A sauna hat is made by felting. Loose Merino wool is washed, combed into a thin web, layered, then worked with steam and pressure until the fibres lock into one dense sheet. That sheet is pressed over a hat-shaped form and dried, then trimmed. There's no weaving and no knitting. A well-made hat has no seams, as it's formed from one piece of felt.
This guide to how sauna hats are made sits under our guide to why merino wool is the gold standard for sauna hats. That guide covers the fibre. This guide follows the fibre from a fleece to a finished hat, and explains what each stage does.
Key Takeaways
- Wool felts because each fibre is covered in overlapping scales. Heat, moisture and pressure make those scales catch on their neighbours until the fibres lock together (The Woolmark Company, retrieved September 2026).
- Around 35 to 40% of a freshly shorn fleece is grease, suint, dirt and sand. Scouring removes it before the wool goes anywhere near a carding machine (The Woolmark Company, retrieved September 2026).
- A felt hat starts as a layered web of fibre that is hardened into a sheet and shrunk into dense felt. The felt is then blocked over a form and dried (The Hat Magazine Archive, retrieved September 2026).
- Water conducts heat more than twenty times as well as still air does, so a dense, dry felt insulates and soaked felt does not (Engineering ToolBox, retrieved September 2026).
why does wool felt in the first place?
Wool felts because of the scales on each fibre. The cuticle is the outer shell of the fibre. The Woolmark Company describes it as overlapping scales that sit in a stacked formation like roof shingles (The Woolmark Company, retrieved September 2026). Those scales all point the same way, from the root of the fibre towards its tip.
Add moisture and heat and the scales lift a little. Add pressure or friction and the fibres slide past one another, tip first, and catch. A fibre can travel in one direction along its neighbour, but it can't easily slide back. Every movement pulls the tangle tighter. Keep working it and the loose mass becomes a single, dense material that holds together on its own.
A wool jumper comes out of a hot wash a size smaller for the same reason. The machine supplies all three conditions at once. A felt maker sets up those conditions on purpose and controls them, so the shrinking happens evenly and stops at the density they want.
Merino felts into a finer material than coarser wool because the fibre itself is finer. Merino wool is generally less than 22 microns across (The Woolmark Company, retrieved September 2026). Finer fibres pack closer together, which gives a felt with smaller air pockets and a softer surface against the skin.
what happens to the fleece before it becomes felt?
The fleece is washed and combed before it can be felted. A shorn fleece is called greasy wool, and the name is accurate. It carries lanolin, suint, dirt, dust and sand, and those contaminants make up around 35 to 40% of the total fleece weight (The Woolmark Company, retrieved September 2026). Scouring is the wash that takes them out.
Clean wool then goes through carding. The fibres are fed into a carding machine, where wire-covered rollers separate them (The Woolmark Company, retrieved September 2026). The tangled fleece comes out the other side as a thin, even web with the fibres loosely aligned. In hat making, that web is wound onto a cone to build up the first fluffy mass of fibre (The Hat Magazine Archive, retrieved September 2026).
Australia grows 90% of the world's supply of the wool that goes into fine apparel. It comes from about 68 million sheep, looked after by 60,000 woolgrowers (The Woolmark Company, retrieved September 2026). The national clip for 2025/26 is forecast at 255.4 million kilograms of greasy wool (Australian Wool Innovation, Australian Wool Production Forecast Report, April 2026). The fleece in a FELT. hat is Australian Merino from that clip.
how does loose wool become a dense sheet of felt?
The carded web is layered, then hardened with steam and pressure. In felt hat manufacturing, the layered fibre goes through a felting process that uses steam and pressure to bind the fibres together. The felt is then rolled to consolidate it (The Hat Magazine Archive, retrieved September 2026). The trade has its own names for the stages. Heritage Crafts lists carding, forming, hardening, settling, bumping, planking, stiffening, blocking and finishing among them (Heritage Crafts, retrieved September 2026).
Hardening is the moment the web stops being loose fibre. Hot water or steam opens the scales, and pressure sets them catching on one another. At this stage the material's still soft and open, closer to a thick blanket than a hat.
Fulling turns the hardened sheet into felt. It's rolled and pressed, over and over, and each pass pulls the fibres closer together. The sheet shrinks in length and width and grows thicker and firmer as it goes. The felt maker keeps going until the sheet reaches the density and thickness they are after, then stops. For FELT. hats that finishing thickness is 4mm.
Density is the thing you are paying for in a felt hat, and it is set here, well before the hat has a shape. Two sheets can start from the same wool and end up as different felts depending on how long and how hard they were fulled. Our guide to how to choose a high-quality sauna hat covers how to feel that density for yourself.
how does a flat sheet of felt become a hat?
The felt is then blocked. A hood of felt is stretched to a standard size and shape and dried in an oven. The surface is then finished (The Hat Magazine Archive, retrieved September 2026). The stretching happens over a form, and the form gives the hat its silhouette. Heat and steam soften the felt enough to take the shape, and drying sets it there.
Each FELT. hat is formed from a single piece of 4mm Australian Merino wool felt. There are no panels and no seams. The Modern is pressed over a bucket form, 16cm tall and 26cm across the rim. The Classic is pressed over a taller form, 21cm tall and 27cm across the rim, for fuller coverage over the ears and neck.
Seams are generally a weak point on a hat that lives in a sauna. Heat and moisture work at stitching session after session, and a sewn hat tends to loosen at the seam while the felt around it is still sound. A hat pressed from one piece has nowhere to come apart. It also holds an even thickness right around the head, with no doubled-up seam allowances and no thin spots where panels meet.
Once the hat's dry it comes off the form and the maker trims the brim. Felt does not fray, because there is no weave to unravel, so the cut edge needs no binding or hem and is left as it comes off the form.
why does density impact how the hat performs?
Because the insulation is the air, and density controls how much still air the felt holds and how well it keeps it still. Still air conducts heat at about 0.026 W/m·K. Wool felt sits at about 0.07 W/m·K, and water at about 0.61 W/m·K (Engineering ToolBox, retrieved September 2026). That gap is why a dry felt insulates and a wet one doesn't.
Those numbers explain two things about a sauna hat. The first is why a dense felt works and a loose one doesn't. A loosely felted hat has big air pockets that the heat can move through, and it slumps and thins where it touches your head. A dense felt holds many small pockets of air that stay put, so the layer between your scalp and the hot air near the ceiling holds through a full session.
The second is why the hat has to stay dry inside. Merino takes sweat into the fibre itself and away from the surface, which keeps the air pockets open. That's one of the reasons we air a hat between sessions instead of washing it, and why it hangs by its loop at home. Our guide to the benefits of a sauna hat covers what that insulation does for you once you are in the heat.
4mm is where we settled for both our hats. Thinner felt loses the buffer partway through a longer session. Thicker felt insulates more but weighs more, holds more water and dries more slowly between sessions. The merino wool guide goes into the thickness trade-off in more detail.
the finishing touches
The last stage is the detail work. On a FELT. hat the logo is embroidered directly into the felt, in thread close to the colour of the wool. A small hanging loop is stitched at the brim so the hat can air between sessions. Each hat is checked, then packed with a drawstring bag to protect it in transit and on the way to the sauna. At home, it hangs by the loop.
Because the material's a natural fibre, no two hats come out identical. Merino wool felt can vary slightly in texture and colour from one hat to the next, and those variations are part of the material.
Our build decision: we design both hats in Sydney and have them made to our specification by a specialist wool-felt maker, from 100% Australian Merino wool. Each one is formed from a single sheet of 4mm felt with no seams, then finished with an embroidered logo and a hanging loop. Both shapes, The Modern and The Classic, are made the same way.
frequently asked questions about how sauna hats are made
how are sauna hats made?
Sauna hats are made by felting wool. Clean fleece is carded into a web, layered, then hardened with steam and pressure until the fibres lock together. The sheet is fulled to shrink and densify it, blocked over a hat-shaped form, dried, and trimmed. Around 35 to 40% of a raw fleece is grease and dirt removed by scouring before any of this starts.
are sauna hats sewn or pressed?
Both methods exist. Some hats are cut from flat felt and sewn from panels, which leaves seams. Others, including FELT. hats, are pressed from a single piece of felt over a form, so the finished hat has no seams at all. A pressed hat keeps an even thickness around the head and has no stitching for sauna heat and moisture to work loose.
what is a sauna hat made of?
A good sauna hat is made of wool felt, and the best of them from Merino wool felt. Merino fibre is generally under 22 microns across, which felts into a dense, soft material with small air pockets. Felt can also be made from synthetic fibre, so look for "100% Merino wool" on the label to know what the felt is made of.
can a felt sauna hat be reshaped?
Yes. Felt takes its shape from heat and moisture, and it will take it again. If a hat has been crushed in a bag, hold it over steam for a few seconds and ease it back into shape by hand. Then let it dry on its own. Avoid hot water and wringing, which would keep felting and shrinking the wool.
why is felt used for sauna hats instead of knitted wool?
Felt holds still air in a dense structure with no gaps, and it keeps its shape without stretching. A knit has open loops that let heat move through and a structure that sags as it warms and dampens. Still air conducts heat at about 0.026 W/m·K, and a dense felt keeps that air in place around your head for the length of a session.
references
- The Woolmark Company. What is the wool fibre? Retrieved September 2026. woolmark.com
- The Woolmark Company. What is Merino wool and how is it made? Retrieved September 2026. woolmark.com
- The Woolmark Company. Scouring (wool processing). Retrieved September 2026. woolmark.com
- The Woolmark Company. Wool processing. Retrieved September 2026. woolmark.com
- The Woolmark Company. Where does wool come from? Retrieved September 2026. woolmark.com
- Australian Wool Innovation. Australian Wool Production Forecast Report, April 2026. Retrieved September 2026. wool.com
- The Hat Magazine Archive. Felt manufacturing. Retrieved September 2026. thehatarchive.com
- Heritage Crafts. Hat making. Retrieved September 2026. heritagecrafts.org.uk
- Engineering ToolBox. Thermal conductivity of common materials and gases. Retrieved September 2026. engineeringtoolbox.com
