Introduction
I’m making a fast ascent on a cold day. I feel the growing stickiness on my skin as perspiration tries to control the excess heat I am producing. But the perspiration cannot evaporate and disperse fast enough through my clothing, so as time goes by, I notice my base layers feeling increasingly moist. I know that soon my clothes will be soaked with moisture. How will I get dry? How will I stay warm if I stop moving?
This is the scenario that active insulation was created to deal with. The purpose of active insulation is to provide an appropriate (usually minimal) amount of insulation for high-output activities in cool or cold conditions, while at the same time offering improved moisture elimination, as compared to traditional insulated garments. If we choose our clothing wisely, then perhaps we can find the sweet spot where heat generated by our activity level is balanced by heat loss to the environment. This can be a difficult balancing act to achieve, but that’s the goal.
I started working on this article as I completed my article on fleece. In that article, I concluded, among other things, that fleece has a poor warmth-to-weight ratio and might not be the best choice for achieving low pack weights. Fleece is probably the original active insulation; it just was not called that because the term had not yet been coined by the outdoor industry. However, fleece has long been used during high output activities to provide minimal insulation value with good moisture handling (relatively quick dry times and good breathability). As I produced the fleece paper, I wondered if there might be better solutions to fill the role of fleece. That led me to start testing the physical properties of garments marketed as active insulation.
This article presents quantitative performance measurements of a number of active insulation garments. Importantly, it delves further into the relationship between air permeability and moisture vapor transmission rate (MVTR) that I discussed in my last article. Since active insulation garments can be created from Polartec Alpha Direct, I have included thermal measurements for four different fabric weights of this fabric from which garments may be created. In the course of producing this article, I concluded that Alpha Direct provides the best active insulation for my activities. That conclusion is not because Alpha Direct provides unbeatable benefits over other options. It does not. The benefits that it does provide, in combination with a high MVTR wind/rain layer brings me the best balance of insulation, breathability, weather protection and ventilation for my activities. I hope, with the information presented here, you will be able to make the best choices for your particular needs.
Active Insulation Origins
As near as I can tell, the active insulation concept was kicked off with the introduction of two products: Polartec Alpha insulation in 2013 and the Patagonia Nano-Air Jacket/Hoody in 2014. Other manufacturers, including Arc’teryx and Rab, were quick to jump on the bandwagon. The key common concepts in these two very different approaches to active insulation are low insulation values and high air permeability.
Traditional insulations, such as down or synthetics, are encapsulated in face and liner fabrics. The fabrics must contain the insulation so its fibers cannot leak out. This is achieved by using tightly woven nylon or polyester fabrics. Often, the fabrics are calendered, a process where one or both sides of the fabric are melted. The process seals tiny openings between the fabric fibers. In general, the better the face and liner fabrics seal in the insulation fibers, the better it prevents water vapor or wind from moving through the garment. For high output activities, these traditional constructions limit the ability of vapor from sweat to escape from the garment. With traditional insulated garments, the chances were good that sweat produced by high output activity would simply wet out the insulation and degrade their insulative value.
Polartec Alpha and the Patagonia Nano-Air series relied on new insulation technology. The objective of the new approach was to provide just the right amount of insulation for the expected activity and conditions while improving the opportunities for water vapor from sweat to escape to the environment.
The new insulations were at least partially self-supporting and also produced relatively low thermal resistance values. These new insulations did not need the extensive quilting required by down or various synthetics to remain stabilized and prevent cold spots that resulted from shifting insulation fibers. The fibers of the new insulations would not readily leak through the face and liner fabrics. Since the new insulations would not leak through the face or liner fabrics, the new garments could utilize fabrics that were loosely woven. Loosely woven face and lining fabrics offer an important advantage: they provide increased air permeability that can facilitate the escape of moisture vapor to the environment. At the same time, they offer a big disadvantage: when used in higher wind conditions, cold air can blow through the garment and severely degrade its warmth.
Patagonia got the concept mostly right with the original Nano-Air. The jacket face and lining fabrics had very high air permeability, making use of the same fabric contained in the Air Shed windshirt (now known as the Houdini Air), while using insulation that did not require extensive quilting, did not leak through the face and liner fabric, and supported good vapor transmission and air movement. I think the significant shortcoming of the original Nano-Air was that it was a little too warm for high-level activities. Of course, there were also complaints that high winds cut right through the jacket.
In my opinion, early Alpha garments were not as successfully introduced. To the best of my knowledge, the first implementation of Alpha was by Patagonia in the L3A military jacket. This jacket did not match the air permeability and vapor transfer capabilities of the Nano-Air due to its heavy face and liner fabrics. I own one of these jackets. I own two other early Alpha jackets: a Mammut Guye and a Rab Alpha Direct. Like the L3A, both used tightly woven face fabrics that are not designed to support high moisture vapor transmission rates. As a result, these early jacket constructions using conventional face fabrics did not take advantage of the performance advances provided by new Alpha insulations – they did not support an adequate rate of moisture vapor removal.
The Secret Sauce for Active Insulation: Air Permeability or Moisture Vapor Transmission Rate?
A previous study explored the relative importance of Air Permeability vs. Moisture Vapor Transmission Rate (MVTR) for removing moisture vapor from a garment. I found that MVTR played an outsized role in maintaining comfort in low-speed activities such as hiking, running, or backpacking. In that article, the garment that provided the best moisture elimination was impermeable to air penetration but had extremely high MVTR. I suggest the same phenomenon is at work for active insulation garments.
In most active insulation garments, loosely woven face and liner fabrics are used. I suspect this approach was taken because manufacturers and users believe that high air permeability provides increased ventilation that can carry interior moisture vapor away. Tests described in my prior article demonstrated that this is not necessarily the case.
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Discussion
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I’d Love to second Brett Peugh’s kind request for a list if at all possible it would be a great resource. Thank You for Your time and energy! : P
Hi Stephen,
Curious about the MTVR of the houdini air vs. houdini. I note that in this other paper of yours, the MVTR of the houdini is considerably lower than that stated in this article (about half the rate). Can you explain the difference, or where I may be mistaken in my comparison?
Cheers,
Erik
Hi Erik:
Good observation. At the time that early article was written, my MVTR test was performed without an interior circulating fan. Shortly after that article was written, I introduced an interior circulating fan, This fan reduces the effective resistance to vapor transmission of the air layer between the top of the permeation kettle surface and the test garment. As a result, the measured MVTR went up for all subsequent tests.  So, the values I have obtained under the present testing procedure for the Houdini is 2250 g/m2/24hr and, the Houdini Air is 3120 g/m2/24hr.
Stephen, Have you had a chance to test the Visp yet? Thanks.
Thank you!
Thank you for your eye-opening article Stephen.
Could you please tell us what are the best wind shirts in terms of mvtr with a low cfm?
Do you have the stats for the Black Diamond Alpine Start (2021) and the OR Helium Wind Hoodie?
Thx
This is fascinating, thank you for your hard work and extensive testing.
Two notes – firstly, in the conclusion you say ” In the winter, temperatures rarely drop below 0 °F (18 °C). In the summer, temperatures can range from 30 °F to 90 °F (-1 °C to 32 °C).” I think that should be (-18°C) not 18°C – sorry I’m being pedantic…Secondly, in your opinion, how much difference would these findings be affected whilst cycling? Clearly you are often moving through the air at higher velocities so the air pressure differential would change significantly, thoughts?
Thanks again, Scott in New Zealand
Hi Scott:
Thanks for reading.
I guess you have discovered that we don’t do metric in the US.
My conclusions about the ineffectiveness of air permeability is based on field use at hiking speeds, testing results and calculations of pressures at low speed or low air velocities, typically 3 mph. Clearly, as air speed increases due to increased forward speed or higher winds, garment air permeability can result in a greater contribution to cooling and moisture/vapor removal.  But it is not always a slam/dunk. Penetrating air must be able to displace interior air. If interior air cannot escape than vapor or moisture removal may well remain inadequate and still require high MVTR.
Wonderful work, Stephen. What a resource your analyses are! In your article on fleece, you listed R values. Here you talk about the importance of MVTR and air permeability. I wonder if you have MVTR and air permeability numbers for the fleeces you tested. I’m especially interested in fleeces like the Patagonia Micro D and the Decathlon Quechua MH100.
Hi Shawn:
I have not done much testing of finished fleece garments. R-value tests require the guarded hot plate and that means the garment must be cut up to perform the test. When I did the Alpha Direct testing, I did test one fleece jacket I have to see how they compared for MVTR and air permeability. The problem is, I have no idea what weight fleece they are, and, as I found for at least Polartec fleece, the fabric weight had no bearing on warmth anyway. I have two fleece jackets and just measured the air permeability. One is pretty heavy and its air permeability is 253 CFM/Ft2. Its MVTR I measured previously and it is 2960 g/m2/24hr. Both measurements are much lower than Alpha direct fabrics. The second jacket is much lighter weight and its air permeability is 204 CFM/Ft2. The only place I wear fleece these days is in the house. In fact, I don’t do that much anymore. Mostly, I wear a Norona Falketind Octa. It is warmer than my heavy fleece jacket and just over half the weight. Very comfy.
So Stephen, or anyone who has done the cross referencing research here – for an ideal alpha combo (low CRM high MVTR?), whats on the top performers list?  OR Helium Wind hoody? (very interesting)BD Alpine Start? (tad heavy?),Patagonia Houdini Air (CRM too high?), (MB Tachyon, etc)?  If someone has already done the aggregate work, what’s the dream wind shirt?  Thanks! (PS, returned my EE Visp before ever wearing it, figuring the DWR would be problematic).
For 3 season use, I carry a Montbell Shakedry jacket and a 2 layer 60g Alpha direct. That will change when winter arrives. I want a water proof breathable shell with the highest possible MVTR. I don’t want to carry a windshirt and a rain jacket. I want one jacket that will perform both functions. I want a midlayer for insulation only that provides great warmth to weight performance with as little resistance to vapor transfer as possible.  There is no right answer to your question. That is because you are already assigning your values to a variety of choices and only you can decide which are best for the kind of hiking you do and the conditions in which you do them. Concerning DWR: Why do you think that EE’s DWR is inferior to every other brand? There are only two jackets that eliminate DWR. The Columbia solution has poor MVTR. The Gore Shakedry solution may suffer elevated wear. If you don’t like those choices, you will have DWR. All DWR will eventually fail. DWR must be maintained to support its functionality.  Is one manufacturer’s DWR more likely to serve you better? I don’t know. Their are lots of reports of DWR failure. Why wouldn’t there be? We at BPL know it will occur. Lots of users probably don’t. Those users may have taken no steps to follow proper care instructions.  However, unless those reports describe how a garment was used and how well it was maintained, you still don’t know if it would work for you. A friend of mine is doing the entire PCT. He has a new EE Visp for the trip. I am looking forward to his experience on this issue.
Thanks Stephen.  I’m sure I’d like one shell for wind and rain too.  Eventually.  I appreciate all your research.  As far as DWR, yes I realize and have experienced it’s limitations many times like most of us.  Certainly proper care helps.  Regarding the Visp, I look forward to your friends feedback as well.  And why I’m a bit suspect of its DWR being perhaps inferior to others, a friend that was less then impressed and Philip Warner over at Section Hiker made me pause. It’s not an inexpensive shell, and I was hoping for one shell to accomplish both wind and rain functions, but decided to wait until something more promising, perhaps without DWR reliance, comes out.
Thanks again. Look forward to all you learn and share.
The Montbell results really surprise me – I expected it to be the complete opposite. Very interesting indeed.
The Thermal Airshed still interests me for no other reason than it might be a much more durable alternative to the Nano Air light hybrid – as much as I like those discontinued layers the loft in front seems to degrade very quickly.
Realistically I think everyone likely wants to maximize MVTR but air permeability will end up being a very personal thing – I run very warm so I prefer shells and insulation with much higher air permeability. If I am sweating profusely due to a complete lack of airflow (or not enough) I am pretty sure I am going to outpace the ability of even a high MVTR garment to dump moisture.
@Stephen Seeber ,
Any reason to think this would be any different for static insulation?
The traditional view has always been that membranes or coatings on a down jacket reduce breathability, thereby increasing the amount of moisture trapped in the down.
But as you mention, the plain fabrics have very low mvtr and air permeability, and membrane or coated options would seem likely to offer better mvtr while maintaining down proefjes and wind resistance.
What are your thoughts?
Stephen, first of all, thank you for this very useful work you are doing!
my question is, if this is the case for active insulation, would the same not be true for ‘static’ insulation?
The current view has always been that adding membranes or coatings to the face fabrics of down jackets reduces breathability, leading to moisture build up inside.
However, your tests show, that at least for certain materials, membrane laminates can  be more breathable than plain fabrics, especially the tightly woven and calendared fabrics used in down jackets.
Do you have any ideas about this? Any future testing planned for this?
Thank you again
Hi Tijaard: Not quite sure what you are asking about, but I will take a stab at it.  Many active insulation pieces are distinguished by low R-value, self supporting insulation and loose weave face fabrics. I suppose you can produce a high loft insulated jacket with a WPB membrane and light face fabric that might provide reasonable breathability. In fact, there are such jackets. I would rather have multiple layers that can be worn in whatever order meets the present conditions than one single jacket with high insulation values and a wpb membrane. Such a garment may work well in specific conditions but won’t work in a broad range of conditions. I hope that answers your question, but repost if not.
Hi Stephen, this is confusing given this conversation is across couple of companion threads! I think what Tijaard is asking is the same as what I was asking on the other thread. How is it that a WPB membrane can have a higher MVTR than a non coated tightly woven fabric? Like Tijaard my understanding is that you most definitely don’t want to use a WPB fabric on an insulated garment i.e. a down jacket due to the reduced breathability (MVTR) of that fabric. What your data is showing is quite the opposite, at least in the case of Shakedry. Is it because Shakedry is such an outlier in regards to MVTR?
Thanks, S
Further to my last question, in the past I suspect that the MVTR of WPB membranes have been so bad that their rates were noticeably lower than uncoated tightly woven fabrics. Perhaps with the advent of high value MVTR WPB membranes insulated garments could use them instead?
Does calendaring a fabric completely seal up the holes? Could this explain the difference in MVTR?
S
Yes, I meant what Scott said.
I was asking about regular high loft down or synthetic jackets (or sleepingbags/quilts) for low activity/rest.
For down parka’s and sleeping bags you need the most breathable fabric possible, s (because the vapor pressure differential on both sides of the fabric is so low, and down suffers when wet).
In the past, the thought was always that ‘plain’ fabrics were better at this. But your testing shows that some laminated membrane fabrics, or coated fabrics, achieve better mvtr than some ‘plain’ fabrics.
In other words:
One of the primary reasons I use fleece is for chilly mornings while backpacking. In Idaho, we have a lot of overgrown trails. The abrasion resistance is important – or at least relative expendability/inconsequence of what might happen to the garment even if it gets a snagged. Is there any single garment that rivals fleece when the three values of warmth, MVTR, and resistance to branches/overgrowth are all important? Something that does all three with lower weight and/or smaller packed volume would be ideal.
Hi Shawn: Hard question to answer because we don’t know anything about the fleece you wear. Fleece I tested has an R-value of .34-.78. We probably can’t know what your fleece is without testing it. The same goes for MVTR and air permeability. The weight you can easily determine. I have a fleece jacket that is probably 200 weight fleece. It weighs 459 grams. I recently measured MVTR of a 100 weight fleece at 2480. An alternative to a fleece jacket could be the Montbell UL Thermawrap. R value .67, MVTR 2880, weight 247G. The air permeability of the Thermawrap will be way less than fleece. You can reference my fleece article as well as the search for a high MVTR and see how various options compare.
The question is really much less about MVTR or warmth than about durability against overgrowth in real use. The thermawrap is far more likely to be ripped than traditional fleeces (TNF TKA, R1, Delta LT, Polartec classic 100, etc) when ‘bushwacking’ or pressing through high overgrown trail, which I encounter a lot. Is there anything lighter or more compressible than, say an R1 or a TNF TKA Glacier 100, that has the surface durability to handle such conditions as well or better?
I have the exact same concern about durability. I ALWAYS have a 100wt Polartec Classic Fleece 1/4 zip that weighs 9 ounces (XL). R1 non hooded XL is around 12 ounces (not ultra light!). No matter what else I have for insulation, the fleece can go over it for protection. IMO, classic fleece is one of the most durable fabrics I have ever used, and about 50% of my hiking miles are bushwhacking.
Another aspect one might not be thinking of when thinking of weight to warmth ratios of fleece, is that a lot of 100wt fleeces have a good amount of stretch in them. One can fit A LOT of grass, moss, leaves, or anything with insulating value into the fleece. In emergency situations, especially when there is no escape from rain, filling a fleece with even wet stuff can add significant warmth and protection from the wind and rain.
Hi Shawn: Sorry, there are lots of possibilities out there but I can only comment on what I have tested. The best I can suggest is to review what I have posted and see if there is something there that makes sense. It is certainly possible that fleece is the best option for you.
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