How Much R-value is Enough?

There is more to an effective insulation system than piling on “R"s.  But can there be such a thing as too much R-value?

The short answer is no.  But the more thoughtful answer is that the appropriate level of R-value depends on where you are building and on the level of return that you are seeking on your investment in insulation.

As we know from the discussion of the relationship between U- and R-value, we must double the R-value to cut conductive heat loss in half.  This is clearly a worthwhile thing to do when we first add insulation but there is a rapid drop-off in return on investment as insulation thickness increases.  At R=12 we eliminate about 92% of the conductive energy loss.  To reach 96%, i.e. cut the remaining loss in half, we must double the R-value to 24.  Is a savings of 4% worth the increased cost associated with doubling the thickness of your insulation?  The answer depends on your climate and the cost of energy but chances are that you are better off spending your money on a system that both air seals and provides r-value.

Case in point: The Canadian province of Alberta requires only an R-10 in residences but they do require a tight structure to go along with it.  More insulation is simply not cost effective for their weather.  And the January low in Calgary, Alberta is 5 degress Farenheit!