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Why More Insulation Isn't Always The Answer

July 20, 2013 | Posted by Mitt Jones

If R-38 is good, R-49 or R-60 is even better, right? Surely the more insulation you install, the more energy and money you save. More insulation does generally reduce heat transfer, which does save energy. In an unfortunate but undeniable reality of physics, however, the reduction in heat transfer goes down dramatically as you add insulation—especially in a fairly mild climate.

Heat_transfer.gifTo see what we mean, take a look at the insulation curves shown here. The curves represent heat transfer through an attic assembly versus the R-value of insulation.

The three different curves show heat loss at three temperature differences between the attic and living space—10 degrees F, 30 degrees F, and 50 degrees F. The curves would look the same with wall and crawlspace insulation. 

Look what happens to the heat loss value as R-value increases. Heat loss plummets with the first R-10 or so of insulation—the first few inches. With the next R-10, heat transfer still diminishes quite a bit, but the drop isn’t nearly as impressive. The trend continues as you add R-value.

If you live in an area with harsh temperatures or want to be especially prepared for occasional extremes, it may well make sense to insulate your attic to R-49 or R-60: The red curve shows that there is still reasonable improvement to be had when the temperature difference goes up.

In the Portland area, the blue line is a pretty good approximation of usual conditions in winter. It represents a temperature difference of 30 degrees F between the living space and attic. At that temperature difference, R-38 looks like a solid, cost-effective choice. Imagine that you’re starting at R-25, though: The improvement you’d notice will be incremental at best, though adding insulation may well be the right choice as part of a larger home performance project.  

For some no-nonsense guidance about how much insulation is enough, take a look at How Much Insulation Is Enough?

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