"good R- Value"

Started by Dennis_Kuhn, February 11, 2005, 01:11:10 PM

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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.
None of us are as smart as all of us.


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