I have measured thermal resistance of a lot of garments. I have often found that the measured resistance of the garment didn’t seem to relate very well to the insulating performance claimed by the insulating manufacturer. How much of this was the result of the garment construction and how much might be related to misleading claims by the insulation manufacturer?
So, I decided to measure the thermal performance of bare, brand new insulation.
It is clear that garment construction can be responsible for the loss of significant insulating value. However, in this study I found that manufacturer’s claims of insulating performance may be dubious. I also found what appear to be surprising lapses in quality control and/or achievable manufacturing tolerances.
Oh, of course, you probably want to know: what is warmer—Climashield Apex or Primaloft? The answer is, as sold by weight per square yard, they seem to be pretty identical. Which lasts better over the long term? Which is warmer when wet? Those and questions will have to wait on an upcoming article in this series.
One of the challenges in studying insulation is that I can only study products that are available for purchase. The insulations that are available for sale online are a small portion of what you see advertised by clothing manufacturers. Many of the proprietary insulations are made by third-party manufacturers and are rebranded by the garment manufacturer. I don’t expect any of the users of proprietary insulations will be willing to provide samples for testing, so you won’t see them here.
I must warn the readers, this is a long article. There are a lot of basics to cover here. There will be additional articles to come that examine more of the issues of concern for synthetic insulation. I promise these will be shorter.
Here is what will be discussed in this article:
- Synthetic insulation categories and types with a brief discussion of advantages and disadvantages.
- What is a Clo? Clo is a measurement of resistance to heat loss. It is a simple concept that creates a lot of confusion. The confusion is warranted. The meaning of clo has multiple definitions and you typically don’t know which is being applied and what test method or variations were used - thus, the confusion.
- The guarded hot plate is one of the methods available for measuring the thermal performance of insulation. We will briefly discuss the guarded hot plate and link you to a more detailed discussion of how a hot plate works and a discussion of the guarded hot plate I built for this study.
- Thermal performance measurements of several market-leading insulations will be presented along with four performance metrics to help judge their performance. Performance results for specific insulations will be considered.
Editor's Note: While we of course recommend that you read the entire article in order to have a complete understanding of Stephen's results, we also recognize that not everybody wants the kind of detail we are about to jump into here. If you want to skip straight to the results, scroll down until you see the section titled "Results of the Thermal Resistance Measurements of Selected Insulations."
Synthetic Insulation Categories and Types
The synthetic insulations we'll discuss in this article fall into two primary categories: Non-woven and knit.
The majority of synthetic insulations fall in the non-woven category. Non-woven fabrics constitute a huge worldwide industry that spans numerous applications. Non-woven fabrics are created by depositing fibers, composed of a wide range of materials, both synthetic and natural, into a continuous web. The material that goes into 3M N95 masks is non-woven. Geotechnical fabrics that prevent soil erosion can be non-wovens. Filter media used to purify your drinking water are non-woven fabrics. Non-woven fabrics can have an extraordinary array of properties based on the materials, manufacturing processes, and chemical or mechanical treatments utilized in their production.
There are a variety of manufacturing processes for non-wovens. The actual fibers may be produced as part of the manufacturing process (Spunlaid, Meltblown, Submicron Spinning) or incorporated into the process as bales of material (Drylaid-Carded, Short Fiber Airlaid, or Wetlaid). If you want to know more about any of these processes, this is a good source.
After the fibers are formed into a web or mat, the resulting web strength is minimal. Web bonding must be undertaken to stabilize the web to achieve adequate structural integrity for its intended application. This is accomplished by bonding the fibers through applications of heat, adhesives, or mechanical processing such as needle punching and compression. During this processing, additional chemicals may be added to reduce friction between fibers, improve moisture resistance, or control the build-up of static electricity.
When selecting the type of non-woven to utilize, designers must consider a number of issues:
- The garment design, appearance, and manufacturing costs. Thick, inflexible blankets of insulation will not lend themselves to sleek, athletic-looking garments. Insulation physical space requirements, pliability, and ease of assembly will impact the materials selection process.
- Quilting Requirements. Structural integrity will vary greatly among insulation types. Insulation rigidity can be achieved during the manufacturing of the web or during garment production through the use of quilting. The use of quilting may be desirable from an aesthetic or marketing perspective, but it reduces thermal efficiency. A quilted seam produces a localized area where the insulation is compressed and provides little or no thermal resistance for the garment. Some insulations are self-supporting so that quilting requirements are minimized or eliminated. Support considerations will have a direct impact on fabrication costs, insulation efficiency, and garment weight.
- Fiber characteristics. The shape, length, and diameter of fibers that comprise the web will influence insulation warmth, resilience, and longevity. Smaller fiber diameters can reduce radiant heat transfer in the insulation but at the cost of the structural integrity of the web. Some insulations are created with a range of fiber diameters and shapes to provide the desired balance of thermal efficiency and loft maintenance.
The three major categories of non-woven insulation include cluster fiber, short-staple, and continuous filament.
Cluster Fiber insulation consists of tiny tufts of fibers. The North Face Thermoball (made by Primaloft) or Rab Cirrus (3M Featherless Down) are examples. These fibers can readily substitute for down in the manufacturing process and lend themselves to garment designs associated with the use of down. These types of insulations require extensive quilting to provide stabilization. None of these products are included in the present study.
Short-Staple insulation consists of short fibers, typically less than 2 to 3 in (5 to 8 cm) in length. Primaloft Gold is a market-leading example of short-staple insulation, of which two weights are tested in this study. Short staple insulation can have a very soft feel, excellent conformability to complex shapes, and very good warmth-to-weight performance. As is the case with a number of short-staple insulations, several diameters of fibers are incorporated into Primaloft Gold. These range in diameter from approximately 10 microns to 30 microns.
The thin fibers allow for the greatest entrapment of air (trapped air provides the insulation in any of these products) but have little strength to resist compression. The thicker fibers provide resistance to compression and allow the web to restore loft after compression. The short fiber insulations have little strength in any direction and can be easily pulled apart by movement, washing, or compression. As a result, quilting is essential for garments that use short-staple insulation. Short staple insulation suffers from loss of insulating ability from use. Depending on the insulation, loss of insulating ability can be rapid and significant. Figure 1 below shows an infrared image that compares surface temperatures of two samples of Polartec Power Fill short-staple insulation.
As re-testing on this insulation proceeded, I noticed that the surface temperature increased by 1 °F, showing reduced insulating ability. I took the sample and ran it in a dryer with no heat for an hour and tested it again. As shown in the sample on the right of Figure 1, the surface temperature increased to 2 °F above unused insulation (left sample). These thermal images demonstrate significant insulating degradation with very minimal use. Another short fiber insulation included in this study is 3M Thinsulate.
Continuous Filament Insulation is constructed from continuous fibers that run across the width of the web. As a result of the continuous fibers, the insulation web can have substantial strength. For example, Primaloft Gold, with short fiber running in all directions, can be easily ripped apart in any direction. By comparison, Climashield Apex, made with continuous filaments, has substantial strength across the roll. Strength in the direction of the roll (the direction in which the web is pulled through the production machinery) is substantially less, as evidenced by the manual force required to pull the fabric apart in both directions. Climashield, owned by Harvest Consumer Insulation appears to be the market leader in this type of insulation. The corporate predecessor of Climashield produced Polarguard, which is no longer marketed.
Climashield is produced with relatively large diameter fibers—approximately 22 microns. Thanks to the larger fibers and continuous filaments, this type of insulation reportedly has better long-term loft retention than short-staple insulations. (This is a subject we will follow up on in a forthcoming article.) Continuous filament insulation can be installed in garments or sleeping bags without quilting. In assembled garments or sleeping bags, this should provide an advantage in thermal efficiency over short-staple insulations. In theory, the warmth to weight ratio may be better in new short-staple insulation with small diameter fibers than with continuous filament insulation. However, this possible advantage for short-staple insulation may well disappear due to faster loft degradation than may be the case for continuous filament insulation.
At this point, the potential warmth to weight advantage of short-staple insulation is not supported by our tests. We find that for 6 osy (oz/yd2) insulation, Climashield Apex has a small warmth advantage over Primaloft Gold. However, for the 6 osy insulations, Apex is about 0.25 in (1 cm) thicker than Primaloft Gold. So, when using Apex, expect a similar initial insulating performance per ounce of insulation compared to Primaloft Gold but in a thicker garment or bag. Our Primaloft samples had softer hand, better pliability, and greater thickness uniformity than our Climashield Apex samples. For this article, we include three weights of Climashield Apex. We also include a German continuous filament insulation: Freudenberg KHT-60.
Two types of knit insulations will be considered in this study. Both are made by Polartec and made from polyester.
The first type includes Polartec Alpha and Alpha Direct. The second is Polartec Fleece. Fleece will be covered in detail in a follow-up article.
Construction is somewhat similar for both: A grid support structure is knit using polyester fibers. The fibers are then knapped to provide loft and sheared to produce a uniform surface.
Alpha offers several unique capabilities. The fabric is self-supporting so that quilting is not required for assembly in a garment. The Alpha knit matrix allows for substantial airflow to support the elimination of excess heat and promote rapid drying. Any face fabrics or liner fabrics used in Alpha garments can also support substantial air permeability because they do not require high-density fabrics to contain the insulation as in the case of down and some synthetics.
One feature not offered by this insulation is high warmth. The insulation is created for high exertion activities and so is not expected to provide high insulation values. It provides the lowest value of any insulation in this study except fleece. The Alpha insulation we tested is about 3 osy but has less than half the thermal resistance of other 3 osy non-woven insulations we tested.
Synthetic Insulations Under the Microscope
Photomicrographs of various insulations are presented below. These were produced so I could determine the fiber diameters used in each type of insulation. The diameters used can help highlight expected warmth and expected resilience. The diameters of fibers are measured and displayed for each insulation.
Member's Only Content
- Synthetic Insulations Under the Microscope
- What is a Clo?
- Development of a Guarded Hot Plate
- Results of the Thermal Resistance Measurements of Selected Insulations
- Quality Control and Measurement Accuracy
- Discussion of Insulation Performance
- Conclusions and What's Next
Member's only version is 7,006 words and includes 22 photographs and/or illustrations.