R-Value in the HVAC World
R-value is a term that you commonly hear out of the mouths of HVAC contractors. So what is it, exactly? Well, R-value is a measure of insulation’s ability to resist heat traveling through it. Technically, it is calculated by taking the ratio of the temperature difference across an insulator over the heat flux through it. But all you really need to remember is the higher the R-value, the better the insulation works.
The R-value is dependent on the insulating material, as well as its thickness and density. Some materials are simply more resistant to temperatures than others, which is why insulation should always be purchased based on its R-value, rather than its thickness or weight.
An insulation’s effectiveness also depends on the location it is installed. If insulation is compressed, its R-value may be lowered (unless it is specifically designed to withstand pressure). Layering heavy insulation on top of lighter insulation can cause compression, as can wedging blankets (or batts) into narrow cavities. Unexpected moisture can substantially lower the R-value as well by weighing down the insulation, leaving gaps and bare spots. For this reason, it’s important to properly install your insulation to achieve the maximum thermal resistance.
The R-value of an insulating material is per inch of thickness. Snow has an R-value of R-1.0 per inch of thickness, while baled straw generally has an R-value of 1.5. Loose-fill insulation (such as fiberglass or cellulose) usually has an R-value ranging from R-2.0 to 3.5. Rigid insulation (such as plastic foam) can run from R-3.5 to 8.0 or higher. Aerogel has an R-value of about R-10.0, and vacuum insulated panels top the list with an astonishing R-value of R-30.0 (or higher) per inch of thickness.
When calculating R-value for several layers of insulation, you simply add their R-values together. For example, an inch of loose-fill fiberglass has an R-value of R-2.5. Two inches would have an R-value of R-5.0, assuming it was properly installed and uncompressed.
So, the next time you hear an HVAC contractor talk about the R-value, you can now know exactly what they’re talking about.
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