Therma Boost Explained
In brief: Therma Boost is a thermal fabric technology from Sweaty Betty, used in its winter running and training leggings. It pairs a stretchy, supportive recycled fabric with a soft brushed inner surface that holds warmth close to the body, so you stay warm in cold conditions without heavy layers. It sits within Sweaty Betty's cold-weather HeatProtect line and is built to move with you while keeping the chill out on winter runs.
What is Therma Boost?
Therma Boost is a thermal fabric used by Sweaty Betty in its cold-weather activewear, most notably its winter running leggings. The idea is simple: give runners and gym-goers a layer that keeps them warm in cold conditions while still moving freely and managing sweat. Rather than adding bulk, Therma Boost uses a soft brushed inner face to trap a layer of warm air against the skin, combined with a supportive, stretchy outer fabric. It forms part of Sweaty Betty's broader cold-weather offering, which the brand groups under its HeatProtect range, with Therma Boost positioned as a warm, everyday winter training fabric.
How the brushed thermal fabric works
The warmth in Therma Boost comes mainly from the brushed inner surface. Brushing raises a soft, fine pile on the inside of the fabric, which traps still air close to the skin and slows the loss of body heat, much as a light fleece does, but in a thin, stretchy form that suits running tights. Because the fabric is engineered for activity, it also stretches in multiple directions to follow the body and offers the supportive, sculpting feel Sweaty Betty leggings are known for. The aim is warmth without restriction: enough insulation to take the edge off a cold morning, without the bulk or stiffness that would get in the way of movement.
What Therma Boost is made from
Therma Boost leggings are made from a stretch fabric built around recycled polyester with elastane for stretch and recovery. Using recycled fibre reduces reliance on virgin materials, in line with Sweaty Betty's wider move towards more sustainable fabrics. The elastane content gives the leggings their four-way stretch and that supportive, hold-you-in feel, while the recycled polyester body carries the brushed thermal inner face. The exact composition can vary between specific styles and updates, so it is always worth checking the individual product page for the precise fabric breakdown of the leggings you are looking at.
Therma Boost for winter running
Therma Boost is aimed squarely at cold-weather training. For winter runners, the appeal is a single warm layer that handles a frosty start without the need to pile on multiple pairs of tights. The brushed inner keeps the legs warm as you set off, while the breathable stretch fabric and sweat-wicking properties help manage moisture once you have warmed up. It is best thought of as a cold-weather rather than an extreme-cold fabric: ideal for British winter runs, brisk morning sessions and chilly outdoor workouts, where you want reliable warmth and full freedom of movement rather than heavyweight, expedition-level insulation.
Therma Boost at OD's Designer Clothing
At OD's Designer Clothing we stock Sweaty Betty activewear, including cold-weather styles built for winter training. Therma Boost is a strong choice for anyone who wants to keep running and training through the colder months in a warm, supportive legging that still moves freely. We offer next-day delivery and free click and collect, and customers in the North West are welcome to visit our St Helens store to feel the fabric and check the fit in person.