Ken,

Yes people have thought about it, Icelander's have done it for over 1000 years, and many traditional infill systems from Europe involve earth and/or clay. It should be noted though, earth provides thermal mass much more readily than it insulates. Icelanders, for example, build their earth walls 3 feet (or more) thick to achieve the necessary insulation needed for their winters, and lack of wood to heat with.

I suggest you look at the traditional houses of Norway, Iceland, and the Faroe Islands where turf roofs are quite common, and they have over the past 1000+ years developed a dependable and durable system for building such roofs.

I also suggest you look into the traditional turf roofs, rather than the modern 'green roof' systems, mostly because these do things that the modern system claims to be impossible, look for example at the pitches of some Icelandic turf roofs which are much steeper than the green roof people say is possible, yet in many cases these fine examples have stood like that since the Middle Ages.

frá Íslandi (Iceland):


Fra Norge (Norway):

Fra Færøerne (Faroe Islands):