The Difference Between Nominal R-Values and Effective R-Values

October 19, 2016

The concept of "consumers beware" takes on new relevancy when it comes to home insulation. And that relevancy becomes acute with respect to R-Values - the industry measure that defines insulation performance. It's not that much different than any other consumer product - like that new SUV that assertively advertises 12 liters/100 kilometers, but performs at 18 liters/100 kilometers in real life. With R-Values it's the same - the Nominal R-Value is the "estimated" level of performance, while the Effective R-Value is the "actual" level of performance in real conditions. It's not unusual for an insulation product to be advertised with a specific R-Value and perform at a different value during the cold months of winter. In fact, even when an insulation product is installed according to the manufacturer's specifications, performance can differ in a wood-frame assembly, compared to a steel-frame assembly. The bad news is that the difference between the "estimated" performance and the "actual" performance can be quite dramatic. And without an expert to advise, a typical homeowner could be exposed to surprises that were unforeseen. For R-Value to be accurate, performance testing must be done under "real world" conditions - for example, in an actual wall assembly or attic cavity, and under real weather conditions. Here, the results would reflect the Effective R-Value of the product - in other words, the real value under real conditions. This would also explain why certain insulation products deliver performance in certain areas of the house, and why some don't. Clearly, having a professional on site would help with the uncertainties, and better define products/installations that would deliver performance. Now, beyond the actual performance of a product, there are some other variables to consider. For example, depending on the "building assembly", insulation will perform differently with wood, concrete, or steel. All of this has to be considered when assessing the real R-Value. Not to mention the airtightness of a space. Airtightness and/or air leakage can radically reduce R-Value, even with the best product on the market. In fact, a poorly sealed roof cavity or basement crawl space can totally compromise the performance of installed insulation, regardless of product type. Moisture-laden air is another enemy of insulation, and can quickly reduce the Effective R-Values (sometimes dramatically). Some industry studies actually show that moisture-laden insulation is so disturbed as to render the performance of the insulation almost pointless. This is just another reason why an airtight space is so critical. As it is, airtightness prevents air movement, but it also prevents moisture from encroaching and accumulating. This is one more variable that highlights the difference between Nominal R-Values and Effective R-Values. It's the big picture that matters. For an average homeowner, the takeaway here is to opt for a professional approach to residential home insulation. The experts are the ones who have product knowledge and installation know-how. They can interpret R-Values and insulation performance so that an installation delivers both short term benefits and long term results - without the need to rework or reinstall down the line.