As it is, assessing the temperature rating for sleep systems is tricky. Everyone’s body is different and the level of insulation each of us needs to feel comfortable at a set temp is variable. However, with sleep systems, one constant we can rely on is that everyone is going to be at, or close to, their lowest metabolic rate. With Clothing layers, we are typically wearing them in variable states of activity, which brings along huge variations in what we need to stay comfortable. I think there are really three main categories of insulation purposes. Obviously there are many differences within each, but I think we could generally split into these categories.
Sleep Layers – As mentioned, these are worn lying down, at your lowest metabolic rate. We need much more insulation to keep us warm at this state than any of the others. Part of this is the extremely low metabolic state, but we also need a higher level of comfort to be able to rest well. These are sleep systems and clothing items used in sleep systems.
Static Layers – These are layers that are worn while relatively inactive, but still up and about. These are your puffy down or synthetic layers like down jackets, sweaters, pants, batting insulated garments like Climashield and Primaloft. We would wear these while we are at a low metabolic rate. Setting up / taking down camp, sitting around camp, stopping for a lunch break, or hanging out waiting for a hiking buddy. You would think that these metabolic rates would be in the same category as sleep, but they are usually not. While these are very low level activities, they do run your internal furnace just enough to bump the level of needed insulation down just a bit. It is generally not enough to have breathability or ventilation be of much concern though. This category gets much more variable than sleep layers because of the variability of metabolic rates while doing these different minor activities. It’s also worth noting that we usually define comfort differently during these things. The amount of warmth and comfort we desire for falling asleep is usually different than what we consider acceptable while sitting on a log eating. We are usually willing to run a bit colder while wearing static layers.
Active Layers – These are the layers you wear while active. The level of insulation needed goes down and the importance of breathability, moisture resistance, and venting goes up. This category is the most variable because of how much range of exertion there is. At maximum effort, many can get away with not really wearing any insulation in very low temps, while someone walking easy on flat ground might be comfortable in a waterproof-breathable rain jacket or static insulation layer in cool weather. Once the rate of exertion ramps up, like jogging or hiking uphill with a pack, then the rain jackets are quickly overwhelmed and the static insulation layers become way too much insulation for a body producing heat. The most important thing in this category is a layering system that can cover as much of that range as possible. That is why the Alpha Direct / Wind Shell combo is so successful. The modular system can go from very little insulation and extremely open breathability with just the Alpha, to good insulation and good wind proofing with the two combined.
The variability in the static and active layers leads to some confusion. One example is when the topic of temp rating for Alpha Direct comes up. That is so hard to even think about. I have been totally comfortable in an Alpha Direct Hoodie on a run in 70 degree heat. I have also been totally comfortable on a run in the same hoodie with a Hyper D wind shell over it at 20 degrees. Alpha is mostly dependent on the outer shell for it’s insulation. It isn’t necessarily the shell alone creating the insulation, but it is a combo of the shell’s insulation and the shell creating the pockets in the Alpha that are needed for it’s insulation. Alpha on its own has wide open spaces between the tufts of fiber. Without a shell, heat goes right out those open spaces and only the tufts between resist heat movement. If the wearer is moving or the air is moving, then it blows right through the open spaces, hits the skin, and travels under the tufts, almost totally negating any insulation. It has a minor amount of insulation when the wearer is still and there is no wind. Heat can escape out the open spaces, but without wind, the tufts do still provide a small amount of insulation. It is a very small amount of insulation, comparatively, and there are much, much better choices in the static insulation category for this type of use. When you place a shell over it, all the open spaces become insulating air pocket, along with the tufts. Wind can no longer blow through to rob that heat and the whole system creates some pretty substantial insulation. Being modular, the shell can be swapped out for different purposes…..high CFM wind shell for higher exertion or a rain shell to handle precipitation.
Another area where confusion arises is in how we classify the static insulation layers. Should the SUL and SDUL 1.5 sweaters be called “4 season”? That is likely going to be difficult to say, but we can get it narrowed down quite a bit. First, the popular comment is “How can it be warm in winter with only 3.8oz of fill?”. Unfortunately, there can’t even really be much discussion about warmth around this because total fill weight is a wildly inaccurate metric. These sweaters do not have many of the down filled features that other garments might have. The fill weight it has is going straight to creating warmth, not covering over a zipper with a draft tube or installing external insulated pockets. It’s also based on the collar version and doesn’t include the fill that would be in a hood. The fit is certainly different from other garments, which means the total area being filled with down is variable. It’s just a very unstable foundation to assess warmth from. Calculated loft, chamber calculations, or even measured loft are much more functional metrics to use as our foundation for assessing warmth. From there, it is really important to take a look at how that amount of fill is being used. design goes a long way towards how much warmth you get out of a given amount of down. Opinions that chime in with “3.8oz can’t be 4 season” are failing to understand the inaccuracy of total fill weight and failing to understand how design affects warmth. Their gauge of how warm 3.8oz total fill is, is likely based on narrow spacing chambers, not the wide, puffy chambers used in these sweaters…..and the 3.8oz figure isn’t necessarily comparing equal insulated area anyway.
From a user feedback perspective, the 1.5s comes back most often with confirmation of comfortable 4 season use. Personally, with an Alpha Direct 90gsm as a base layer, an SUL 1.5 is pretty comfy at 10 – 15f, assuming I am covered appropriately on my other body parts. I wouldn’t consider it enough for sleeping in without anything else, but for hanging out, setting or breaking down camp, doing some chores around camp, etc…..it is very warm at that temp. Overall, I tend to run a bit cold. I’m usually colder than males I’ve hiked with. I run a little warmer than most females I’ve hiked with. Everyone’s picture of 4 season is likely different, but I think those temps fit into most people’s picture of 4 season. No, I’m not going to use it as a sleep system at that temp. No, I’m not going to use it for 4 season use in Antarctica. No, I’m not going to call it a 4 season alpine climbing parka…..but for most 4 season backpacking purposes, it fits pretty firmly into the 4 season category.