Insulation Layer

The purpose of the insulation layer is to keep you warm. There are a number of factors which effect how much insulation a person will need to be comfortable for a given set of conditions.  The most significant is the level of activity, or what researchers call Metabolic Equivalent (MET). Below is a table from Hal Weiss’s Secrets of Warmth which summarized the results of  US Army study to determined recommended insulation (expressing in terms of inches of loft) for various effective temp (which include wind chill):

Effective TempSleepingLight WorkHeavy Work

Inches of loft is a decent approximation, but there are other factors that govern how warm clothing is. For example, there have been a number of tests which indicate that down garments that are overfilled provide more insulation that standard fill and that often synthetics lose more loft than insulating ability as they age. There are also other elements that can be incorporated that boost warmth without changing the garment thickness such as vapor barriers and reflective barriers like Columbia’s Omni-Heat.

As the table indicates, the amount of insulation you need is inversely proportional to your activity level. One of the most common mistakes is using too much insulation during heavy work that results in sweat and then feeling chilled. I think it’s good to feel slightly cool while active and then add insulation when activity level drops. It is often useful to have multiple insulation layers which can be varied to match the conditions and activity levels.

It’s also worth noting that different parts of the body might need different levels of insulation. For example, when I am doing just moderate activity my legs are comfortable with 1/2 the insulations I need for my torso. Several manufacturers are starting to use body mapping to create garments which vary insulation and breathability.

People base metabolism level varies which is often reflected in people talking about running “hot” (me) or “cold” (my wife). The amount of insulate listed above should be considered “average”. You might find that you need more or less. For example, I found that I need and 1/2 the listed insulation doing “heavy work”, 2/3 the insulation listed for “light work”, and I need the listed amount for sleeping. If you haven’t had adequate food and water, you are short on sleep, or if you are physically exhausted, you will need additional insulation because your metabolic systems will not work efficiently. Activity levels can shift your metabolic rate, even after the activity has ended. For example, I have found that after backpacking for 15 miles with a lot of elevation changes that I need less insulation in camp that evening, even though I am no longer hiking.

As the table above indicates, it’s important to consider both activity level and target temperature when selecting clothing. There are three common approaches used by people when engaged in multi-day activities such as backpacking :

  1. Bring clothing that are warm enough to be comfortable doing light work at anytime of the day or night, and a sleeping bag/quilt that is warm enough for the expected night time conditions. This allows someone to do camp chores at their leisure, be comfortable most nights using just their sleeping bag, and allows their clothing to supplement their sleeping bag if the night turns out to be surprisingly cold. Typically insulation is at least two pieces so the insulation isn’t too much during the day when active,
  2. Go as light on clothing as possible. Typically this mean bring a clothing system that is slightly warmer than what’s needed heavy work. This is an approach used by some hard core ultra lighters and thru-hikers. The theory goes that they are hiking all day with minimal stops. When they stop the metabolism is still running hot so little insulation is needed. Eat dinner early before it has cooled too much, and then hike at a moderate pace (avoid sweating) until you are done. Stop and immediate get into the sleep systems. The sleep system (bag or quilt) will be warmer / weight than clothing. The early morning cold is overcome by immediately hiking (hopefully up hill… think about this when selecting a stopping location) and eating breakfast later. The sleeping system can be worn like a cape if warmth is needed during the day.
  3. Have your clothing provide the majority (if not all) of the warmth, even when sleeping. The theory goes that insulation in the form of clothing is more versatile than just a sleeping bag or quilt because you can make use of it anytime. Often an elephant foot sleeping bag is used. In very cold conditions where activity level is low this can work. Generally though I don’t recommend it.  A sleeping bag / quilt will provide more warmer / weight than clothing, and clothing that is warm enough to keep you comfortable when sleeping will be way too warm when you are active.

My Choice

Often I use a Uniqlo Down Vest which provides enough warmth for most of my activities. I like the freedom of movement a vest provides.  When just sitting around I found the combination the vest, a heavy-weight base layer, hiking pants, rain jacket or windshirt, neck gaiter, and down baklava keeps me comfortable down to 20F though my arms are get slightly chilled. [Note: I seem to run warmer than many people so this might not provide enough warmth for you.] When expecting particularly cold or unpredictable conditions I use a Western Mountaineering Flash Hoody, though it’s almost always too warm. Someday I will replace it was something, maybe a Patagonia Micro Puff.  When the conditions are extreme mild I use a Patagonia Thermal Weight base layer + wind shirt. I also have a Patagonia Down Sweater that I use around town because it has my company’s logo.


Down is still the premier insulation material. Down offers more warmth / weight than nearly any other material. It’s also amazingly compressible for easy packing. When properly cares for, down can last decades.  There are just two downsides with down. The first is when down gets wet it quickly loses it’s insulating ability. The introduction of hydrophobic treatments such as DriDownDownTekNikwax Hydrophobic Down in theory addresses this problem. These treatments are amazing in the lab, but there isn’t consensus as to whether these treatments are superior to high quality down in the field. The second is that down is not particularly good insulation when the wearer is engaged in high energy activities because moisture tends to accumulate in the down, and the shell which contains the down has low air permeability. This suggestions that down is best suited to  static “activities”, such as sitting, standing, or engage in light work.

Montbell is my recommended supplier of down garments due to their wide range of features and fill amounts at competitive prices. The Montbell Inner Down Jacket (now called the Superior Down)  and the upgraded models Ex Light and Plasma ignited the market for ultralight down garments. If you want something perfect for you, check out Goosefeet Gear because they will customize a garment to your exact specifications using the finest materials. For an easy to find, off the rack options, I often recommend the Mountain Hardware Ghost Whisper. For people on a tight budget I recommend Uniqlo (600 fill).  For lists of high quality down jackets which get updated regularly I would recommend the lists maintained by Adventure Alan. greenbelly, switchback travel.

An experiment done by folks, it seem that even when you soak well designed ultra-light down garments with highly breathable shells such as the Western Mountaineering Flash Vest, that in less than an hour the Flash vest will have more loft than any synthetic vest which weights the same as the Flash when dry. For folks who are considering a WM Flash Vest, keep in mind that you will lose a fair amount of heat from the V-neck and the lack of a collar.  You should consider using a neck gaiter to protect what the Flash leaves unprotected. 

For an insulation layer above to freezing I encourage people to consider a vest rather than jacket. Vests  tends to provide more warmth / weight, leaves your arms unencumbered for better mobility, and can harness your body’s natural vascular constriction for preserving warmth (as your arms chill your body sends less body which means less heat lose).

These days there is a large number of down jackets and I am not actively testing or following the market closely. One list of ultralight dandy’s down jacket guide is reported to be useful. I do wish he would use real measurements of iclo rather than estimating “warmth” from his equation (I believe montbell would end up scoring higher if he did).

Synthetic High Loft Insulation

In most cases I would recommend using down insulation over synthetic high loft garments. High loft synthetic insulation used to have two advantages over down which are largely gone: the cost gap has been shrunk due to improve market efficiencies for down and water resistance thanks to hydrophilic treatments for down. There are two applications that synthetic high loft insulation I believe still has a place. The first is when you are regularly transitioning between high and low activity levels in wet conditions. The second is when engaged in aerobic activities (aka heavy work) in conditions which are too cold for a base layer with a shell. BackpackingLight wrote a good Market Summary of Synthetic Insulated Jackets (2018).

The Belay Garments for Activity Transitions

Andy Kilpatrick’s The Belay Jacket is a nice explanation of an idea popularized by Mark Twight.  Simply put, you have a warm garment than is layered over all the clothing you are wearing while active to warm up during periods of lower activity level or when you have become too cold to function effectively. By putting the garment on over the rest of your clothing means you don’t lose heat by removing your shell to add additional insulation. You also create an environment which can allow the inner garments to dry out from your body heat.

These sorts of garments are typically made from materials like Primaloft, Climashield Apex, or Polarguard. See comments on various insulation material. Belay clothing are coming on and off in the middle of an activity, so it likely to be exposed to more moisture than even hydrophobic down can handle which is why high loft synthetics are ideal.

Active Insulation

In the last decade a number of garments have been released which make use of high loft synthetic insulation which have been designed for high energy activities. These garments are typically worn rather than fleece providing similar warmth and breathability at a lower weight, better mobility, and is more compressible. I have only modest experience with these.

Polartec Alpha combined with an air permeable shell is excellent as an active insulation layer. I don’t have extensive experience with Alpha, but several people I trust have used it extensively for the last few years and have very positive reports. I haven’t purchased something for myself because a heavy base + shell have been sufficiently warm for my cold weather, high energy activities.

Less breathable, but effective in colder conditions are the Patagonia Micro Puff using PlumaFill insulation and the ArcTeryx Atom LT which uses light synthetic insulated combined with PowerStretch side panel and underarm


Fleece (typically 100, 200 or 300 weight) have been the most commonly used insulation layer for the last thirty years.  Reasons to use fleece include: durability, breathability, and fleece dries very quickly. Finally, compressed fleece retains it’s warmth which is very useful in pants that you might use to sit on a cold surface or gloves when holding ice tools.  100wt and 200wt fleece were designed to be used as active insulation.

Polartec Thermal Pro High Loft (which first appeared in the Patagonia R2/R3) is the nicest fleece material in terms of comfort and warmth / weight.   I found that Thermal Pro which was the same weight as a 200wt fleece, was warmer than a traditional 300wt fleece.  There was a nice discussion about fleece as a staple for backpacking. I am very fond of combining a light fleece with a high loft vest.

WindPro is one of the few forms of fleece  that is somewhat wind resistant while still being adequately breathable.  I generally recommend against using WindStopper and Windbloc fleece because they don’t breath well and aren’t as warm for weight as other fleece when using a shell.

Other Insulation Options

Several companies have tried making products which would let you vary insulation by changing the amount of air in a garment. None of these products have lasted more than a year or two. This include Gore’s Airvantage the Aerovest designed to emergency insulation, and a jacket made by Klymit which uses compressed Argon rather than human blown air. I would be highly skeptical of any products like this. 

Another emerging solution is micro heaters (or coolers) being developed by companies like Aspen Systems primarily for military applications. I think there is a lot of development left before these sorts of systems would be good for backpacking trips.

There are materials like aerogel which often incredible insulation for a given weight at stratospheric pricing. I have seen several clothing items made using aerogel, but so far, none have had staying power in the marketplace.  Time will tell if this can be made sufficiently durable and affordable while delivering performance benefits in the real world. My experience with a sleeping bag and boot inserts were disappointing.

Technical Details

For even more information, check out  Richard’s best combinations for backpacking. I think it’s very useful to note that as the temperature drops, activity level significantly changes the absolute amount of insulation. The colder it gets, the more you want different insulation for when you are active, when you are are at rest, and when you sleep. Richard also has a very useful New Paradigm for understanding garment warmth. I think Richard’s graph is quite interesting and it matches my subjective experience in the field except that I have found the Thermawrap Jacket to be closer to the 200wt fleece. I also found the original micro puff to be a bit warmer, but the more recent models don’t seem to be as warm which would match up with Richard’s chart. But all of my observations are subjective vs Richard’s objective measures. I wonder if there are facts that Richard isn’t testing for, or if my subjective experiences were shaped by my expectations. For more information see the discussion thread about high loft materials CO values and engineeringtoolbox clo clothing page. The paper Application of Nanofiber Technology to Nonwoven Thermal Insulation has some interesting observations for people interested in various properties of a number of insulation materials.

Low Cost

You most likely already have a fleece.  If not, it is possible to buy a 200 weight fleece at discount stores for $10-20 which will be adequate for cool-moderate temperatures. You might also look at using the US Army’s M-65 Field clothing liners which can often be purchased for less than $20. For colder weather add a good wool sweater or second fleece.  Uniqlo  down garments are fairly inexpensive and sometimes go of sale. Costco sometimes carries low cost down garments, and  L.L. Bean, Eddie Bauer, or Lands End sometimes have specials on primaloft or Polarguard jackets.

Leave a Reply

Your email address will not be published. Required fields are marked *