Damart thermal underwear
Damart thermal underwear is a set of fitted base layers in which a knitted wool or wool-blend fabric traps a still layer of air between your skin and the outside world, and the insulation you feel comes from the fibre itself, not from the thickness of the cloth. A single garment in the range holds a 2-layer construction, a 200 g/m² to 300 g/m² fabric weight, and a fibre content that puts merino or a merino blend at the insulating core, so the question of which layer actually warms you reduces to which fibre does the trapping.
Damart is a UK value retailer that has sold base layers through its own catalogue and store network since 1988, and its thermal underwear line sits between the bare-minimum synthetic base layers and the premium merino garments that cost 3 to 5 times as much. If you are comparing what is on the shelf against what you can order elsewhere, the honest starting point is to look at the fibre, not the label, and the main alternatives fall into a few clear families. Thermal underwear by retailer tells you how the fabric was knit and what weight it carries, but it does not tell you whether the wool content is 100% merino or a 60% merino blend cut with a polyamide shell. Amazon thermal underwear and Decathlon thermal underwear are the two most common benchmarks, because both let you read the exact fibre split on the product page, and a 50/50 merino-polyamide layer from either retailer will warm in roughly the same way as a Damart layer that lists the same blend. The one fibre that keeps surfacing in the conversation is Angora for thermal layers, which some heritage brands still use in a small percentage alongside merino to add a fluffy secondary fibre that raises the loft of the knit.
What the base layer does when you wear it
The base layer works by managing 3 things at once: keeping the skin dry, trapping warm air, and moving moisture away fast enough that you do not sit in a damp patch of fabric. Damart's thermal underwear range builds all 3 of those functions into the same garment, so the base layer is also the insulating layer, and the middle and outer layers you add on top never touch your skin. In a 2-layer outfit the base layer carries about 70% of the warmth budget, because the air it traps is the air that never moves, and the outer layer only blocks wind. The fabric weight matters here: a 200 g/m² shirt is the right call for a 5 °C day, and a 300 g/m² version is what you reach for when the temperature drops below 0 °C. The fibre blend decides how well the base layer holds that trapped air when it gets wet, because merino wool absorbs up to 30% of its own weight in moisture before the wearer feels damp, a figure a pure polyester layer hits at roughly 5%.
The fibres that do the insulating
Merino wool is the fibre that does most of the insulating work in the Damart range, and it earns that role from its diameter, which in the fine grades used for base layers sits between 18 and 24 microns, fine enough to feel soft against skin and small enough to pack a high number of fibres into a square centimetre of fabric. Each fibre is a crimped tube, and the crimps stack on top of each other to create thousands of tiny air pockets per square inch, which is the actual insulation. Angora adds a different kind of loft: the fibre is smoother and finer than merino, around 10 to 14 microns, and when a small percentage of it is blended in, it puffs the knit up without adding weight, which is why some heritage blends carry 5% to 10% angora alongside 85% to 95% merino. Polyamide, the nylon-like fibre in most blends, is not there to insulate; it is there to give the fabric 10% to 20% of its shape retention and to speed up drying by wicking moisture to the surface.
| Fibre | Diameter | Role in the layer | Typical share in blend |
|---|---|---|---|
| Merino wool | 18 to 24 microns | Primary insulator, traps air, absorbs moisture | 50% to 100% |
| Angora | 10 to 14 microns | Secondary loft, adds fluff per gram | 0% to 10% |
| Polyamide | 15 to 20 microns | Shape retention, wicking, abrasion resistance | 0% to 50% |
The blend ratio is the number to read on the label. A 100% merino base layer will feel warmer and drier against the skin than a 50/50 blend at the same fabric weight, but it will take 2 to 3 times longer to dry after a long wet session, and the polyamide in the blend is what keeps the garment from stretching out at the cuffs and hem after a season of wear.
How the knit structure changes the warmth
The same fibre in two different knits will warm you differently, because the stitch pattern sets how much air sits inside the fabric loop by loop. Damart's thermal underwear range uses a 2-layer knit on its midweight pieces, where the inner face is a smooth, flat knit that lays close to the skin and the outer face is a raised loop that catches more still air. A 3-layer knit, which appears on the heavier 300 g/m² items, adds a middle purl layer that breaks the direct path between the two faces, and that single structural change adds an estimated 15% to the thermal resistance of the same fibre at the same weight. The gauge of the knit matters too: a 12-gauge merino shirt packs more yarn per square inch than a 7-gauge version, so it feels denser and blocks a little more convective loss, but the heavier 7-gauge knit holds more air in each loop and wins on pure still-air trapping when the wind is calm. Reading the gauge number on the swing tag, if the retailer lists it, tells you more about the hand feel of the layer than the fibre percentage alone.
Which layer wins in which cold
The right base layer depends on the temperature band you are working in and on how hard the activity makes you sweat. Between 5 °C and 15 °C, a 200 g/m² merino base layer is the correct choice, because the activity heat you generate is enough to fill the air pockets and you only need the fibre to keep that air in place. Below 0 °C, the 300 g/m² 3-layer knit takes over, because the still-air layer is thick enough to slow the heat gradient from your skin to the outside air. For a stationary task in 10 °C cold, a 50/50 merino-polyamide blend is the better pick, because the polyamide wicks the sweat you are not burning off fast enough to keep the merino dry, and a 100% merino layer would hold that moisture against your skin for 20 to 30 minutes before it started to wick on its own. The angora-blended pieces sit in the middle of that range: the extra loft from the angora gives them a warmth-to-weight ratio that is close to the heavier all-merino knit, but at a fabric weight that is closer to 250 g/m², so they are the layer to reach for on a 0 °C day when you do not want the full 300 g/m² bulk.
How to judge the fabric before you buy it
To judge the fabric, hold the layer up to a light and count the visible loops per square inch, then pinch a fistful and feel whether the fibre springs back or stays compressed. A merino base layer that has been washed 10 times and still springs back to its original loft is holding its crimp structure, which is the structure that traps the air. A polyamide-heavy blend will spring back faster because the nylon fibre is more elastic, but it will not hold the same depth of loft after compression, so the warmth you feel in the shop is not the warmth you feel on a cold morning after the garment has been in a rucksack all day. Check the care label for the wash temperature: merino takes 30 °C on a wool cycle, and a 40 °C wash will felt the fibre and collapse the crimp permanently. The blend percentage on the label is the number to compare across retailers, because a 60% merino layer from any brand will insulate in roughly the same way as a 60% merino layer from another, and the 20% to 40% polyamide or angora that fills the rest of the blend is what differentiates the hand feel and the drying time, not the core warmth.