The Search for a Fleece Replacement – A Close Look at Active Insulation and the role of MVTR

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.