Insulation Helps, Tips, and Rules-of-Thumb
Insulation star # ratings system by Fesmire. The more #, the better (the lower the k-value) performance and the higher the cost of the total system as-installed.

##### = MLI or LCI-HV. High vacuum. 0.1 mW/m-K. Multilayer insulation.
#### = Bulk fill. High vacuum. 1 mW/m-K. Glass bubbles or perlite powder.
### = LCI-SV. Soft vacuum. 2 mW/m-K. Best in world.
## = LCI-NV. No vacuum. 13-15 mW/m-K. Near aerogels.
# = LCX. Exposed. 18 to 24 mW/m-K. Similar or better than the best spray foams.

Stars one through five cover a very wide range of thermal performance. Star one or two is for ambient pressure systems. Star three is for soft vacuum (about 1-10 torr) systems. Star four or five is for high vacuum systems.

Note 1: These k-values are for 293/77 K boundaries. The k-values for 293/273 K boundaries or normal refrigeration will be higher according to the higher mean temperatures.
Note 2: LCI = Layered Composite Insulation (can be tailored for HV, SV, or NV); requires vacuum jacket or sealed enclosure.

Thermal Conductivity Units and Conversions
k = thermal conductivity [mW/mK] or k = thermal conductivity [Btu-in/hr-ft2-degF]

R = thermal resistivity = 1/k

k = 144.2/R and R = 144.2/k

Note that R are always per inch of thickness. For example, fiberglass in your home is always going to be around R=3.3. If you have a thickness of 9-inches then the equivalent total thermal resistance is 30, but the R is still 3.3.

A one-inch thick oak board is R =1 (and the corresponding k = 144.2 mW/m-K).

Caution! There are different meanings of "Thermal Conductivity"
Really, there are different types of thermal conductivity with respect to thermal insulation materials. The key distinction here is that thermal insulation materials are generally low density, porous materials. That is, they are mostly "nothing" inside (free space) relative to metals, ceramics, and plastics. We can then talk about four different types of thermal conductivity depending on the insulation material(s) and the test environment:

  1. lambda (Greek symbol for lambda) = thermal conductivity by solid conductivity for a "pure" material (see ASTM C168)
  2. k = apparent thermal conductivity = thermal conductivity for a "pure" material with more than one mode of heat transmission, not just solid conductivity, so that the total effect is "apparent" (see ASTM C168)
  3. ke = effective thermal conductivity = thermal conductivity of a complex material or combination of materials under a large temperature difference (see ASTM C1774)
  4. ks = system thermal conductivity = like ke but as the total thermal insulation system is rendered in the field application = "real world total cryogenic system" (see ASTM C1774)

These are some guidelines to help you wade through the different terms and definitions that litter the thermal landscape. The ASTM C1774 is a new for cryogenic insulation and helps to bring some clarity to this issue. The following data, whether they are called k or k-value, generally fall under the definition of ke as defined above.

Thermal Conductivity Values Lookup Table
Material
 Environment
 k (mW-m-K)
R (hr-ft2-degF/Btu-in)
MLI
 high vacuum
 0.05
 2,884
Glass Bubbles
 good vacuum
 0.7
 206
Perlite Powder
 good vacuum
 1.1
 131
LCI-SV
 soft vacuum
 1.6
 90
Aerogel blanket
 soft vacuum
 3.0
 48
Aerogel blanket
 no vacuum
 11
 13
Aerogel particles
 no vacuum
 14
 10
Polyurethane foam
 no vacuum - new
 20
 7.2
Polyurethane foam
 no vacuum - aged
 26
 5.5
Note: These values are for boundary temperatures 293 K / 77 K with a residual gas of nitrogen and about 1-inch thickness test specimens.

Temperatures
Popular boundary temperatures in cryogenic systems are as follows: warm boundary temperature (WBT) = 293 K (68 degF) and cold boundary temperature (CBT) = 77 K (-321 degF). Here are the corresponding temperature difference and mean temperature for the boundaries of 293 K / 77 K:

Tdifference = 216 K (389 degF)

Tmean = 185 K (-127 degF)

These temperatures are important to get straight as you are comparing data from different sources. Always know your boundary temperatures and your environment. And the test thickness and density are also good to know!

Common Materials
Here are the materials that have ASTM International specifications and are included in the cryogenic temperature range:
ASTM
 Material
 Classification
 Density
kg/m3
 Tmean
K
 k-value
mW/m-K
 Environment







C549
 Perlite powder
66
 185
 30.2
 No Vacuum
C552
 Cellular glass
118
 185
 29.2
 No Vacuum
C578
 Polystyrene, rigid cellular
 Type XIII XPS
 26
 185
 23.8
 No Vacuum
C591
 Polyisocyanurate, preformed rigid cellular
 Type IV
 32
 185
 23.4
 No Vacuum
C1482
 Polyimide, flexible cellular
 Type I, Grade 2
 7.1
 185
 27.3
 No Vacuum
The apparent thermal conductivity values (k-values) for these materials were interpolated for the specification data at a mean temperature of 185 K (-127 degF). All data are for an ambient pressure (No Vacuum) environment, of course.


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Reference: Fesmire, J.E., Standardization in Cryogenic Insulation Systems Testing and Performance Data, Physics Procedia, Vol 67, 2015, pp. 1089-1097, http://dx.doi.org/10.1016/j.phpro.2015.06.205.


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Example of open cell material such as can be found in a number of insulation materials.