208g (unisex)

Features: The Ultimate is a light and warm (winter weight) base layer with a mock neck and close-fitting cut. While most performance base layers use polyester or merino blends, the Ultimate is mostly woven from acrylic and nylon, with seven per cent merino and some polyester and polyurethane. The outer fabric has a smooth weave, while the inner face is soft and fleecy. ZeroFit says this fleece, which it calls ‘Heatrub Technology’, brushes against the skin ‘to generate heat instantly’, as well as trapping air next to the skin. 

Fit: This is a small fitting top. While I normally wear a size small, I was a medium in the Ultimate and could upsize to a large for a looser fit. The garment is cut long, so it tucks in well. 

Comfort: The body-hugging fit and mock neck make the Ultimate very cosy. I didn’t get any skin irritation under my pack straps as I do with merino base layers, but the fabric was slightly itchy when I got hot, and the fabric blend gives the garment a synthetic feel that reminded me of polypropylene. 

In use: The manufacturers claim the Heatrub Ultimate to be the world’s warmest base layer, recommended for use in temperatures ranging from -10–10℃. My first test was on a steep 250m track I regularly walk up. I also wore an ultralight rain shell and the air temperature was around 4℃. At that temperature and with a moderate-to-fast pace, I was uncomfortably hot when I got to the top, despite having removed the shell about halfway up the 15-minute climb. Although I was hot, my skin remained dry, so the fabric wicks well and breathes adequately. The mock neck is a benefit in cold wind, or when stationary, but is less comfortable for high paced activity for heat cannot be dumped through ventilation as it can with a half zip. 

Value: In the context of warmth for your dollar for its overall weight, the Ultimate is unbeatable (although you can buy a less warm, but more feature-rich garment for a similar price). 

Verdict: I found the Ultimate to be as warm as an average mid-layer, but with the advantage of less weight and bulk. It’s too warm for energetic pursuits such as rugged tramping or bikepacking in anything but the coldest conditions. There is another model called the Move which is designed for more active sports and would be more suitable to fast-paced activities. For downhill skiers, outdoor photographers, fishermen, or more static outdoor use, the Ultimate could be ideal.

Plusses: Very warm, cosy to wear, super light.

Minuses: Only comes in one style, no zip.