The purpose of the insulation layer is to keep you warm. (I have some notes about layering clothing). There are a number of factors which affect 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 (ResearchGate PDF / Google Site). 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 Temp	Sleeping	Light Work	Heavy Work
+40F	1.5″	0.8″	0.2″
+20F	2.0″	1.0″	0.27″
0F	2.5″	1.3″	0.35″
-20F	3.0″	1.6″	0.40″
-40F	3.5″	1.9″	0.48″

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 cool (even slightly chilled) while active. So long as you have adequate food are rested you activity will warm you up. When engaged in high energy activities I will wear clothing that I initially feel cold. After around 10 minutes I am cool but ok. Around an hour into the activity I am comfortable to a bit warm at which point I ventilate a bit. I add insulation when activity level drops. Doing this avoids sweating. Sweat makes it much harder to keep warm long term. It is often useful to have multiple insulation layers which can be varied to match the conditions and activity levels.

Inches of loft is a decent approximation of warmth, but there are other factors that govern how warm a garment feels. 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.

My Choice

I live in the San Francisco Bay Area which has particularly mild weather. I have found that a hoody made from 90gsm Polartec Alpha Direct (Macpac Nitro Hoody) is the perfect insulation for our conditions. It offers very little insulation when uncovered (especially when the wind is blowing or even air movement from walking) but can be surprisingly warm when worn with a wind blocking shell, making it an excellent active insulation for stop and start activities when combined with a shell. Merino tee + Nitro Hoody + Shell, hoods on, kept me warm to around 40F when walking at a slow pace on level ground in the shade, and comfortable in warmer weather by venting (or removing) the shell. I have a Montbell Plasma 1000 Vest (Cumulus Minilite much better value) which extends my comfort range to cover all the conditions I regularly encounter. I also have a Patagonia Micro Puff Hoody (iclo ~1.3, slightly less than Montbell Plasma Jacket) which extends my comfort by roughly 25F that gets used in colder conditions such as winters in the Sierras or I am going to be facing cool & continuously wet conditions.

Down

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.  Down is more expensive up-front, but cost competitive over the long run. The biggest issue with down is that moisture significantly impacts insulation performance.

The introduction of hydrophobic treatments such as DriDown, DownTek, Nikwax Hydrophobic Down in theory addresses the problem of moisture. 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. For example, an experiment done by BackpackingLight.com folks found that even when you soak a 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. Of course, that hour would be miserable and you would need to be a condition to generate enough heat to avoid hypothermia. 

Besides external moisture, one needs to manage internal generated moisture: sweat and perspiration. As a result, down is typically not ideal when engaged in high energy activities because moisture tends to accumulate in the down, and the shell which contains the down tends to have low air permeability. This suggestions that down is best suited to  static “activities”, such as sitting or standing.

I think concerns about moisture are typically over-blown. It’s only an issue if you are caught in a rain storm without real rain gear, if you are doing heavy work which has you sweating for an extended period of time (hours), or are in sub-zero F conditions for days where your perspiration can freeze on it’s way through your insulation. In the last 30 years I haven’t had my down clothing significantly impacted by moisture. Of course, I have spent most of that time the the relatively dry “Western United States” and when there was a high risk of moisture (like climbing in bad weather) I did opted for a synthetic belay jacket.

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)  followed by the 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. If you don’t want to special order, check out Cumulus Equipment. 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 ultralight down garments.

For a good list check out ultralight dandy’s down jacket guide. 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), but it’s still quite useful and arguably the best data that can be easily found. There is some good information on theultralighthiker everything you ever needed to know about down jackets. The lists maintained by Adventure Alan, greenbelly, and switchback travel seem pretty good summaries of decent options.

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 is less of a worry 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 but now somewhat dated Market Summary of Synthetic Insulated Jackets (2018). The other reason some people will choice synthetic insulation is because they are vegans or are concerned that the ducks or geese are not treated humanely. There are a few companies that use down which is harvested from nests or other means which avoid plucking the animals. Many companies are using down from animals that were killed for their meat… the down would have been wasted if not collected. Today, I think the Patagonia Micro Puff Hoody is the best option but it’s pricy.

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 down can handle which is why high loft synthetics are ideal.

Fleece

Fleece (typically 100, 200 or 300 weight) have been the most commonly used insulation layer for the last forty years.  Reasons to use fleece include: price, 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 in my opinion.   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.

“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 Direct combined with a separate highly breathable shell is excellent as an active insulation layer. I have been using a Macpac Nitro Hoody since 2021. It’s warmer and lighter than Polartec PowerGrid found in heavy bases like Patagonia Thermal Weight Capiline, about the same warmth as a good 100wt fleece, though it’s more fragile. A big advantage of Alpha over a traditional fleece is how air permeable it is and how little water it absorbs / how quickly it can dry. Very light insulate without a shell, surprisingly warm under a shell. I can feel air flow at walking speed in still air. I am comfortable sitting in 68F wearing the Nitro and a tee-shirt, but when I pull up the hood and layer on a shell I am comfortable down to 40F doing light work (walking <2mph).  I expect when combined with an EPIC shell it would provide excellent cold weather rain protection when active. There are a number of pullover hoodies made my cottage gear companies such as timmermade, superiorfleece, senchi designs, and farpointe. There are a few jackets with zippers, typically using heavier versions of Alpha Direct including the Rab’s Alpha Flash, OMM Core, and the Ursus-UL Jacket. A materials not quite a performance at Alpha Direct, but more durable and often cheaper is Teijin Octa (called AirMesh when used by Mountain Hardware).

Active Insulation jackets that have had good reports include the ArcTeryx Proton and the Montbell Thermawrap which uses light synthetic insulated combined with a DWR 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 for 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 (first created in 1931) 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 pad and boot inserts with aerogel were disappointing offering on advantages over more conventional materials at a higher price. The latest version of this mimics polar bear fur. Time will tell if this formulation can make it out of the lab into products that actually work.

Several companies are reportedly making “graphene” jackets with claims of amazing performance. I know graphene has some amazing properties, but I believe these products are more hype than substance. The only real user review I found was not exactly positive.

Additional Factors

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.

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 blood which means less heat lose).

Strategies for Clothing and Sleeping when Backpacking

As the table at the top of this post 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.

Technical Details

For even more information, check out Richard Nisley’s posts such as best clothing 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. I find that Richard’s graph roughly matches my subjective experience, though I need less insulation when active. Richard also has a very useful New Paradigm for understanding garment warmth and his even more detailed Revised Paradigm. which will provides a sense of the relative insulating ability of garments. The open textbook Body Physics provides the mathematical models for how our bodies and the environment interact. For more information about how to measure the performance of materials see Thermal Performance Measurements of Synthetic Insulations by Stephen Seeber and for additional values see 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.  Costco sometimes carries low cost down or synthetic garments, and there are a number of reasonably inexpensive jackets from Decathlon. Uniqlo down garments are fairly inexpensive and sometimes go on sale.  L.L. Bean, Eddie Bauer, or Lands End sometimes have specials which make their puffy jackets and vest very reasonably priced.