A Simple Outdoor Clothing System Layer 3: Midweight Insulating Jacket

img_0215A midweight insulating jacket is one of the most versatile pieces of clothing you can add to your outdoor gear closet.

This is the fourth in a series of articles about a simple outdoor clothing system. An overview of the system can be found here.

For more information check out the pages on baselayers and lightweight fleece insulation.

A midweight insulating jacket fills several needs in the outdoor closet. On trips in mild weather, this layer can be your warmest insulation. Paired with a wind shell it will keep you warm on cold summer nights or frosty mornings in late spring. In truly cold conditions you can layer this jacket within your clothing system, over your lightweight fleece insulation and under a winter soft shell. For most folks, a fuzzy fleece or light insulated jacket is also something that will get a lot of use in everyday wear around town.

Jackets in this category will be made either of polyester fleece or highly breathable high loft synthetic insulation. As you might imagine, their are advantages to each, but since we’re trying to keep it simple we’ll do our best to narrow down the choices. If you don’t want to come along for the techno-geek stuff you can skip to the bottom and read the Buy This, Not This section.

Two high-loft fleece jackets from Mountain Hardwear. The jacket on the left is made of Polartec High Loft fleece. The one on the right from Polartec Thermal Pro. Both are lighter, warmer and more compressible than traditional fleece jackets.

Polyester fleece has been the go-to insulation for outdoor recreation since Patagonia introduced Synchilla fleece in 1994. Synchilla, the original version of Polartec fleece bunting, dried quickly and provided better warmth for weight than the wool insulating layers that it replaced. The new fleece worked better than anything that had come before and soon became a staple of outdoor gear closets everywhere.

An older fleece jacket with a knitted face. This jacket will function in a layering system but is heavier and bulkier than other options.

Polartec’s original polyester fleece was further refined when Patagonia and Malden Mills worked together to develop new fleece fabrics that would become the core of Patagonia’s Regulator insulation system.

Regulator fleece was introduced in 1999 and featured both Polartec Power Grid and Polartec Thermal Pro fabrics. Power Grid (Patagonia R1) is a light weight wicking fleece that can double as a base layer. This fabric is discussed in detail in the post about light weight insulating layers. Thermal Pro (Patagonia R2) is a higher loft fleece that looks like shaggy fur. When they were first introduced, Patagonia had an exclusive on these fabrics. They are now widely available from a range of outdoor clothing companies.

Polartec has continued to innovate and now makes a fleece fabric called High Loft that is woven in a pattern that looks like rows of bricks or a shaggy caterpillar. This fleece takes the high loft concept behind Thermal Pro one step further by incorporating more open space in the pattern of the weave. This improves breathability, air permeability and compressability while reducing weight.

The benefits of the newer high loft Polartec fleece fabrics are numerous. These fabrics are warmer for their weight than conventional fleece because they have more insulating loft. They’re also more compressible and more air permeable than traditional fleece. These factors add up to more warmth, easier packing and a broader comfort range than you get with a conventional fleece jacket of the same weight.

The US Army issues a high loft fleece jacket as part of the ECWCS Gen III cold weather clothing system. As far as I can tell, the ECWCS Gen III Level 3 fleece jacket is loosely patterned on the Patagonia R2 fleece. It uses a high loft Polartec fleece for the main body and Polartec Power Grid fleece in the underarm areas. Thousands and thousands of these fleece jackets have been issued to soldiers as part of the standard Army combat uniform, so there are tons of them available on the second hand market for about 20 bucks.

Two other options in high loft jackets are the Patagonia R2 jacket and the Mountain Hardwear Monkey Man and Monkey Woman jackets. The Monkey Man currently uses brick grid Polartec High Loft in the main body with a stretch fleece similar to Power Stretch in the underarm and cuff area. The R2 Jacket is made from Polartec Thermal Pro in the body with Power Grid inserts. I prefer the performance of Power Grid to Power Stretch (see discussion here), but either jacket is a good option if you are looking for something a little more stylish than the Army surplus option.

Windproof Fleece
Avoid windproof fleece if you’re going for simplicity and versatility. Windproof fleece fabrics like Polartec Windbloc or Gore Windstopper incorporate a semi-permeable membrane within the fleece fabric. This membrane is a thinner and more breathable version of the membrane found in waterproof/breathable raincoats. Despite the increased breathability over waterproof membranes, windproof fleece still traps moisture and functions poorly as part of a layering system.

If you drop down the rabbit hole of technical semi-permeable fabrics you’ll find a lot of information about how breathable these fabrics are. Various tests exist to measure how readily water vapor will pass through a fabric, and some of the breathable membranes come in with very impressive numbers. Unfortunately, the lab tests don’t seem to mesh with subjective experience in the real world. In my experience, semi-permeable membranes are sweaty, regardless of the numbers.

There are likely two reasons for the discrepancy between lab measurement and subjective experience. The first has to do with dewpoint and condensation. The second with air permeability.

Our clothing system is a micro climate of temperature and humidity that shields us from the outer macro climate. Inside our clothing it is warm. Outside the temperature is typically cooler. The idea is for moisture to pass through our clothing from warmer to cooler and for our next-to-skin layer to stay dry and comfortable.

For any given temperature and humidity there is a dew point–the temperature at which water vapor will condense into liquid. In a fully breathable clothing system body temperature pushes this dew point away from our skin and toward the outside of the system. If your layers are air permeable, the outer environment will penetrate into your layers somewhat. This creates a shifting bubble of warmth within your layers across an relatively even temperature gradient.

Semi-permeable membranes can short circuit this effect. These membranes are fully windproof, so in a cold environment the inside of the membrane is sharply warmer than the outside. This abrupt temperature change can cause condensation inside the membrane. Once the liquid water condenses on the inside of the membrane it can’t pass through the fabric. At this point the fabric more or less stops being breathable.

The lack of air permeability in windproof fleece also inhibits drying of your layers through evaporation. In theory, completely windproof layers should help to keep you warmer by eliminating evaporative cooling, but in the real world it doesn’t seem to work out that way. A little bit of evaporation in your insulating layers can help speed drying and keep you feeling more comfortable.

Bottom line? A certain amount of air permeability appears to improve the performance of a cold weather layering system. Windproof fleece inhibits this air permeability and is out of place in a versatile layering system.

Two mid weight jackets made from breathable high loft synthetic insulation. The the jacket on the left is made from Primaloft Silver Active. The jacket on the right from Polartec Alpha.

Breathable High Loft Synthetic Insulation
Up until recently, synthetic high loft insulation was a poor choice for a layering system. The main reason was poor breathability. In the past, insulated jackets had to be made of tightly woven, downproof fabrics. These fabrics were often calendared (that is, melted) on the inner face to prevent the tiny fibers of synthetic insulation from migrating outward through the shell of the jacket. As you might imagine, a synthetic fabric that is melted and fused together doesn’t breathe very well and offers very poor air permeability.  Not a good choice for a layer in a versatile clothing system.

A few years back a new family of synthetic insulations came onto the market that offered improved breathablity. These, “active insulation” materials bind the insulation fibers together so they can’t wiggle out through the outer fabric of the garment. This allows for the use of much more breathable and air permeable fabrics. Fabrics in this insulation family include Polartec Alpha, Toray’s Full Range and Primaloft’s Silver Active, among others.

Polartec Alpha is more of an expanded woven fleece fiber than a traditional synthetic insulation. Other materials use insulating fibers that are bonded together with proprietary processes. The end result is the same–puffy insulation that can be built into garments that are much more breathable than previously possible.

Think of this category of insulation as somewhere between fleece and traditional synthetic puffy jackets. Breathable synthetic insulation offers the compressability of traditional puffy garments along breathability and air permeability that approaches that of fleece. They are extremely versatile, offer a broad comfort range and work well in a layering system.

Buy This, Not This
We’re trying to keep this simple, and the simple answer is to buy an Army surplus Level 3 fleece jacket. You can get one of these for about 20 bucks and it’s made of Polartec Thermal Pro and Power Grid–top notch stuff. The surplus fleece will get the job done for the least amount of money. If you’re on a budget this is the way to go.

If you’re looking for a commercial version that looks a bit sharper and hasn’t been halfway worn out by heavy use, take a look at the Patagonia R2 jacket, Mountain Hardwear Monkey Man or similar fleece jackets from other manufacturers. Look for Polartec Thermal Pro or High Loft fleece in the body and Power Grid or Power Stretch in accent panels.

To my mind the fleece jackets are more versatile than the synthetic puffy jackets. They have a broader comfort range because of their higher air permeability and I spend a lot more time wearing fleece than I do my Alpha Jacket. That said, if you’re in the market for a synthetic puffy jacket, consider a jacket made with one of the highly breathable active synthetic insulations like Polartec Alpha, Primaloft Silver Active or Toray Full Range. These will function as midweight insulation within a layering system and can be worn around town as a cool weather jacket.

Don’t buy a windproof fleece jacket or vest, they don’t breathe well enough to use within a layering system. If you’re buying a new fleece jacket, don’t buy a heavier, bulkier fleece aimed at casual wear. Finally, don’t buy a synthetic insulated jacket made with traditional, less breathable fabrics. These work fine as outerwear for sedentary activities, but they don’t breathe well enough to layer under a shell.

After a couple seasons of using both breathable high-loft active insulation pieces and high-loft Polartec fleece I’ve come to the conclusion that the active insulation pieces perform better within a winter layering system than their fleece alternatives. The reason is that active insulation jackets allow you to dispense with a windbreaker within your clothing system in deep cold conditions. In deep cold and windy conditions, fleece is best layered with your windbreaker top under your softshell to improve wind resistance and heat retention. Shelled active insulation pieces dispense with the the need for this additional layer within the system. They also slide more easily beneath outer shell layers and help the entire system of clothing to fit better and more comfortably. I still prefer high loft fleece as the most versatile option across all seasons, but if you have the option of owning both, you’ll likely find the active insulation jacket functions better within your winter clothing system.

The PCU Protective Clothing Uniform: A Buyer’s Guide and Clothing System History
Polartec Products Page
Primaloft Silver Active

Outside Magazine–How Does Breathable Midlayer Actually Breathe?

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6 thoughts on “A Simple Outdoor Clothing System Layer 3: Midweight Insulating Jacket

  1. Yaron Raz

    Hey Brian

    Firstly, thank you for these amazing articles. I’m reading and slowly finally understanding this complicated outdoor clothing. It’s eye opening. Can’t wait to re organize my closet! I’m sure I’ll read it all at least one more time.

    I do have a question about the mid weight insulation layer, why not using here a light down jacket? I know the downside when it’s getting wet, but for winter ski touring I’m not expecting any rain anyway. It is less durable, but i always use it with a shell layer on top.

    Again thank you and looking forward to learn more from you

    Kind regards from Switzerland

    1. Hi Yaron,

      Thanks for the kind words. That series was fun to write and I learned a lot along the way.

      The main reason you wouldn’t want to layer with a light down jacket as your midweight insulation is vapor transfer. Down jackets are made with downproof fabrics. These fabrics are calendared–put through heated rollers that fuse the fibers of the fabric together. They are not breathable.

      What you’ll find if you’re wearing a down jacket is that you’ll sweat up the jacket and that sweat will be trapped inside your clothing system. It will also start to work on the down and flatten out the jacket over time, so the jacket will loose loft and insulating value over a multi-day trip.

      It might seem like a lightweight synthetic insulated jacket could work here in place of down. Problem is that conventional synthetic puffys also use downproof fabrics. So you’re right back in the same boat (although the synthetics won’t flatten out from your perspiration).

      The only real options for the midweight layer in a fully breathable/air permeable system are fleece or active synthetic insulation like Polartec Alpha or similar products from Patagonia or Primaloft as outlined in the article.

      Hope this helps! Thanks for reading and commenting!


  2. Krishna

    Hello Brian,

    Thanks for this wonderful series of articles on layering. I never heard of the PCU system before and now after having read your articles and then studying the PCU Visual User Guide, I have developed a good understanding of the layering concepts. I very much appreciate the effort you put into this series.

    I have a few questions about the midweight insulating jacket in particular –

    1. Some of the Polartec Thermal Pro jackets I’ve come across from other brands have a knit sweater like face. One example is the Filson Ridgeway jacket. The Polartec Thermal Pro jackets that you mentioned such as the Mountain Hardwear Monkey Man and Patagonia R2, have a shearling like face. Would the jacket with the sweater like face be any less effective than the one with the shearling like face?

    2. Is the active insulation jacket equivalent to the softshell + lightweight fleece combination from a purely weather resistance point of view? Similarly how would it compare to a softshell + high-loft fleece combination?


    1. Hi Krishna,

      Thanks for joining the conversation.

      My thought on the hard face fleeces is that they’ll likely be a little less air permeable than the high loft open knit options. They’ll likely be heavier and less compressible as well.

      Regarding active insulation pieces, I see these as about equivalent to midweight fleece plus a windbeaker. Layered under a soft shell they will be warmer than the other to fleece options you mention if the wind is blowing.

      1. Krishna

        Thanks for the elaboration on the active insulation layer. I’ve been reading up on the newer Patagonia MARS which seems to be the next generation of the PCU system. It looks like the high-loft fleece has been replaced with an active insulation layer, the Patagonia Nano-Air.


      2. Sounds right. The previous version of this jacket was made with Polartech Alpha and was called the Level 3 A. It’s highly sought after “snivel gear”. I bought a version off the Patagonia site several years ago and its what I use in my layering system now.

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