Working in the outdoor apparel market, it's not often that I get to see really exciting innovations in fabric technology. Sure there have been some huge leaps since most big brands started refining and designing outdoor gear back in the 70s (think Patagonia's Synchilla Fleece and the whole journey of waterproof, breathable membranes that have given us modern rainwear) but Polartec's Alpha Active Insulation is truly something to write home about. The fabric has been around for a few years now so forgive me if you're already very much on the bandwagon with this one but I'm always reluctant to review something until I've had the chance to test things like durability and performance in a range of conditions which can only really be done over lots of trips and an extended period of time.
So what is it? Polartec Alpha was a fabric initially designed for the special forces in the US to provide a more versatile insulation layer that could perform across a wider range of exertion levels without the wearer needing to constantly shed and/or add layers as they moved from low to high intensity activity and back again. This clever fabric achieves this by being incredibly quick drying; moving moisture away from your body and featuring a high degree of breathability. This means body heat is dispersed efficiently before you start to overheat. Even used alone in a garment without an added outer shell, the fibres effectively trap enough warmth to keep you comfortable at surprisingly low temperatures. Combined with a windproof outer fabric or layered with a shell, I'm yet to wear anything that keeps me as warm for such little weight.
This brings me to the next benefit of Alpha, It's super packable. When I started transitioning from hiking into fast packing and trail running, I found it hard to find a midlayer in my wardrobe that would give me adequate protection but be small and light enough to carry in a hydration vest or small day pack. I was often heading into alpine environments so I needed something with a really high warmth to weight ratio that would perform even when wet. I've found Alpha the best option for this. Paired with a lightweight rain shell and a basic thermal top, I've got a really bomber layering system without feeling like I'm carrying unnecessary weight.
Lastly, as I mentioned earlier, durability is always a key consideration for me with fabric choice. If it's lightweight and breathable but can't put up with a reasonable amount of wear and tear, I don't see it as a great investment. I'm not an elite athlete and I hate the idea of throw away gear so I have pretty high expectations for how long something should last and how much I can abuse it in that time. I've found that Alpha as an insulation has stood up really well which when you first look at and feel the fabric with all of its visible holes is a little hard to get your head around. It doesn't snag, rip or become misshapen after washing and unlike some other fleeces I've worn, it doesn't leave fluff or cause pilling on your base layers underneath.
In the last three years I've been wearing a Macpac Pisa Hooded Jacket as my go to mid layer for tramping and climbing. This is made of Polartec Alpha Insulation with a lightweight DWR treated nylon ripstop outer shell. As a general rule I find it perfect for moderate to high exertion winter activities (tramping, climbing and mountain biking) but actually too warm to trail run in outside of the coldest months in New Zealand. It's the most packable piece I own for the warmth and I only deviate from this now if I'm planning on very low exertion, cold weather trips where a high pile, bulkier fleece is worth taking for the extra warmth (eg camping in winter or if I plan to sleep outside with a more minimal sleeping bag).
The only issue I had with the Pisa Hooded Jacket was that the added shell still made the overall jacket too bulky to go in my trail running pack as I still needed a properly waterproof jacket anyway. Cue the Macpac Nitro Pullover. This is essentially as minimalist as you can go and comprises a single layer of Alpha Insulation in a hooded pullover style for a total garment weight of 140g (Men's Medium). You lose the windproof aspect of the more substantial Pisa but you gain a super packable, lightweight trail running option that paired with a water proof shell should keep you toasty in all sorts of conditions.
The only negative I've found with this fabric over other options is that it lacks any real stretch and as such you'll need a slightly more relaxed fit than with other active mid layers. I haven't found it at all uncomfortable personally but if you have wide shoulders I would take into account alleviating any potentially tightness across your back by buying a piece that fits comfortably through a complete range of motion. The Macpac Pisa for example is designed as part of the New Zealand Alpine Team range and as a result has good reach at the shoulders for climbing.
If you're looking for a midlayer that's going to provide you with more versatility than traditional fleece options but still offer the protection of solid wet and cold weather performance I think you should check out this fabric. I've linked the two Macpac versions we've been using above or check out some great examples from some of my other favourite brands below.