15+ Years
Designing and installing saunas since 2010.
100+ Projects
In London & the surrounding areas
100% Bespoke
Every sauna custom fitted to your exact space on site.
The outdoor sauna with cold plunge combination is genuinely the pinnacle of home wellness. Done properly it's transformative; done cheaply it becomes an expensive garden ornament that deteriorates within three years. Here's everything you need to know to get it right.
The Philosophy of the Space
Before any specification, the most important decision is conceptual. Are you building a sauna that happens to have a plunge pool next to it, or are you building a dedicated wellness sanctuary where both elements are part of a considered whole?
The answer changes everything — the layout, the materials, the drainage, the landscaping, the budget, and critically, how much you'll actually use it. The best outdoor sauna installations we see are the ones where the client thought about the ritual first: where you undress, where you cool, where you rest between rounds, where you warm up again. The sauna and plunge pool are just two points in a cycle that also needs a changing area, somewhere to sit outside between sessions, and ideally a shower or cold rinse point.
Get the ritual right and you'll use it every week for years. Get only the hardware right and you'll use it occasionally until the novelty fades.
The Outdoor Sauna
The Building — Not a Shed, a Sauna Room
The single most important distinction in outdoor saunas is the difference between a garden shed conversion and a purpose-built sauna structure. They look similar from the outside. They perform completely differently.
Our sauna building structures are robust and long lasting.
Foundations:
A concrete slab or adjustable steel post foundation, not timber bearers sitting on soil. Timber-on-soil rots within 5 years in the UK climate. The slab needs a DPC (damp-proof course) between it and the structure, and it needs to be level — sauna doors that drop out of square are a persistent problem in poorly founded garden installations.
The outer shell cladding and weatherproofing:
The exterior cladding is designed to handle the UK climate: damp, frost cycles, UV, and the particular challenge of being adjacent to a water source (your plunge pool). The best options are:
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Thermowood (thermally modified timber) — heat-treated to remove sugars and moisture, making it dimensionally stable, rot-resistant without chemical treatment, and genuinely long-lasting. Thermo Pine, Thermo Radiata or Thermo Spruce are the standard choices for outdoor sauna exteriors. Much better than untreated timber or pressure-treated pine.
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Western Red Cedar — naturally rot-resistant due to its oils, beautiful silver-grey weathered finish over time, excellent thermal properties. Slightly softer than thermowood so more susceptible to denting, but the premium aesthetic choice for a garden setting.
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Black-stained thermowood — very popular in contemporary Scandinavian-inspired designs. The charcoal exterior against green garden landscaping is striking and ages well.
The roof Flat roofs on garden saunas are a maintenance problem. We always install a pitched roof — even a gentle mono-pitch — that handles UK rainfall properly and gives the structure better proportions. EPDM rubber membrane is the most durable flat roof option if the design demands it. Metal standing seam roofing (zinc or steel) is the premium choice for a pitched roof and looks exceptional.
Structural insulation The walls, floor and ceiling of the sauna room inside the structure are built as a proper sauna — not just a timber-lined shed. This means:
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Mineral wool / foil faced insulation to at least 100mm in walls, 150mm in ceiling.
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A continuous aluminium foil vapour barrier on the warm side (hot face) of the insulation, properly lapped and taped at every joint. No gaps, no tears.
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50mm air gap between the vapour barrier and the interior cladding for heat circulation.
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Insulated floor — either a raised timber floor over insulation, or insulated screed if concrete.
The vapour barrier is the most frequently skimped element in budget outdoor saunas. Its failure leads to moisture in the wall structure, timber rot from inside, and eventual structural failure. You won't see it happen for 3–4 years, and then it becomes expensive.
The Interior — Where the Experience Actually Happens
Timber cladding
For an outdoor sauna that gets heavy use and temperature cycling (cold exterior, very hot interior), thermal stability matters more than in an indoor installation.
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Thermo Aspen — the premium choice. Heat-treated for stability, golden-brown colour, odourless, non-resinous. Won't weep resin during hot sessions, won't warp through seasonal cycling
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Abachi — particularly good for benches. Extremely low density, stays cool to the touch even at high temperatures. When you're moving between hot sauna and cold plunge repeatedly, bench surfaces that don't brand your skin matter
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Avoid untreated pine or spruce for the interior — they look fine initially but resin migration in high-heat sessions is unpleasant and they darken unevenly over time
Ceiling — We like to slope it.
A vaulted or mono-pitched ceiling — rising from the door end to the back — creates natural heat stratification that actually works for you. The hottest air pools at the highest point (above the upper bench), giving you a temperature gradient you can use: sit higher for more heat, lower to cool slightly. Flat ceilings create hot spots and waste heat.
Bench configuration
For an outdoor sauna that will primarily be used in the context of hot-cold cycling, consider:
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L-shaped bench layout rather than parallel benches — allows one person to lie flat while others sit, and creates a more social configuration
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Upper bench height critical — the upper bench surface should be 90–110cm from the floor, positioned so a seated person's head is 20–30cm below the ceiling. Too close and the heat is overwhelming; too far and you're wasting the best heat
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Lower bench at 45cm — functional for sitting and as a step, but also low enough to be genuinely cooler for clients who want a less intense session
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Bench depth — minimum 60cm for sitting, 90cm minimum to lie comfortably. Many outdoor saunas have shallow benches that make lying down awkward
The Heater — The Heart of the Sauna — Wood-Fired vs Electric
For an outdoor sauna used in the context of hot-cold cycling and serious wellness use, the heater choice matters enormously. This is a genuine decision and both have strong arguments.
Wood-fired:
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The most authentic experience — the crackle of fire, the smell of burning wood, the particular quality of radiant heat from a wood-burning stove is different from and preferred by many to electric
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No electrical installation needed (relevant if your garden lacks power)
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High stone mass on good wood-fired stoves means excellent Löyly quality
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The ritual of building the fire is part of the experience for many users
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Takes longer to heat (60–90 minutes vs 30–45 for electric) — requires planning ahead
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Best options: Harvia M3, Narvi NC, HUUM Hive Wood — all proven in outdoor settings, all capable of producing genuinely excellent Löyly
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Requires a proper flue — stainless steel twin-wall insulated flue with correct clearances and a rain cap. Don't economise on the flue
Electric:
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Faster heat-up, WiFi control (pre-heat from your phone before you go outside), programmable schedules
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More consistent temperature control
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Requires a dedicated electrical circuit run to the garden — factor this cost in from the start
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HUUM HIVE or Harvia Virta for outdoor installations — both have large stone capacity for premium Löyly quality
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Saunum integration — if you want the best electric experience in an outdoor sauna, a Saunum climate control unit transforms the evenness of heat and Löyly quality markedly
Our recommendation for a premium outdoor wellness setup:
If you want the authentic experience and don't mind planning your sessions 90 minutes ahead, wood-fired is hard to beat. If you want spontaneous daily use — deciding at 9pm that you want a sauna and being in it by 9:45 — electric with WiFi controls wins.
Stone mass:
Whatever heater you choose, more stones means better Löyly. The steam produced by a heater with 40kg of stones is softer, more enveloping and lasts longer than the sharp, aggressive steam of a heater with 15kg. For outdoor use where the sauna will be opened frequently (people moving in and out to the plunge pool), high stone mass also means better temperature recovery after door openings.
Ventilation — Outdoors Doesn't Mean This Matters Less
A common misconception is that outdoor saunas ventilate themselves. They don't — the sauna room is still a sealed insulated box with people breathing in it. The same CO2 management principles apply.
Correct outdoor sauna ventilation:
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Fresh air inlet low on the wall beside or behind the heater — drawing in genuine outside air, not recirculated air from within the structure
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Exhaust outlet on the opposite wall at mid-height, ideally with a closable vent so you can dial the ventilation rate
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In a wood-fired sauna, the flue creates natural draft that assists ventilation — but this alone is not sufficient for multi-occupancy sessions
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Consider a small mechanical extract if the sauna will be used commercially or by larger groups regularly
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The Cold Plunge Pool
The Cold Plunge Pool
This is where outdoor installations most frequently disappoint, because buyers focus budget on the sauna and treat the plunge as an afterthought. A cold plunge pool done properly is a significant installation in its own right.
What Makes a Good Cold Plunge Pool
Size and depth The minimum viable cold plunge for genuine hot-cold therapy is large enough to fully immerse to shoulder depth when seated, and deep enough to submerge limbs. That typically means:
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Minimum internal dimensions: 1.2m × 0.8m × 1.0m deep (for a solo plunge)
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For two-person use: 1.6m × 0.9m × 1.0m deep
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Depth matters more than footprint — full body immersion is the goal, not paddling
Construction options
Stainless steel:
The premium choice for a permanent outdoor cold plunge. 316-grade stainless steel is fully corrosion-resistant, hygienic, easy to clean, and will last indefinitely. A properly fabricated stainless steel cold plunge looks exceptional, holds temperature well with insulation behind the panels, and integrates beautifully with high-end landscaping. It is the most expensive option but the only one we'd describe as genuinely lifetime quality.
Concrete / Gunite:
The pool construction approach applied to a small volume. Fully bespoke shape and finish, can be tiled to match surrounding landscaping. Requires proper waterproofing (the same principles as a swimming pool shell), drainage, and coping. Typically the most expensive approach but gives total design flexibility — flush with a deck, stepped entry, bespoke depth profile.
Fibreglass shells:
Pre-formed, faster to install than concrete. Quality varies enormously — the best fibreglass plunge pools are excellent, the worst delaminate and fade within 5 years. Look for thick gelcoat, full structural fibreglass layup, and UV-stable pigment.
Repurposed stock tanks / IBC containers:
The social media version. Fine for experimenting with cold exposure. Not a serious outdoor wellness installation.
Temperature control — the most important specification decision
A cold plunge pool without active chilling is dependent on ambient temperature and manual ice addition. In the UK, this means it's genuinely cold in winter (sometimes too cold — below 5°C is contraindicated for most users), warm and ineffective in summer (tap water at 18–20°C is not a cold plunge), and completely unpredictable in between.
An active chiller unit is not a luxury — it is what makes a cold plunge pool function as intended year-round.
What we supply and install has the following:
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Target temperature range: 8–15°C is the therapeutic sweet spot for most users. The ability to set and hold a specific temperature is what a chiller delivers.
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Chiller sizing: matched to the water volume and the expected ambient temperature range. Undersized chillers run constantly and fail prematurely.
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Heat pump chillers: the most energy-efficient option — the same technology as air conditioning, running in reverse. COP (coefficient of performance) of 3–5 means 1kW of electricity delivers 3–5kW of cooling.
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Integrated filtration: the chiller unit should integrate with the filtration system — cold water below 15°C inhibits bacterial growth significantly, but a stagnant cold plunge still needs filtration and UV or ozone sanitation.
Filtration and sanitation
Cold water inhibits many bacteria but does not eliminate the need for sanitation, particularly in a shared or commercial use scenario.
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UV sanitation — a UV lamp in the circulation loop kills bacteria and viruses without chemical taste or smell. The gold standard for cold plunge sanitation
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Ozone injection — complementary to UV, ozone breaks down organic matter in the water
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Minimal chlorine or bromine — if chemicals are used, very low doses are sufficient in cold water and should be monitored with a proper controller rather than manual dosing
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Turnover rate — the full water volume should be filtered at least 4 times every hour. Size your pump accordingly
Insulation
An fully sunken uninsulated stainless steel or fibreglass cold plunge pool in a UK garden gains temperature rapidly, particularly in summer when you actually want to use it most. Insulation behind the panels (spray foam or rigid board) dramatically reduces chiller running costs. A cover is not important as the heat gain is mainly through the walls, but a shade or openable canopy on top looks good and avoids direct sunshine when not needed.
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