"good R- Value"

[Dennis_Kuhn]

I'm insulating my Catherdral ceiling (Victoria's Cottage) with R-38 Fiberglass.  This is more than the energy code for my house calls for.  But is it really good?  I know I could beef it up by putting foam board over that.  But?  

I'm reminded of a saying my friend has, "The Best is the Enemy of the Good".  

That really sums up what the decisions have been like for this building project.

The walls are R-21 with 1/2" of Dow board on the outside.

[John Raabe]

Your R-21 walls have R-5 at the studs and the foam helps considerably to raise that value.

In the roof R-38 is very good and the rafters are probably about R-10 so the foam will give you less bang for the buck than it did on the walls.

Your quote is right-on as the law of diminishing returns says that the first R-5 does the most work, the second R-5 cuts the remaining loss in half. To cut it in half again takes R-10. The next halving takes R-20, then R-40... at that point there is very little heat getting out and mountains of insulation are needed to cut that tiny amount in half.

[JeffnTN]

That is very interesting.  I have never seen it looked at in that manner.