Step 30: Footnote on R values and radiant barriers
So any question of doubling depth and doubling R value is not relevant. The question - if there is one - is whether increasing the depth of the quilt increases the Comparative Energy Saving Equivalent R Value, and the two data sheets explain the difference between SF19 (19 layers) and SF40 (36 layers).
Essentially, the latter is 50% more effective than the former. Hope this helps.
Note that as explained in earlier steps, I have added quilt to an existing 120mm layer of fibreglass. So I now have 120mm fibreglass (R= about 6), a layer of chipboard (R= about 1), and the SF40 (R= about 2).
But taking into account its radiant properties, the SF40 has an equivalent R value of 9.6.
For more explanation see http://en.wikipedia.org/wiki/R-value_%28insulation%29 (especially 4.3: The limitations of R-values in evaluating radiant barriers).
Also http://www.theyellowhouse.org.uk/themes/insula.html for a chart of R values of various materials.