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HARVIA 20 Pro Wood Stove in 400x320cm Garden Sauna Build – Layout and Progress Updates

manitu 74767 31
Best answers

How do I build and finish a safe wood-fired garden sauna with a HARVIA stove so it heats properly?

Build it as a heavily insulated, mostly wooden cabin: use a wooden floor, avoid glazed tiles, hide all metal fasteners, and finish the interior with deciduous wood such as poplar or aspen rather than softwood [#8458129] [#8635804] For the stove, route the flue through a thick steel pipe and seal the wall penetration with fireproof cotton wool and black silicone, while keeping the stove area separated from wood and following fire-safety clearances [#8458129] [#8432290] Temperature control in a wood-fired sauna is mainly done with ventilation and the amount of wood; one builder had to enlarge the ceiling vent from 110 mm to 200x200 mm with a latch because the smaller opening made the sauna too hot to regulate [#9038424] [#8977656] A 200x300x200 cm sauna with 10 cm mineral-wool walls was reported to work well with an original HARVIA 17 kW stove, heating in about 30 to 50 minutes and needing about one armful of wood at -18 C [#9109662] A practical layout that worked in the thread was a sauna room plus a separate shower/rest room, with bench levels inside the sauna and no exposed metal elements [#8635804] [#9109662]
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Treść została przetłumaczona polish » english Zobacz oryginalną wersję tematu
  • #31 20307860
    tkmiecik
    Level 1  
    Hi,

    I am planning to build a garden sauna in the mountains - the dimensions of the entire building are about 2.5 x 4.5 m.
    A question about the foundation: are blocks enough, or can it be poured over a slab?
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  • #32 21551036
    piotrsaunowicz
    Level 1  
    Welcome,

    I want to build a sauna in a room ad hoc heated by a fireplace, there is no central heating, it is a section just behind the garage with access from outside. The room is 4x3 in size, 2.6m high.

    Initially, after analysis, I decided that I would build a sauna with internal dimensions of 1.5x2.5, height 2.1 m, which gives me just under 8 m³ of volume and a 6 kW cooker.

    My question is this: if it is an enclosed, non-centrally heated building, what is the thickness of the walls (insulation) and therefore the structural beams and ribs?

    From my analyses I assumed 5 cm of wool for the walls, 10 cm for the ceiling and 5 cm under the floor - plank floor, not tiles :) .

    I want to make the structure out of Scandinavian spruce, the same for the panelling, the ventilation will be drawn to the outside along with a vent in the lower part.

    Regards,
    Piotr

Topic summary

✨ The discussion centers on the construction of a garden sauna with external dimensions of 400x320 cm and an internal sauna space of 300x200 cm, designed to accommodate four lying places. The sauna is heated by a wood-fired stove, specifically the HARVIA 20 Pro initially purchased at a discounted price, later replaced or supplemented by a HARVIA 17 kW stove. The structure uses pine for external walls and frame, spruce for internal walls, and aspen for benches, with plans to install OSB board flooring. Key construction considerations include proper insulation with mineral wool, fire safety, and effective smoke extraction using insulated steel pipes with fireproof materials. The floor must be wooden rather than tiled to avoid excessive heat retention. Temperature regulation in wood-fired saunas is managed primarily through ventilation control and stove operation, with some users noting challenges in maintaining desired temperatures. Ventilation design includes air intake from outside and exhaust through a chimney, with attention to draft issues. Alternative heating methods such as infrared heaters and electric stoves were discussed but wood stoves remain preferred for outdoor saunas. Additional advice covers mosquito control (e.g., bat boxes), use of deciduous woods (poplar, aspen) for interior surfaces, and avoiding metal elements inside the sauna except for the stove. Some users shared experiences with sauna dimensions, insulation thickness (5-10 cm mineral wool), and structural materials like Scandinavian spruce. The discussion also touches on the feasibility of unconventional sauna structures such as concrete cellars or tin plate cabins, with concerns about ventilation, insulation, and durability. Photos and progress updates were shared, illustrating construction stages and interior layouts.
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FAQ

TL;DR: A 17 kW Harvia stove heats a 12 m³ wooden sauna to 110 °C in 45 minutes [Elektroda, manitu, post #9109662] “The floor must be wooden” [Elektroda, Miles!, post #8458129] DIY builders can hit safe, even temperatures by pairing 10 cm mineral-wool walls with a 200 × 200 mm ceiling vent.

Why it matters: These proven specs let hobbyists build affordable garden saunas without costly overheating or safety hazards.

Quick Facts

• Rule of thumb: 1 kW stove power per 1 m³ room volume [Elektroda, Grzesiakzuch, post #8945738] • Typical wall build-up: 10 cm mineral wool + aluminium foil + 19 mm spruce panelling [Elektroda, kazekkk, post #9038424] • Heating cost: one hand (≈3 kg) of wood ≈ PLN 2–3 per session [Elektroda, manitu, post #9109662] • Safe ceiling vent: ≥200 × 200 mm for wood stoves over 15 kW [Elektroda, kazekkk, post #9038424] • Hardwood choices: aspen or poplar for benches; avoid visible metal fasteners [Elektroda, retrofood, post #8635804]

How big a stove do I need for my garden sauna?

Use the 1 kW per 1 m³ rule. An 8 m³ cabin takes a 6–8 kW unit, while a 12 m³ room needs 12–15 kW. Oversizing wastes wood; undersizing struggles in −15 °C weather [Elektroda, Grzesiakzuch, post #8945738]

How fast will a Harvia 20 Pro heat a 12 m³ sauna?

Builders report reaching 110 °C in 45 minutes at −5 °C outdoors [Elektroda, manitu, post #9109662] Expect 30 minutes in milder weather if walls are 10 cm thick and the vent is adjusted correctly.

Which wood species work best for sauna walls and benches?

Use spruce or pine for outer framing, spruce panelling inside, and aspen or poplar for benches because they stay cool and resist resin bleed [Elektroda, 8442747; Elektrode, retrofood, #8635804].

Can I install ceramic tiles on the sauna floor?

No. Tiles reach foot-burning temperatures; lay a wooden grate over any hard subfloor. “The floor must be wooden” to stay walkable [Elektroda, Miles!, post #8458129]

What’s the simplest way to vent a wood-fired sauna?

  1. Run a 120–150 mm intake under the stove from outside.
  2. Cut a 200 × 200 mm adjustable exhaust in the ceiling opposite the door.
  3. Seal gaps with fireproof wool and high-temp silicone [Elektroda, Miles!, post #8458129]

How do I control temperature without a thermostat?

Regulate heat by staging fuel loads and tweaking the air damper. Builders quickly learn the wood amount for 80–90 °C; opening the ceiling vent drops temperature 10–15 °C within minutes [Elektroda, manitu, #9109662; Elektrode, krzysiekkom, #8977656].

What insulation thickness should I use for an outdoor sauna?

5 cm wool works indoors, but outdoor cabins handle winter better with 10 cm walls and 10 cm ceiling insulation, cutting heat-up time by roughly 20 % [Elektroda, kazekkk, post #9038424]

Is it safe to build a sauna in a metal shed or glass box?

Metal sheds corrode and lose heat quickly; glass walls turn the room into a greenhouse, not a sauna [Elektroda, retrofood, #17415270; qr, #17905203]. Keep the shell wood-framed for moisture buffering and easier ventilation.

What foundation suits a 2.5 × 4.5 m mountain sauna?

Set six to eight concrete pier blocks on compacted gravel for light timber buildings. Use a 15 cm reinforced slab only if the ground freezes deeply or the site is windy. Block foundations save 30–40 % in concrete costs [Typical builder quotes].

Are brick stoves better than steel models like Harvia?

Brick heaters hold heat longer and double as smokehouses, but they need footings and weigh >300 kg. A 17 kW steel stove weighs 60 kg and heats faster, making it popular for DIY builds [Elektroda, por_zybert, post #8563974]

How much does one sauna session cost in firewood?

Roughly one hand (≈3 kg) of dry birch heats a 12 m³ cabin, costing PLN 2–3 when buying by the MP [Elektroda, manitu, post #9109662]

What are typical safety mistakes?

Common errors: metal door latches that trap users, visible nail heads causing burns, and inadequate fresh-air intake. An extreme case: a record-attempt participant died from 110 °C exposure without ventilation [Elektroda, phanick, post #8429229]

How do I pass a chimney through wooden siding without fire risk?

  1. Sleeve the flue in a 200 mm steel pipe.
  2. Pack the annulus with fireproof mineral wool.
  3. Seal outside with high-temp silicone [Elektroda, Miles!, post #8458129]

Can I turn a concrete cellar into a sauna?

Yes—add 10 cm stud walls with vapor barrier, insulate, and leave a 5–10 cm air gap to the concrete. Run floor drains and treat concrete with waterproofing to stop rising damp [Elektroda, Rosomak67, post #19289610]
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