trip them. Some electricians may be uncomfortable with this arrangement as RCDs seem to be the norm now, but the wiring standard AS3000:2017 does specify an exception for sauna heaters and since everything is wired properly with good earth and correct cable size/type I am very comfortable with following the advice of the experts who make these appliances.

The interior of the finished sauna showing the modern heater and single bench layout:

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Wiring and light fittings above 1m is supposed to be heat rated to 125 degrees inside a sauna, I bought a porcelain sauna light fitting for this purpose but unfortunately it was broken during the installation. I am going to trial an outdoor LED light instead, it is shielded from direct heat by a wooden screen and seems to be fine for intermittent use, with 95% of the integral cable being outside the cabin anyway:

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There are a few details left to sort out, but it works and I am very happy with the result. Looking forward to the winter cold.