The North Faces Thermoball is taken into the hills for testing by gear editor Chris Townsend

This unusual jacket is designed to marry core warmth with breathability and also to be stretchy so that it can fit closely. To achieve this, the jacket has Thermoball synthetic insulation on the front, shoulders, lower back and upper arms. The sides, centre of the back, underarms and lower arms are made from stretch fleece.

Thermoball is made for The North Face by Primaloft and consists of small clusters of synthetic fibres that mimic down and which are contained in small boxes of fabric. It’s closer to down than any other synthetic insulation I’ve tried and warm for the weight. It dries quickly and is water resistant too. The North Face says the warmth is equivalent to 600 fill-power down. The stretch fleece is The North Face’s own one and is similar to Polartec Powerstretch.

The Momentum jacket has an ‘active fit’, which means body-hugging. While the stretch panels do mean it moves with the body I’d still prefer a larger size. There are two lower pockets that are cut off by a pack hipbelt (the jacket can be stuffed into one of these), a snug collar and a hem drawcord.

The Momentum is very warm, especially when worn under a shell jacket, which it needs to be anytime there’s more than a gentle breeze as the fleece panels are not even slightly wind-resistant. This helps with the breathability of course – The North Face says it’s to “ensure optimal moisture wicking during aerobic activities” – but it does waste the windproof properties of Thermoball in my opinion. Also, for me it has to be very cold before I can wear it under a jacket without overheating despite the breathability.

In Britain’s windy weather I don’t think the fleece/synthetic insulation combination works that well. Thermoball is certainly excellent as a form of insulation but I much prefer the fully windproof Thermoball Hooded Jacket that I reviewed and recommended back in the October 2014 issue, as this can be worn as an outer jacket except in heavy rain. It weighs a bit more but costs the same and is far more versatile.

Reviewed April 2016