R Value of Insulation Blankets

The kind of insulation used in the construction business – that pink fiberglass stuff that normally is used in attics and walls- is well known. These insulations, mostly fiberglass, are provided with ‘R’ values, which the manufacturer adds as a measure of the effectiveness of the insulation.

The ‘R’ value is a relative number, meaning that the higher the ‘R’ value, the higher the effectiveness of the insulation, so that an insulation with an ‘R’ value of 30 will perform better, keeping the home colder in the summer and warmer in the winter, than an insulation with an ‘R’ value of 20. Correct?

Well, if one is looking to insulate the home, then it is right. An ‘R’ value is established by the “thermal resistance” of a material at a given temperature. In the building and home industry, it is usually calculated at 75 °F (24 °C). Since a regular building will linger around this temperature, the ‘R’ values for building sector applications will offer a good idea of their performance. However, when it comes to insulating exhaust systems, engines and other industrial applications, temperatures can differ and get very high.

The ‘R’ value of an insulator at 500 °F (260 °C) will vary from its ‘R’ value at 750 °F (390 °C). So when comparing insulation materials, or attempting to establish how effective a given insulation is, for these type of applications one would have to know the ‘R’ values spanning the temperature range. Knowing the ‘R’ value at the temperature where building insulation is rated will have limited or no use.

‘R’ Value and ‘K’ Value – Definitions

So as to gain a better understanding of a ‘R’ value, and its inverse, the ‘K’ value mean, it is beneficial to look at their definitions. ‘K’ value is a measure of a material’s “thermal conductivity”, which is defined as “the time rate of steady state heat flow through a unit area of a homogenous material induced by a unit temperature gradient in a direction perpendicular to that unit area”.¹ In other words, the ‘K’ value is the actual measurement of heat conveyed through a specific material.² Its unit of measure is Btu-inch/hour per square foot per degree F (or W/(m.k)).

The ‘R’ value, a measure of a material’s “thermal resistance”, is defined as “the quantity determined by the temperature difference, at steady state, between two defined surfaces of a material of construction that induces a unit heat flow though a unit area”.³ Simply put, it is a sign of a material’s ability to resist heat loss.⁴ It is the inverse of the ‘K’ value, multiplied by the thickness of the insulation.

For instance, if a material’s ‘K’ value is 0.25, then the ‘R’ value for 2” thick material would be 1 / 0.25 x 2, or an ‘R’ value of 8. In brief, the lower the ‘K’ value, the higher the ‘R’ value and the higher the effectiveness of the insulation. A ‘K’ value will be constant at a given temperature, while an ‘R’ value will vary based on the thickness of the insulation.

Other Factors to Consider

Ok, you might say, once someone knows the ‘R’ values in the temperature range of a specific application, does that mean that the higher ‘R’-rated insulation will be the more effective insulator for that application? Well, the answer to that would be only a partial ‘yes’.

The problem with using ‘R’ values to rate an insulation material’s effectiveness is that the ‘R’ value is just one of a number of critical variables to consider that can affect insulation performance. For instance, how does an insulation material perform when it gets wet? Some insulation materials will stand up well to moisture over time and retain their integrity, while others will deteriorate. For applications where insulation blankets may come into contact with the elements, choosing an insulation with a high ‘R’ rating but which does not perform well when wet is obviously not the correct solution no matter how high is its ‘R’ rating.

Brett Herman, Vice President of Sales and Customer Service, Firwin

Resistance to compression is another variable that can impact insulation performance.

“The geometry of the components we insulate are often complex – elbows, silencers, catalysts, SCRs, valves, flanges”, said Joash Katsivo, Firwin’s Sales and Design Engineer. “Because of this we use dense insulation that resists compression even when formed to the shape of the component, thus retaining its insulation value. A low-density insulation may have a high ‘R’ value, but is easily compressed. If we were to wrap such an insulation around some of the exhaust components we cover, it would compress to very thin and lose its insulation value”, added Joash.

Other variables that may impact the effectiveness of an insulation are:

• Temperature rating – What is the temperature range within which a particular insulation is effective? Apparently, a high rated ‘R’ insulation will not be effective if the application’s temperature drops outside the established temperature range for that insulation.
• Vibration resistance – Some insulation materials are better at enduring vibration than others. So an insulation with a high ‘R’ rating which is not resistant to vibration would not be the ideal choice for a high vibration application.

Putting It All Together

We find that when a customer is asking about an ‘R’ value, what he really wants to know is what material will meet the goals that he is trying to achieve by using insulation blankets. For instance, often the customer is concerned with personnel safety, and wants to ensure that the outside touch temperature of the insulation blanket will be within safetouch limits.

Brett Herman, Vice President of Sales and Customer Service, Firwin

“What we will do in this case is gather the variable information that is necessary to formulate this calculation. This information includes not only the material’s ‘K’ / ‘R’ value, but also other variables that can affect the insulation blanket’s performance - i.e. ambient temperature, air flow, and pipe diameter. We can then provide the customer with the outside temperature he can expect using certain insulation materials and thicknesses. This can also be illustrated graphically using Heat Flow Charts, so the customer can get a visual representation of how the outside surface blanket temperature will be affected by various material and thickness combinations”, added Brett.

So what, if any, use do ‘K’ and ‘R’ values have in industrial settings where Firwin removable insulation blankets are typically found?

I would look at ‘R’ and ‘K’ values as indicators to be considered alongside others when trying to determine the appropriateness of an insulation. Ultimately, the customer needs to inform us what it is they are trying to achieve, and then we can advise them accordingly.

Brett Herman, Vice President of Sales and Customer Service, Firwin

References

1. Gordon Hart, Insulation Outlook, March 2009, “Understanding the Value in All These Values”, p10.
2. David South, The Monolithic Dome, “ ‘R’ Fairy Tale – The Myth of Insulation Values”, p1.
3. Gordon Hart, Insulation Outlook, March 2009, “Understanding the Value in All These Values”, p12.
4. David South, The Monolithic Dome, “ ‘R’ Fairy Tale – The Myth of Insulation Values”, p1.

This information has been sourced, reviewed and adapted from materials provided by Firwin Corporation.

For more information on this source, please visit Firwin Corporation.