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Community and Q&A

Converting Thermal Conductivity Values

bluesolar | Posted in Energy Efficiency and Durability on

Hi all – I’m intrigued by Armacell’s insulating foam material made from recycled PET bottles. However, I don’t understand their units for thermal conductivity, or how to convert them to R-values. I’d appreciate your help.

They give their units as W/(m•K). An example value is 0.034 for their least dense version. I thought the metric R-value unit was (K•m²)/W, and I don’t understand their unit.

They also give an alternate unit of².hr.°F.

I don’t know what that is, or what some of those periods are for. I thought R-value in American units was °F•ft²•h/BTU.

How can I convert these units into R-values?

Thanks for your help.

p.s. Here’s a screenshot of their spec sheet with these units:

And here’s their product website:

JD Composites built a fascinating house in Nova Scotia out of SIPs made from Armacell’s PET foam.

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  1. Jon_R | | #1

    That's a conductance value . Thickness (in meters) / conductance = RSI.

  2. Expert Member
    Peter Engle | | #2

    Conductance is the inverse of resistance. To get the R-value, divide 1 by their advertised value.

    As in: 1/C=R

    If you flip their units over, you will see that they are the same as the ones you are familiar with. The only other difference is that you have to include the thickness of the material in your calculations. The R-value units you mention above are for insulation of a specific thickness. Their units essentially give you conductivity per inch (or meter). Jon's formula above is absolutely correct.

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