It really isn't about an R-value. Unless the structure was small enough to heat by body heat.

What is necessary is passive solar heating.
So, south facing sunlight accumulators and heat sinks.

And it is really hard to compare R-values to a below ground house that stays 60° all by itself.

But, it is possible to make an active solar heating with water heating panels on the roof and radiant floor heating/earth heat sink.

To answer your question, you would need about 12" of glass or foam insulation.