wrote:

Does anybody know what increase in effective r-value you might get this way?

Very good, with lots of layers.

I live in the north and don't use AC, so reducing heat loss is generally
more important a consideration. I searched for a while with google
but didn't find anything very relevant.

http://www.rima.net/handbook/HandbookAll0504.pdf

See document pages 23-27...

You might enjoy solar heat with airflow from a low-mass sunspace and
shiny hot ceiling mass over the living space, as in the Barra system.

Nick